FT-757GX Service Notes
~~~~~~~~~~~~~~~~~~~~~~FT-757GX Service Notes

~~~~~~~~~~~~~~~~~~~~~~

 

last update June 30, 2022

 

Some folks call this the ‘GX I, but there is no official model 1 of any

thing. Its just a FT-757GX.

 

We’ll start with a quick review.

 

The radio is noted to be ground breaking at the time as having every

option included – keyer, metering, IF Shift/Width, calibrator, all mode

and full general coverage too. The TX Finals are near bullet proof.

Metering is done by a real meter also.

 

The downside is that it is quite compact and is a challenge to fix in

some ways. There are no parts to speak of and while many are being parted

out on eBay these parts too are 30+ years old. Overall this isn’t too bad,

but parts that wear like shaft encoder are unavailable and any from a

parted out radio are likely to be worn also.

 

In the author’s opinion, the biggest problem with this radio is the “Local

Unit” otherwise known as the synthesizer. Its noisy and therefore reduces

the receiver performance some. It does not render the radio useless; its

just that the FT-757GX is readily beaten by more modern HF radio examples.

 

No one makes a radio that looks like this any more. Modern radios are all

SMD these days and not easily serviced at home. Neither is this radio

cost effective to send out for repair. Its not worth much and any real

repair that requires a board to be pulled to replace a part will exceed

in labor at $70/hr what the radio is worth.

 

For the few hams left that are capable this radio can be serviced at home.

Its still a good performer for general use and to those Radio Amateurs

that regard this as a hobby, its a good radio for mobile, cottage or

portable. In other words, a perfect General Use radio.

 

 

 

Authors Note

~~~~~~~~~~~~

 

To date I have received two pieces of feed back. This piece has been up

for four years.

 

One was from a fellow ham in W6. Thank you David H for your suggestions

to include the signal generator and the emphasis regarding the -8V supply.

 

The other was from a CB’r and technician berating me for my sarcasm

towards those that CB. He considers the modern CB person much more skilled

than I allude to. He specifically asked me to “tone it down”.

 

NO way.

 

First my opinion of those that CB is…. well, I don’t have one. I’ve

never bothered with it personally and to me it comes across as a cult. I

don’t mean that negatively. The folks that engage in CB seem to me to be

more interested in results than radio as a hobby. In other words while

CB involves a radio transceiver the attraction is one of belonging and

its socializing.

 

Last and in specific relation to the FT-757GX I’ve bought a few of them

in what might be referred to as “anonymous sources”. Read: eBay and flea

markets. In almost EVERY case there are traces of a CB history. The

“magic switch” has been thrown, the radio reeks of stale ciggies, the

knobs are yellow and there are streaks of goo running down the sides.

The ALC is backed off and anything that looks like a “drive” adjustment

is fully clock wise. Any soldering is done in blobs. In one case the

radio arrived and displayed “27165.0” upon first power up. It “swung”

close to 150 watts with modj and, as expected, sounded like crap.

 

So no. I am not going to edit out one word.

 

Thank God for CLR.

 

This “CB Butchery” is not limited to the FT-757. I have an FT-707 here

that was modded to work on CB. To do that, you change out some 10 meter

band crystals. The fellow used plug in crystals, probably from an FT-101.

The FT-707 are wire in. No problem, we’ll just drill out the holes with

the ol’ Black and Decker. Only have 1/16″…. it’ll have to do. No

problem… we’ll just bridge that gap with more solder.

 

The person that did this thought soldering and acetylene welding were

equivalent.

 

There are 4 crystal positions to choose from. Of COURSE he whacked the

lower 2. The upper 2 cover above 29 MHz. If you were using any brain

cells…. any at all you’d swap these out. On top of that rather than

bin the 10M crystals a forward thinking person would stash them inside

the radio.

 

But noooooooo……

 

Then there’s the FT-100D. It had the “Mars Mod”. Someone also discovered

the Service Menu and cranked every adjustment that looked like power,

modj, drive, compression, 10M and even sensitivity to “255”.

 

My experience concludes that the CB environment does not encourage technical

skills. Unfortunately older ham gear tends to fall into the hands of some

of these folks and the radio seems to suffer for it. In other words, I have

never ever witnessed a piece of ham gear that has escaped the CB environment

come out better than it went in.

 

I really don’t care about what folks doing CB do. None of my business. This

all started from a cautionary word regarding a new to you FT-757GX and its

possible history.

 

More so than ever before that caution is alive and well today.

 

 

Last and for the record Netty Electronics is just hosting this article.

It is NOT authored by Netty Electronics.

 

 

 

 

A Used FT-757GX

~~~~~~~~~~~~~~~

 

The problem with 30 year old HF radios is that they can have 25 years of

neglect. The first 5 years of a radios life is where it is respected and

has resale or trade in value. After that it appears to the market as a

“back up radio” and after that it starts to acquire the status of “beater”.

People stop to care anymore.

 

The FT-757GX is a perfect CB radio. It can be made to go on 27 MHz with

the flick of a switch… literally. Now all the attributes that made this

radio attractive at the Dealer in 1983 apply to the CB crowd in Year 2000.

The CB crowd has little to no radio knowledge other than mutual support

and that is often driven by rumor, myth and innuendo. Radios get modified

for more “swing”, punch, modj or horror – RF output (a modified FT757 will

“swing” 150 watts). These folks have little skills to do repair and will

crack a radio covers at the drop of a hat and proceed to modify a radio

with nothing more than vice grips and a 300 watt Black Beauty iron.

 

NO wonder 27 MHz sounds so horrible.

 

Simply put – if you are looking for an FT757 be very careful of where it

can come from.

 

The FT757 has a row of adjustment pots along the rear panel. The knobs

are often missing and the knobs are unobtainium.

 

The original model did not have that many factory changes although there

are a few. In particular Yaesu improved the CW QSK that was incorporated

by Series 5 and some Series 4.

 

The shaft encoder being a mechanical device that has been in constant use

for 35 years will show various extremes of wear. Usually the tuning knob

will wobble “some” without ill effect. In extreme cases the radio will

frequency jump. Don’t call Yaesu – they don’t have any.

 

The most common fault with these radios is the failure of the front end

PIN diodes. Usually they will be found shorted. Failure of these diodes

(depending on which ones, of course) often is illustrated by the FM RX

being dead or the Squelch control acting “funny”.

 

The S Meter is lit by 2 wired in bulbs. The illumination is inadequate

even on a new radio. Today all of them have burnt out at least once and

what they get replaced with can be very inventive. No bulbs are available

from Yaesu. Their orientation is critical for proper illumination. But

bulbs – reasonable replacements – ARE available.

 

The top cover is the Final heat sink. It is large, heavy and has a thermo

controlled blower. It also has gaps that allow small items to enter.

Thats often where I find the missing screws.

