|Heathkit HW-8 coil repair project (done to help out a USA ham)
Total parts and labour on my part only $15.00 - good for
repairs/projects for gear of a sentimental value.
|On the left is the original HW8 coil. It has an upper and a lower ferrite slug for adjustment of the tuned
circuit. -- The USA ham sent me this coil and I could not get the slug to move. So what I did.. I found a
similar coil form in my large collection of slug tuned coils and I gave him the materials necessary to
either rebuild the old HW8 coil or perhaps build up a "clone" or similar facsimile of the original coil.
|Measurements were made of the original HW8 coil. The upper coil was about 1 uh (.915
uh) and the lower coil was about 1.5 uh.
I made sure the tester (a cheap Chinese tester I bought from ebay) was giving proper
measurements by calibrating it against some known standard inductances that were
similar in inductance to the coils I was measuring.
With these cheap testers -- you have to keep an eye on them. Sometimes the readings go
off the rails and you have to hit the CALIBRATE BUTTON on the tester and /or reset it by
shutting the power on and off. This is very much a TRUST BUT VERIFY OPERATION.
|Above two pictures shows the calibration procedure. I take a known value of coil (around 1 UH) - (the yellow coils in the
picture) AND I make sure the Chinese tester readings are about correct. NOte: SOMETIMES there are glitches in the
operation of these inexpensive testers you can buy off ebay. Sometimes they need to be "reset" and the measurements
taken again. These are not "labratory grade" testers but they are good for hobby work if you are careful to verify the
|The upper coil measured .915 uh - The lower coil measured 1.563 uh.
|Below: the new coil form that I am sending for customer to use in his repair/rebuild. The new coil form is actually a used coil that has
slugs that move freely whereas the old HW8 coil has the upper slug stuck. The customer later wrote me and told me the HW8 coil had
the upper slug "glued"in place and he had to cut the slug out of it thus ruining the coil form. He used my new coil form to build a new
coil for his HW8 -- AND IT WORKED. ALSO IT LOOKED like it was a factory item when he got finished rebuilding it.
|click here to go to my START page of Netty Electronics.com
You may be pleased to know that I received your parts package earlier in the week, thank you. Canada Post seems to be able to consistently
deliver a package without damage. I'm really glad you are up there providing your service. Thanks for the fair price and fast turnaround.
This evening I finally found time to sit down and have a look. I just went ahead and chopped apart the original part , since we had measured it
anyway. Turns out there was a gob of some kind of superglue holding that busted slug in place. Some "genius" had glued it down, presumably to
hold his "alignment". Poor practice, in my opinion, but it was basically ruined. We never would have budged that slug. I did save the other, good
slug that was at the bottom of the inductor. If I ever need a replacement, at least I'll have it. It's in an envelope, carefully marked as to its origins.
The parts you sent worked very well. The plastic base with ferrite was a perfect match to the motherboard, so I carefully removed the single ferrite
which was original to the part and cleaned up the four contact posts. The dual slug-tuned inductor you included was too round and fat to fit inside
the casing of the base, so I shaved it down square with a razor and jeweler's files until it fit snugly. Then I added a spot of gorilla glue. It looks for
all the world like it just came out of a factory. Nice! My back-up plan was to tear down the original part and re-use the base, but your replacement
worked so nicely that I just went with it.
From there it was easy to wind on the two windings and solder them to the base posts. To my deep satisfaction, as soon as I powered up the
HW-8, I could hear AM modulation - which is normal when the oscillation is not occurring. A few turns of a 2mm twiddle stick and the crystal circuit
began to oscillate! Very nice. You were right about adding a couple more turns of 28-gauge wire. It definitely helped move the effective
inductance range into the middle of the slug's adjustment area.
One interesting tidbit for others who may work with this. I measured the inductance of the 7 MHz coil at 0.960 µH. However, once I soldered it into
the circuit, I measured it again at 0.724 µH. Once I get this radio perfectly aligned I will measure the in-circuit inductances of the four channels and
report those. It might be helpful to someone who wants to align the radio without removing/de-soldering any elements. The darn thing is going to
be tricky, since the two coils definitely have mutual coupling. The lower one has to be good, then the upper one knocks it out, so it's a lot of back
and forth to get things right. I can definitely see why dual slug tuning disappeared from common use. Good grief!
Purists might fuss a bit, as the replacement inductor was slightly taller, and I could not quite seat the shield against the board as tightly as the
original. But I tell you, it's a minor detail. The shield goes down low enough to completely cover the inductor and I cannot detect any coupling with
any other part of the circuit. The coil form just touches the inside lip of the shield, but it is fully enclosed. From the outside, it looks exactly as if the
original HW-8 parts are in place. Perfect! Everything else on this HW-8 is factory original. There are no modifications at all - poorly put together,
perhaps, but all original. I feel like this repair is forgivable, since it was never going to work on 40m otherwise.
So the next step is to actually re-align the radio. There may be other problems in the circuit still to overcome, but this one is repaired. I'll possibly
need to remove one turn or so as I align it, but since I have not yet secured the windings with nail polish, that will be easy. I'll report back the final
number of each winding I needed to get an ideal alignment.
Thank you so much for your help. Without that part I would not have been able to proceed. I will happily endorse your service to
anyone who