 

This is a 30+ year old radio and around now the electrolytic are starting

to dry out. Unlike the TS-50 these capacitors are not leaking out, they’re

just drying out. Relatively speaking they are easy to replace and not

worth troubleshooting beyond the first one that goes. Modern capacitors

are smaller and have a lower ESR. Once the board is out, may as well have

at as many as you can.

 

The FT757 has a memory back up battery. Chances are its near dead if it

original or possibly replaced by some interesting methods involving a small

soldering iron. I’ve also seen original memory batteries still soldiering

on and providing back up sitting at 2.65 volts.

 

 

The above is not in any way to represent Service Notes, but rather is a

short list of what are the nuances of a used FT757GX.

 

 

My Opinion of the “MARS Mod”

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I see no reason why anyone would do this.

 

Hams have “Ham Bands”. This radio was never good enough to operate on

60M. Don’t do it. If you run V/UHF with a transverter, this radio is NOT

a weak signal radio – not at that level.

 

If you are a CB’r, then use a CB set.

 

And last, the Preppers. You’re kidding – right? Its the End of the World

and you’re going to broadcast to the World your presence? There is jock

talk out there about an EMP. Do you have ANY idea what an EMP will do?

Two weeks after it goes off you’ll be eating your pets. You’ll be too

tired and too weak to lift a microphone. Get real. Sure have fun with

your games and run around in camo and a back pack full of MRE. The last

thing you would want is to tranmit your location.

 

Oh? The folks that want open transmit “in case of an emergency”. Who you

gonna call? Ghostbusters? Get a life.

 

An HF transceiver that has GC Receive does not have General Coverage

transmit through a modification that is “clean”. There is a reason why

Yaesu put band pass filters in the FT-757GX for each HF ham band. Yaesu

managed to second purpose their HF transceivers to a CB to add to their

sales. I question the ethics of doing so. There was a reason the FT-101

was the “serious CB Radio” of choice in the 70’s. There is a reason for

“The Switch” in the FT-757GX. Much the same reason as a matter of fact.

 

Let me lead by example. EVERY FT-757GX that has blessed my bench has had

The Switch thrown. After its had a bath and cleaned and serviced and had

any mod that wasn’t Factory removed just before the covers go back on for

the last time The Switch is thrown back.

 

I like my license. I don’t WANT or NEED to transmit out of the Ham Bands.

 

 

WHY the FT-757GX?

~~~~~~~~~~~~~~~~~

WHY would anyone in their right mind laud the virtues of the FT-757GX? It

is a 35 year old radio. Just about anything – even the cheapest dealer HF

radio – will eat it.

 

First… its true. I have an FT-891 here and for a $600 USD radio the

FT-891 will leave it in the dust functionally. I would hope so.

 

BUT…. consider this –

 

– with the exception of DATA, the 757 does all the modes an 891 does

 

– real knobs

 

– no menus

 

– a real S Meter and metering

 

– the FT-757 has a keyer

 

– speech processor

 

– RX IF Width and Shift

 

– a really nice illuminescent display, but only to 100 Hz

 

– built like a tank. The PA transistors are WAY over rated for 100W.

 

– carrying handle

 

– through hole parts. You CAN fix it yourself

 

 

Note that no where am I saying the FT-757GX is a better radio. What I

have listed here are the high lights of what makes it such a fun radio

to have around.

 

Thats all – just fun.

 

Absolutely, there is a downside too which I won’t list. Its an OLD radio.

There is NO dealer support, parts, or in many cases folks that will fix

this radio on a cost effective basis.

 

Note also it is an HF radio. Signals are usually quite strong and over

95% of the places you would use this radio will be limited by the back

ground noise anyway. If you get your radio – any radio – and connect an

antenna to it and the noise goes up in the speaker you have all the

sensitivity you can use. Are there some radios where this doesn’t happen?

You bet. A Heathkit HR-10 is stone deaf on 15M.

 

An FT-757GX will pass this test easy…. on any band and without the pre

amp. The pre amp is useless anyway. Details to follow as to why.

 

To be honest, I would not have the FT-757GX as my only radio. It is though

one of my radios that isn’t going any where. Its just too much damn fun.

 

 

 

The FT-757GX II

~~~~~~~~~~~~~~~

Obviously this radio followed the original model. It looks quite similar

and is exactly the same size. It shares all of the above cautions and

features of the original model, including part numbers.

 

Its main difference is the ‘II does not have Shift/Width. Instead, this

model has IF Shift and IF Notch. The latter is completely manual.

 

Aside from that, I think one of the major changes with the second model

was the improved computer control. Its still crude and don’t forget that

“Computer Control” meant DOS and at best a 286. This radio was 10 years

before any version of Windows. Windows 3.11 doesn’t count and never did.

 

In the authors opinion the original version with IF Shift/Width is much

more useful. To each their own.

 

 

Your “new” FT-757GX

~~~~~~~~~~~~~~~~~~~

You bought this used radio at a flea market or perhaps on eBay. Here you

are in the late afternoon and you can’t wait to power this puppy up. It

really doesn’t matter just what it is. It could be a Drake or a Kenwood.

The seller told you on his mother’s grave that this was his back up radio

and was only used lightly.

 

And you believed him… or you wanted to.

 

There is nothing wrong with what you are about to do – that is power it

up and play radio. But if you think for a moment this is where it stops

you are in for a surprise. What you have at this moment is an orphan and

guess what you have adopted? It has the status of “stray” and right now

your new acquisition needs to be vetted.

 

First thing to do is make notes. You need to go over the radio physically

and electrically. Read the manual. If you didn’t get one then down load

it from the Web. You need to play with it either on an antenna or into

a dummy load. As you do, make notes of what works and what doesn’t. This

should be good for about 2 hours. When you are done, you have a base line

of where to start to get this puppy back on line.

 

Unhook the radio and CLEAN it. Pull the knobs and get rid of the grime in

the knob grip areas. Clean the controls. Do what you can to get rid of

30 years of ciggy and cigar smoke. Make note of all the stripped cabinet

screws. Check it for power out and transmit range. Specifically the

FT-757GX is a prime candidate for open transmit and transmit output in

excess of 100W. If you care at all, that crap needs to get addressed.

 

 

 

Issues and Repair

~~~~~~~~~~~~~~~~~

Certain traits and faults are unique to the FT757. Specifically this

radio used steering diodes to switch signals, audio and the front end

BPF. It is these diodes and specifically the BPF where a lot of strange

operation from diode failure can take place.

 

The diodes in the BPF are severely under rated and prone to failure in a

strong signal environment such as Field Day. Their failure is often as a

short, and its this that leads to some interesting failure modes in the

radio itself. The good news is that a common 1N4148 is a good replacement.

These are NOT PIN diodes, but they serve well and provide adequate

isolation and low loss. This is after all HF. The bad news is I’ve seen

Yaesu use Schottky diodes in some production runs and you should not

mix them. 1N4148 have a forward voltage drop of about .65V and the

Schottky diodes are around .27 V. A simple VOM reading will tell you

which is used. In the case of the Schottky diode, use 1N5711. Now it

could be that I have encountered a radio thats been badly repaired, but

I doubt it.

 

This only seems to be in the radio’s front end. Similar diodes are used

in many other places in the radio but their failure rate is much less.

 

The specifics of the above will be addressed in further detail.

 

As you go through your FT-757GX you may encounter faults that are

difficult to run down completely. You get enough fault info but not quite

enough to nail it. This is the dangerous area of “repair”. You run through

scenarios in your head and say “Yup – its gotta be….”. This is where you

reach that Y in the road. It leads to fragging the radio or making things

worse. That is, finding the REAL fault more difficult.

 

You NEVER fix anything by modification. Thats both philosophical and

“Golden Rule”. As part of level setting the radio to nominal if your new

to you FT-757GX has had “mods” you need to figure out why they are there

and then get them out. There are very few mods out there that will make

an FT-757GX better.

 

Many folks have the knowledge to address repair at some level or another

but lack the equipment to do so. You can address a lot of repair using the

Marker and a good voltmeter. NEVER attempt alignment using signals from

the antenna. It simply does not work.

 

 

PA Transistor Replacement

~~~~~~~~~~~~~~~~~~~~~~~~~

The FT-757GX and other solid state radios of this vintage use bipolar

transistors in the RF Deck. In the case of this radio it is a 2SC1971,

2SC2395 and 2SC2879. ALL of these transistors are “New Old Stock” at RF

Parts. RF Parts also will not ship International any more.

 

The same applies to the 2SC2904 used in many iCom radios except this

particular transistor is NLA at RF Parts as of April 2022. Out of stock,

never to be seen again. Good luck with your IC-735 and IC-737. Perhaps

a few more too.

 

Modern radios mostly use the Mitsubishi Power FET transistors. And no,

you cannot use these transistors in a bipolar RF deck without some mods.

Maybe this is best left for the RF Engineers amongst us, although their

response might be “Why waste your time on a 40 year old radio”? After a

certain point that response has a lot of merit.

 

This leaves you to substitute something else while it is still available

or try eBay or Aliexpress. Much the same thing. You’ll be purchasing

mystery transistors from China. Many of these transistors are the same die

and not necessarily to spec – whatever they are. I suspect somewhere in

this swamp real 2SC2879 exist, but it is at best a crap shoot.

 

I would search out a working pull first.

 

 

 

One Last General Comment

~~~~~~~~~~~~~~~~~~~~~~~~

 

Its about aging.

 

It is amazing how many folks go to a flea market or eBay and as soon as

the radio is in their hot little hands begin to use it, pretending that

its as if it just came out of the box from the Candy Store.

 

Life doesn’t work that way, and the sooner you deal with reality the

happier you will be.

 

Mechanical parts age, break and wear down. Think switches on pots, band

switches, relays and front panel switches. Controls for volume, gain and

VOX delay wear. And worst of all, the rotary encoder gets sloppy and takes

on a life of its own. You rotate the Main Tuning clockwise and the

frequency decreases. The memory back up battery discharges and then barfs

all over the circuit board and the goo eats the circuit traces.

 

Meanwhile the components age. If its a tube radio… especially one with

vacuum tube finals the heat has cooked some resistors out of tolerance,

dried up the electrolytics and possibly has baked the circuit board

brittle.

 

If its a solid state transceiver the resistors are likely OK, but the

electrolytic capacitors aren’t. They dried up long ago. The problem is in

two areas. Obviously the value will have shifted some but there is a worse

item – ESR. Effective Series Resistance. Even if the capacitance is good,

thae resistance in series with it has doubtless gone up. Its this that

will make recovered audio sound constricted and woolly.

 

Specifically where the FT-757GX and ANY other radio of its age be ready

to replace every electrolytic you see, especially those used in the audio

and decoupling areas. They have exceeded their service life twice over,

and modern electrolytics are so much better to begin with.

 

 

Specific FT757GX Failures

~~~~~~~~~~~~~~~~~~~~~~~~~

 

1) Service manual errors

 

2) Intermittent

 

3) Disassembly

 

4) No FM; FM permanently squelched.

 

5) Drift

 

6) Memory battery

 

7) Low RF output on some bands

 

8) Poor or no receive on some bands

 

9) PLL alignment

 

10) Pre Amplifier Gain

 

11) Attenuator

 

12) CPU Reset

 

13) Reverse Voltage

 

14) Display Problems

 

15) Meter Lighting

 

16) Power On Problems

 

17) Pulling the Circuit Boards

 

18) Final Amplifier and the “MARS Mod”

 

19) Capacitors

 

20) Cooling Fan Grinding

 

21) Top Cover won’t fit

 

22) Radio Dead

 

23) RX Alignment Check

 

24) AM TX Set Up

 

25) S Meter is Loose or Sticks

 

26) PA (Final) Transistor Troubleshooting

 

 

1) Service Manual Errors

 

Setting the CW BFO offset crystal. The manual wants you to set the CW

trimmer TC2004 to 8.215.900. Ignore this adjustment. TC2004 is for

LSB. The actual CW adjustment is covered under Point I in the Service

Manual.

 

The manual tells you to adjust VR2006 for 100W out. On some RF Boards

its marked as VR05. Its near VR07. Until I figured this out I spent

a lot of time looking for VR2006. There isn’t one on some circuit

boards.

 

Some.

 

 

 

2) Intermittent

 

This is quite common in many old radios. In the case of the FT757 it

is often caused by oxidized pin headers. This is especially evident if

the display sometimes goes weird on you. The fix is simple and this

should be a common practice for every flea market and eBay refugee.

 

CLEAN THE RADIO

 

You need to do this as soon as the radio is in your hot little hands.

 

You need to pull out every pin header, hit it with some deOxit and

reseat. EVERY pin header. It will take you an afternoon. There are a lot

of screws to undo – especially to get to the PA deck.

 

Pull the knobs and get them into some warm water and dish detergent.

 

Windex, or better…. Invisible Glass.

 

I once had an FT757 that must have been in a barn with a heavy cigarette

smoker. This thing stank and was covered in brown dusty goo. The dial

skirt had smears of yellowy brown guck. The heat sink channels were

filled with a thick layer of brown gunk. There was only one thing I knew

of that would touch this…. CLR. I spent hours on it. I powered it up

the moment I took it out of the delivery box and knew immediately what I

was in for when the display read 27.165.0. It was advertised on eBay as

“works well but the display is intermittent”. I needed rubber gloves to

tune it.

 

The display problem was fixed with deOxit.

 

CLEAN THE RADIO

 

 

 

3) Disassembly

 

Improper disassembly can ruin the radio.

 

The top cover is heavy and held in place with just 4 – 3mm screws.

Attached to this top cover are many small wires and a few coax cable

runs.

 

The bottom cover screw removal is straight forward, but the bottom

cover itself will not come off without some GENTLE expansion.

 

The top cover will allow itself to be flipped over the back, but there

are a few wires and cables that are attached. They are completely

unforgiving if the top cover ever gets away on you. There is a large

cable harness tucked into the rear chassis joined to the top cover,

plus a number of wires connected to the back of the front panel. If you

have the bottom cover removed, the top cover will flip out on to a large

book about 1/2″ thick nicely. From this point on you need to move the top

cover and the rest of the radio as one unit.

 

The front panel is on a short rail. It is held in place by 2 screws on

either side of the main chassis. Once loosened and NOT removed, the

front panel may be slid forward about 3/4″. Don’t forget to undo the

ring around the Key Jack. If its missing, some one has been dancing

on the RF Deck.

 

When you go to put the radio back together note the large exposed harness

at the back of the radio coming out of the PA Deck. You will not be able

to get the top cover back on unless this harness is tucked away properly.

If the top cover will not settle such that you can screw it down without

force, then you need to have a look and see whats going on and why.

 

A SPECIAL NOTE ON THE COAX CABLES

 

NEVER remove a chassis mounted coax cable by pulling on the cable. EVER.

 

The coax cable is just press fitted into the little connector at the end.

Remove the cable from its socket by pulling on the “ears” that are

actually part of initial assembly crimp. Once you rip the connector off,

you’ll never get it back on.

 

There is an aluminum cover over the PA unit. Note that there are THREE

countersunk 3 mm screws. Two are in the center near the front and one is

at the back in the corner right on the edge. If the top cover will not

seat properly when you go to put the radio back together, you have a

normal screw where there should be the counter sunk screw, most likely.

 

 

 

4) No FM RX

 

This is VERY common to this model radio and usually is indicative of

other faults such as poor receive on some bands. Measure Pin 15 on the

MC3357 on the IF board while in receive. It should be near zero. Chances

are its high – perhaps 5 volts. DO NOT adjust the squelch level pots. The

shorted PIN diode is squelching the FM IC and providing some low level TX

voltage on RX to really confuse things.

 

You will likely be drawn to believe that Q68 the TX 8V switch is leaky.

Chances are its fine. The fault is a shorted PIN diode in the BPF. There

are a number that it could be, but do not despair. The fix will take some

time but it will not be expensive.

 

Proceed to Point 8) for PIN diode detail.

 

The squelch circuit for FM does not use the same method for the IC that

the manufacturer documents. There is an adjustment for the squelch

control, and there is also a master trim pot for recovered audio. The

radio has an all mode squelch thats AGC controlled, more or less. The

same for the scanning function as it too is driven from the AGC.

 

 

 

5) Drift

 

Its usually caused by the 15 MHz master oscillator. Part of the fix

for this involves changing the PLL crystals. Yaesu has no stock on

these items. You are reduced to changing the trimmers for ceramic

models. Mouser is your best choice and sometimes Dan’s. You want the

2 pin 6 mm variety. Yaesu no longer has the trimmers available.

 

There have been reports of these radios drifting, but I have yet to

experience it despite a number of FT-757GX that I have had.

 

 

 

6) Memory Battery

 

The radio is very inconvenient to use without a memory back up. Upon

powering up it will default to 7.000.0 MHz if the battery has gone South.

Every time.

 

There is a trick repair on the Web that allows for its replacement

without pulling the Local Unit. On the other hand while the original

memory battery is available and expensive, there is a common Lithium

battery that you can use in its place if you are not afraid of doing

a proper repair and pulling the Local Unit.

 

A near drop in replacement is a CR1220 Lithium battery. It comes with

solder tabs. Its possible, at least in theory, to snap the tabs off the

existing dead battery and solder the new battery to the stubs. Chances

are you’ll melt the solder on the circuit board but it might be worth a

shot.

 

 

7) Low RF Out On Some Bands

 

This is similar to 8) that follows but the symptoms are that the receive

is OK but the TX output is down either on a band basis or every where.

Once again, depending on what diodes are done the FM mode on RX could be

permanently squelched.

 

More or less, if a particular band is good on RX, it should be good on

TX. That is the BPF is common on both RX and TX. This is the circuit

block that has all the coils and is diode switched. The diodes switch

the appropriate BPF, but additional diodes steer or connect the selected

BPF between RX and TX.

 

Before attempting to fix low or no TX on a particular band you should

determine the health of the RX. Fix the RX first. If you determine that

the RX is fine, then your potential diode switching problem is limited

and much easier to find.

 

Part of evaluating the RX is seeing if the RX is squelched on FM. Does

the pre amp work?  If the RX is squelched on FM, quite likely one or more

diodes in the BPF and area are bad…. usually in a shorted condition.

 

If a particular band is dead on RX and TX then there is a good chance

that the BPF selector diode(s) are gone plus the BPF input coil. Not only

does the coil tune, it is also a DC path for the select diodes such as

D07 and D08. In this case L04 and L08.

 

You can use an ohm meter to test the coils. In most cases the diode

will show a short. That is instantly a bad diode but it can also be in

an open state. The diodes in question are 1SS83 for the BPF rated for

300 Vrev and 1SS53 elsewhere rated for 25 Vrev. Don’t let these ratings

fool you. Either diode has been found in a failed state in this radio.

 

 

8) Poor Or No Receive

 

In situations where the receiver is just simply dead, check the lamp fuse

on the RF Board. It can be replaced by just about any bulb of the same

size. Its not critical. Either you were in a strong RF environment, or

you used the transceiver for a dummy load or perhaps the bulb just died

from old age – who knows why it opened. Unconfirmed specs on the bulb

are 8V at 100 mA.

 

But before you put the covers back on, check the radio out. It could be

that whatever took the bulb out could have also cleaned out some of the

front end PIN diodes. Also check that you have FM RX and it squelches

properly.

 

If you find a band thats dead, but its diodes are good then you need to

ohm out the BPF coils, for they are the ones that complete the diode

conduction path. For instance for the 40M Band diodes D11 and D12 switch

the BPF array for 40M. Coils L14 and L15 not only are tuned to 7 MHz,

but they are the conducting path for the diodes.

 

Diodes D112 and D113 protect the RX during transmit. If the bulb is open,

these diodes are immediately suspect.

 

Diodes D03, D04 and D06 form signal routing to and from the diode

switched BPF. If the bulb is open, D03 and D04 could be bad. Consequential

damage could have D06 compromised and therefore have weak RX and TX

exhibited.

 

I have never seen D01 fail, but if it has you’ll need to replace it to

find out if Q01 and Q02 have been taken out.

 

Some of these diodes, in particular D112 and D113 may be located on the

bottom of the board. You’ll need to pull the board to get access. You

may see a blank component at the marked location on the circuit board.

If you need to pull the board, you may as well make the pre amp mod.

 

Before you start replacing PIN diodes you’ll need to see if they are

just silicon diodes or Schottky. If you encounter the latter you need

to use 1N5711 for PIN replacement diodes. Mixing them up may turn out

bad as the usual silicon diodes have a higher voltage drop and may stop

the others from conducting.

 

 

8) Part Two

 

The FT-757GX uses an MC3357 as the NBFM receiver. Referencing the Motorola

Data Sheet will give you a good idea on how the chip works, but Yaesu

does not use this chip exactly as per the Data Sheet. Yaesu uses Pin 15

to mute the IC. Almost ANY voltage on the pin will mute it. This is

likely happening if there is no white noise in FM RX with the squelch

open. Its likely happening because the TX 8V line has some voltage on it.

 

Measure Pin 15 of the FM chip in RX. It should be zero or even slightly

negative. If its more than a volt, the chip is being muted, most likely.

The “usual” cause is a bad diode (or even diodes) in the BPF. In some

cases its an “age thing” and I’ve fixed it by putting a 680 ohm 1/4 W

resistor. The TX 8V line uses little current and this resistor will force

an 11 mA load on the line, hauling any “leakage” to ground.

 

 

 

9) PLL Alignment

 

Is more or less straight forward from the Service manual. If there is a

crystal that won’t align, replace its trimmer with a 7mm ceramic. It DID

adjust at one time, so its either the crystal or the trimmer, most likely.

May as well blame the trimmer as replacement crystals are not to be had

any way.

 

The 15 MHz crystal frequency adjustment is critical for the display to be

accurate.

 

The IF Shift and Width controls must be centered. That is the lines on the

knobs must be pointing straight up, and this is only possible if the knobs

are positioned properly on the control shafts.

 

 

 

10) Pre Amplifier Gain

 

The pre amplifier has far too much gain to be useful. I question its

value to begin with since all it appears to do is just increase the back

ground noise. This is not the fault of the amplifier but rather is a

fault of the synthesizer.

 

You can reduce the gain of the pre amplifier and improve its AGC action

by removing R22 and installing two 1N4148 in its place. Solder the diodes

in series and insert the assembley in place of the resister. Cathode to

ground (bar end).

 

You can also elect to simply not use the pre amp. Its basically useless

on a healthy FT-757GX.

 

 

 

11) Attenuator

 

The attenuator has a 20 dB loss, which in use to me is excessive. Change

R04 to 220 ohms for a 10 dB loss or 360 ohms for about 15 dB. 360 ohms is

not common but is a common value in 5% tolerance. Feel free to use a 330

or 390 instead.

 

The board in question is the TX LPF Board.

 

 

 

12) CPU Reset

 

Sometimes after repair the CPU will lock up. Sometimes you might want to

clear memories.

 

Turn the radio off. Push in both black square buttons on the rear panel

(Linear and Marker). Turn the radio on. Turn the radio off. Reset both

buttons to their out position.

 

This DOES NOT APPLY to the FT757GX II.

 

 

 

13) Reverse Voltage

 

Many Yaesu HF radios of this vintage have a master relay thats engaged

when power is applied. The power on relay is on the RF deck and has a

steering diode in series with the relay coil. The radio simply will not

power up with reverse voltage applied.

 

No damage to this radio with reverse voltage.

 

 

 

14) Display Problems

 

The radio uses a custom display electro luminescent vacuum tube. I have

found the tube itself to be robust and encountering one that is dim by

it being at fault is rare. More often than not its a fault of the voltage

inverter and that is usually a fault of the inverter or the filter caps

for the negative supply.

 

The radio does have a negative 8V supply line driven by the display

inverter. Failure to have this -8V line is the responsibility of many

operational problems but is most likely observed in a weak display. The

inverter is unlikely intermittent, Dealing with the display and the

inverter is a bitch as it involves mucking with the front panel.

 

A lot of display problems are resolved simply by cleaning the radio.

 

I have had intermittent displays. In this case it was caused by the

solder failing on a pin header socket on the synthesizer. The wiring at

the front of the synthesizer board can get compressed by the top cover

and over time the pressure fatigues the solder joints on the PIN header.

 

 

 

15) Meter Lighting

 

Illumination of the front panel meter is poor. Its always been poor. It

was poor when the radio was new. Now, 35 years later it probably isn’t

any better, if not worse.

 

The radio originally had these bulbs in little blue booties that were

press fit into the clear plastic ears on either side of the meter. As

the years have gone by there have been many inventive ways of mounting

new bulbs. I have tried clear LEDs and the success is at best moderate.

 

It seems that there is no substitute for a real light bulb in this

application. Yaesu no longer stocks these bulbs. Grain of Wheat bulbs

will not fit properly. One needs to be careful to use a low wattage

bulb as the heat from the bulb will do damage to the plastic mounting

receptacle. A 12V bulb at 40 mA looks to be ideal and a good place to

start is a 2174 from Bulb Town. Its a T1 3/4 size bulb – 5.5 mm dia

by 8.6 mm long. Something like .22″ by .35″.

 

The meter lights up from the bottom and is never very bright.

 

 

 

16) Power On Problems

 

The radio will not power on with the MOX button pushed in. There is no

indication that anything is amiss, and you are lead to believe that the

power on relay or its switching logic has failed.

 

The master power on switch is not directly involved with powering the

radio on. That is done through a fairly sophisticated transistor switch,

10A relay and some TTL logic. The switching transistor is a Darlington.

The whole thing is steered through a silicon diode that prevents the

relay from closing with reverse voltage.

 

Both the MOX switch and reverse power appear the same – the radio will

not power up. If it won’t power up, it can’t be damaged. This may not

be the case in a mobile installation as likely the radio is grounded

to the chassis. Some mobile installations can fail in a spectacular

fashion depending on how the power is routed. A few scenarios can be

prevented by NOT connecting the DC cable negative to the negative

battery terminal.

 

A properly working FT-757GX will cycle the fan upon power up. The fan

will spin for maybe 1/2 second and then stop. Likely you will only be

aware of this with the radio opened up for service.

 

The 4 pin power connector uses all the pins. There is the usual 2 pins

allocated for 13 VDC power, but the 2 other pins, when shorted, allow the

final amplifier to function. This power connector was used in other Yaesu

of this vintage such as FT-107, FT-747 and FT-707. Its known as a Cinch

Jones S-304.

 

 

 

17) Pulling the Circuit Boards

 

The first time I had to do this I thought that I’d never get it back

together in a working state. It was a simple problem – the AGC capacitor

had opened up. All AGC was FAST. I found out that it really wasn’t THAT

bad to do.

 

Since then I have had the Local Unit out so many times and back in that

I can almost do it blind folded. Its on this board that the memory back

up battery resides. I have seen the Youtube on how to deal with this

without pulling the Local Unit, as if having to do this was something to

dread. Today, I’d pull that board without hesitation and have a new

battery in there quickly. No mercy. Pull the board. Replace the battery

with a CR1220. Do a proper job.

 

To get at any of the 757 boards you need to at least pull the bottom

cover. That means 8 – 3mm appearance screws, 2 screws for the side feet

and 2 screws for the handle. When I pull a radio apart I have multiple

bins for the hardware. This would all go into its own bin and I’d start

a new bin for the next “layer” of dismantling. I use old pill bottles.

 

If you just want access to the RF Unit you only need to undo the four

screws at the top cover/ heat sink. Once you do that, you need to be

very careful in handling the radio and heat sink. They need to move

together.

 

To access the RF Unit and Local Unit you need to loosen the 4 screws on

either side of the front panel such that you can slide the front panel

out about 3/4″. This will give you room to extract the RF Unit Circuit

Board. Before you undo any of the circuit board screws you need to gently

pull all the pin headers and the coax cables. Note the previous caution

on dealing with the coax cables.

 

To gain access to the Local Unit circuit board you need to unscrew all of

the above, plus loosen the top cover/ heat sink and fold it back. Next,

remove the shield and then uncouple all pin headers and coax connectors.

Remove the 3 pin header from the voltage regulator bolted on the side.

Near that IC on the circuit board are 2 – 5 pin headers. Mark one of

them so they don’t get swapped. Note that some wires were above the

shield. Remove the 4 small knobs at the back and undo the ring around

the KEY jack. Slide the board out by moving it back towards the front

panel and over the side with the regulator. You may need to push in the

two switches to get clearance from the rear panel.

 

To gain access to the PA unit and the LPF you need to do all the

preceding, fold the top cover back and undo all the screws holding the

shield in place. Its obvious how it is removed, but be VERY careful of

all the wires and coax cables surfacing out from the cut outs. Note

which screws came from where – they are not all the same.

 

The front panel is removed by removing the 2 screws on each side of the

chassis and removing all the cable headers. The main tuning knob is a

slip on – it just pulls off. Behind the main tuning knob is the knob

drag mechanism.

 

The keyer sub board on the top cover is just screwed in. Be careful not

to loose the 3 slide knobs.

 

 

 

18) Final Amplifier and the “MARS Mod”

 

The PA transistors are 2SC2879 and are capable of more output than the

radio’s rated 100 watts. Actually, through mis adjustment the radio is

capable of almost 150 watts. That doesn’t mean you should do this. That

does not mean the transmitter will be clean at this power and it also

means that you have lost any cushion against a mis matched antenna.

 

The radio is also dead simple to do the “MARS Mod”. Its just a simple

matter of throwing a switch. It completely baffles me why you would want

to do this. EVERY FT-757 I’ve encountered has had the switch thrown for

open transmit. One of them came to me in the mail and when I fired it

up for the first time the display read 27.165.0. Uh huh. It also looked

like it came from an Opium Den and smelled like it too.

 

Its a 100 watt radio. Keep it that way. Its also a Ham Band transceiver.

Thats where the transmit should be used. Only there.

 

Get it?

 

Oh yes. I have a few of these radios. All of them now have the open

transmit disabled. I cannot think of any reason why a Ham would open up

the transmit on a transceiver other than “Angst Against the Machine”.

 

There is a control on the back of the radio without a knob. Its the “AM

Carrier” control and usually you will find it fully clockwise. Like many

SSB transceivers the FT-757 does AM TX by injecting a carrier on top of

a normal SSB transmit. From the Balanced Modulator to the SO239 the radio

is a linear amplifier for low level generated AM. The absolute maximum

carrier the radio can support is 40 watts and still stay linear. I set

this control to 25 watts carrier.

 

Part B – an exception.

 

You can use the FT-757GX as a signal generator.

 

The 4 Pin power connector uses just 2 pins for power. The other 2 pins

are jumpered and provide 12V to the final transistors. If this jumper is

missing there is no “transmit” but there is output on the LL Out phono

jack on the back of the radio.

 

You can “make” a signal generator by throwing the “MARS Mod Switch” AND

disconnecting the jumper on the power plug. Better still buy a Cinch

connector and make up your own “RF Generator Power Plug” for your

FT-757GX.

 

The output from this phono connector should NEVER be applied to the input

of a receiver. Its documented as 100 mV P/P. I would also NOT trust the

frequency accuracy.

 

 

 

18) Shaft Encoder and Parts Availability

 

A good assumption with an FT-757GX is that there are no factory parts.

Thats not literally true in some cases like the power on relay or the

meter light blue booties, but the real meat and potatoes parts like the

shaft encoder and Local Unit crystals are long gone.

 

The good news is that yaesu Service Manuals are on line, and in some

cases a newer model uses much the same parts and they ARE available.

I have yet to test it, but my gut feeling is that a shaft encoder

from an FT-840 or FT-890 might work just fine for instance. I haven’t

tested this (yet).

 

In quite a few cases a later radio uses the same part under a different

part number.

 

And last – there is always a junker around from which to raid. In the

case of the FT-757GX the production run was quite stable. The shield

over the Local Unit changed over time but that is nothing compared to

the sea of board changes the FT-107 went through. You are pretty safe

buying a “working pull” from just about any FT-757GX. In extreme cases

you might loose or get a factory QSK mod, but thats about it.

 

Most of the transistors used are “nothing special”.Netty Electronics may

have some in stock. The MOSFET transistors in the RX IF and the 2SK125

in the balanced mixers are available and rarely fail.

 

The major exception to all this is the 2SA1193. These are NPN Darlington

transistors used in the TR switching and in the power on relay. They

are unobtainium. I’ve had success using MPSA13. NOTE that this is not

a pin for pin replacement. The basings are different. Fortunately these

transistors rarely fail unless the 8V line is shorted to ground.

 

 

 

19) Capacitors

 

At one point I had 2 working 757, one beater and one in an undetermined

state. I took the last one off the shelf and fired it up. It didn’t play

all that well. To get a comfortable volume I had to advance the control

further than I should have to and the audio that came out the speaker

was kind of muffled and wooly. This “sounds” like capacitors to me, so I

decided to do an experiment. I’d pull the RF Deck and change all the

electrolytic capacitors I could get my hands on and then see what the

results are.

 

Modern low voltage electrolytic are MUCH smaller than the ones of 30

years ago. In addition Yaesu used a lot of electrolytic rated for 50

and 25 volts. This resulted in them being huge. Most of my replacements

are rated for 16V which is perfectly adequate.

 

I didn’t bother with the 10 uF around the LPF area but instead

concentrated on “audio”. This included the 470 uF for speaker coupling

and as many of the 22 uF and 1 uF in the audio section around the Speaker

IC and phono jacks as I could get to. I also needed some 100 uF, 4.7 and

10 uF caps to complete the task. In total, I probably changed close to

20 electrolytics on the board.

 

I put the board back in the radio and applied power. The difference was

dramatic. The audio is bright and crisp and there is plenty of it. So this

is what a new FT-757GX sounds like?

 

W0W!

 

I confess that I had already done this to an FT-107 last year and the

results were similar, but not as dramatic. On this particular 757 I

would say the results were “Night and Day”.

 

 

 

20) Cooling Fan Grinding

 

The cooling fan sits in brackets and they are mounted on one end to the

PA heat sink and to the other – the thin aluminum cover. The tolerance to

the bottom of the heat sink to the fan disk isn’t much, and the fan must

hover over heat sink near perfectly parallel to it. You will know

immediately of trouble in this area upon power up. When the radio is

powered up the fan will rotate for a few seconds. The radio should power

up dead quiet with the volume turned down.

 

If it doesn’t you’ll need to flip the PA and its heat sink out and remove

the aluminum cover. Its there you will have the fan exposed. One end of

the bracket will be bolted to the heat sink and the other will be in the

air. Spin the fan with a small screw driver or power up the radio. The

fan should turn freely. If it is binding you need to gently bend the

bracket. Inspect the cover and make sure it isn’t dented and that it lies

flat on the table top.

 

 

 

21) Heat Sink Won’t Fit back On the Chassis

 

You have one or both of the following situations….

 

You took the PA cover off and you didn’t put the screws back in where

they came from. The top cover has 3 counter sunk 3mm machine screws

and they sit in a row along the back from the RHS. More than likely

you installed a 3mm cap head screw along this line.

 

The wiring harness isn’t properly tucked away at the back of the shield.

Its either sitting on the shield or you have a wire sticking out and

you’ve jammed the top cover over it.

 

 

 

22) Radio Dead

 

As previously noted you cannot kill this radio by applying reverse power.

No doubt this is one of the reasons for an FT-757GX surviving all these

years.

 

Make sure the MOX switch is not engaged when you apply power. This is yet

another smart think on Yaesu’s part. The radio simply will not power up

with the MOX switch engaged. This was mentioned in 17) above.

 

If you hear the power on relay close check that the cooling fan spins and

then stops. This is a sign of *some* life, at least. Does the meter back

light illuminate? If you back off the RFG does the S Meter advance? This

does not mean that the RX is healthy; it just means its not completely

dead.

 

Nothing will happen until that power relay closes. It is closed by a

transistor switch and a series voltage steering diode. You need to get this

relay to close before anything happens.

 

If you get the relay click and the cooling fan to spin but the radio is

still dead…. the next thing to check is the inverter for the display.

Its located on the back side of the front panel. In addition to powering

the display tube, the inverter provides -8V for the radio. You can even

have a working display and a dead radio. Basically the FT-757GX low level

signals are powered by an 8V LM7808 regulator. The AGC amplifier, an

6555 (LM358) is also powered by -8V from the regulator. No -8V, no AGC

and no RX. Non negotiable.

 

The CPU is a Motorola 68050. Its proprietary to the FT-757GX. If its sour

I’d just go on eBay and get a new LO board.

 

Last but not least right at the beginning of this note I did tell you to

clean the radio. This also included pulling every connector and running

some DeOxit on the connectors. You DID do that, didn’t you? I have gone

through many FT-757GX and bought a few advertised as “intermittent….

for parts or repair”. In EVERY case the radio was fixed with DeOxit or

the male posts on the solder side has had a soldering failure.

 

 

23) RX Alignment Check

 

The Noise Blanker uses the RX IF strip ahead of the crystal filters for

its signal processing. In addition it has its own AGC in dependant of the

RX. A simple check on any band is to engage the Marker and NB. Tune in

the Marker. The voltage on G2 of Q13 should measure about .85 volts. It

is negative AGC. If you adjust T2008 out of alignment you will see the G2

voltage increase.

 

Most of the RX alignment can be done with the Marker and the S Meter. You

can also use the paragraph above and tune G2 for minimum. A trick is to

control the input by mistuning ONE IF transformer near the 47 MHz crystal

filter. Use the S Meter and tune the RX IF chain for max, all the while

using T1005 to keep the S Meter around S6. The last step is to tune T1005

properly using a low level signal.

 

Needless to say the alignment of the NB IF is critical to a well working

NB, and the one in the FT-757GX is quite effective.

 

 

24) AM Operation

 

I need to add in a special entry for the above mode. CB is still alive

and well in 2021 and has over the years expanded from 23 channels to 40

channels, all of 27 MHz and now all of that and a bit of the 10M Amateur

band. You’ll notice the latter especially if there is “skip”.

 

Like most Amateur Radios, the FT-757 transmits AM by injecting a carrier

with an SSB function. There are a number of ways to do this, but most

often its done by unbalancing the low level balanced modulator. As this

signal is generated for it to get to the antenna it will follow the same

path as if it were on an SSB mode. As far as the radio is concerned it

IS. What this means is that all of the amplifiers in this chain must be

linear and stay that way in this process.

 

On the FT-757 there is an access hole for a trimmer adjustment. Its the

“AM Carrier”. You will usually find it full clock wise. This is a strong

clue of a CB history. Many Hams go through life never transmitting or

caring about AM transmission. The manual has some hocus pocus on how to

adjust this trimmer. I have a simpler method. On the AM mode, set the

Carrier to NO MORE than 40 W out. Bare in mind this is a 100% duty cycle

mode. When you speak into the mic, the output should increase upwards to

about 60W. NEVER down. Down is an indication the mic gain or carrier is

set too high.

 

Thats it. At 40W carrier on AM you have maxxed out the capabilities of the

PA transistors.

 

This method of generating AM is generally not the method used in a real AM

CB set. In this case the radio has a real audio amplifier that modulates

the final and driver transistors through a transformer. This is called

“high level plate modulation”. The term originates from Vacuum Tube radios.

 

Exceeding the above guide lines will be readily apparent through horrible

transmitted audio and how “Skippy” wipes out the band when he comes on.

I’m not condoning the FT-757GX on CB, but if you must do it, at least do

it right.

 

 

25) S Meter is Loose or Sticks

 

The S Meter really shouldn’t be sticking since its not exposed. It sits

in its own plastic case thats part of the bezel assembly. Usually what

causes it to stick is because its no longer solidly attached and is

floating behind the front panel. We can fix this, but be prepared for an

interesting operation.

 

The front panel is actually a plastic cover that goes over a sheet metal

sub panel. The sheet metal sub panel mounts all the front panel controls

and switches and soft push buttons. AND the S Meter. You will need to

remove this plastic cover (read “front panel”) to change the Shaft Encoder

or service the knob tensioner.

 

The plastic front panel will come off easily after removing all the knobs.

The main tuning knob is a slip on. There is no grub screw in the knob.

Once all the knobs are removed the front panel comes off by removing the

2 3 mm screws on each side and the 3 behind the tuning knob. Be prepared

for the push buttons to fall out.

 

The display mounts behind the metal sub panel.

 

The S Meter and bezel with the purple cover for the digits mount in front

of the sub panel. Only one screw holds it in place and more than likely

the plastic tab which is part of the S Meter/ Bezel Assembly is likely

broken. Forget trying to fix it.

 

Unsolder the S Meter tabs and cut the lamp wires where they are soldered

to the bare wires sticking out from the rear of the sub panel on the

display board. If you are lucky enough to still have a tab unscrew its

mounting from the sub panel.

 

If the tab is broken, forget any idea that you can fix it. You can’t

glue it back together – it won’t hold. I thought of making a tab from

some copper craft wire, and epoxy-ing it to the Bezel. You will notice

that this bezel assembly fits into the plastic front panel. It needs to

mount on the sub panel precisely and chances are you or I do not have

the skills for the required precision. You need to do what Yaesu should

of done and glue this bezel assembly to the front panel. Why didn’t

Yaesu do this? Why did they bolt the bezel to the sub panel? I’m not sure

but I think it was done so as not to make the front panel an expensive

part. Had they have glued the bezel assembly in it would of included an

S Meter.

 

You will note that on either side of the S Meter at the back are 2 “wings”

for mounting of the S Meter back lights. It is here that I see a lot of

inventive butchery. Each wing is a wide tab with a large hole in it for

one of two miniature lights. The hole is larger than the light, and the

reason for that is that Yaesu mounted the light in a blue bootie and

THAT slippled into the hole in each wing. The reason for the bootie is

to keep the bulb heat away from the plastic. Most of the time I find

these tabs partially melted. Last I checked, the booties are still

available from Yaesu. In extreme cases, there is no tab and hole for the

bulb. Its all been melted away. You have little choice but to clean up

the mess as best you can and glue the bulbs in. Gorilla Glue works nice

and it now cmes in Clear.

 

If you glue the bezel to the front panel, be a pal – write a note and

leave it inside the radio. You’ll forget what you’ve done or the next

owner will pull the front panel and end up ripping the leads off the S

Meter.

 

Last, how do I get all those push buttons back into the front panel and

get them to stay without losing my sanity while I put the front panel in?

 

Well, you do have a good picture of the front panel, don’t you? Of course

you do. You did pick up all the push buttons off the floor when you took

off the front panel, didn’t you? Of COURSE you did, and you took inventory

didn’t you? Of COURSE you did.

 

Here’s what you do. Its incredibly simple.

 

Put all the buttons back into the front panel. Take some fresh Masking

Tape  and run it across the buttons on the front panel. Now you can put

the front panel back on. Yes, you could of used the masking tape when

you removed the front panel in the first place, but that would be too

easy wouldn’t it?

 

Just checking to see if you read this all the way through.

 

You’re welcome.

 

 

26) PA (Final) Transistor Troubleshooting

 

You have an FT-757GX on your hands and its TX output is low. Where do I

start to fix this problem?

 

Is this radio new to you or did it suddenly develop this problem? If its

new to you, of course you will suspect the 2SC2879’s are gone. If it was

working fine and suddenly went sour then you might have a clue where the

problem might be. For instance the relays in the TX LPF could be bad or

the T/R relay. You could also have a fault in the ALC limiting drive, and

so on.

 

But lets prove or disprove the transistors in the PA section in the top

cover/heatsink first.

 

After you have the top cover off and the cover plate removed set up as per

the manual for final bias adjustment. They should be drawing the current

specified without any adjustment. Sure… its been years, but the idle

current should still be close. If it isn’t, we need to look a little

further. Move the bias adjustment. You should be able to adjust the idle

current between zero and 3A on the PA transistors.

 

If you have a scope by all means see whats on the collector of the pre

driver, PP Driver pair and on the PA transistors. Pull the coax from the

2N4427 LL Driver. Using that coax as input and a scope on the 2SC2879

collectors 100 mV from a signal generator should produce LOTS of

output – perhaps 10 to 20V of RF. After all, this PA chain takes about

1/2W from the 2N4427 and makes 100W – a gain of better than 23 dBm.

 

So far, no joy. Time to pull the 2SC2879. and try them on a transistor

tester. They should produce a low signal gain of about 25.

 

OK, even that pans out. How do I test the drivers? Well, the drivers need

to produce 7 to 10W out for the PA stage.

 

Solder a 0.1 uF cap between the base and collector pads of the (removed)

2SC2879. Each transistor. Button it back up and connect a dummy load.

Operate the radio as you normally would. Full power on FM should produce

at least 7W out. No output, the 2SC2879’s could be good. You DID reconnect

the cable from the 2N4427 didn’t you? 7W out or more…. its not looking

good for the 2SC2879.

 

But how does this trick work? Well, the driver transistor output is trans-

formed down to 1 turn on the secondary. The PA collector primary is 1 turn.

All we’re doing is going down to 1 turn and coupling across directly to a

1 turn primary on the output.

 

 

 

Summary

~~~~~~~

 

This has been my experience with this vintage Yaesu. The radio STILL

does a good job on HF even 35 years after its release. Perhaps its not

a Killer – if even it was 35 years ago, but its still perfectly usable.

I won’t defend it by saying “there are worse”. Thats not saying very

much – is it?

 

The attraction to me is that these radios are “easy” to service. There

is some SMD in there, but you have to look for it. Its 99% through hole

assembly. Its built like a tank and the finals, if run at 100 watts are

bullet proof. It has a real S Meter and there are NO accessories to buy.

It came out of the box in 1984 ready to Rock’n Roll as is. Yes, a modern

radio – say some thing as lowly as an IC-718 will run with it as an equal

if not “better”, but between the two the 757 has much more charisma and

charm. Plus…. I can fix it and align it.

 

This radio will never be sought after or remembered fondly like a TS-830

or an FT-902DM. It will forever be looked upon as “an old HF transceiver”.

You can get a good hint of this by the number of them being cut up and

having the pieces sold on eBay. There is no shortage. This is indicative

of both lack of parts, general failure with age and it being not cost

effective to send out for repair. It also suggests the overall lack of

skills to fix these radios and possibly the desire to do so in the first

place.

 

While we may be in a minority there are still folks out there in “radio

land” that really are not that attracted to modern radios despite their

obvious functional superiority. A Yaesu FT-891 will blow the doors off

an FT-757GX.

 

But its not the same.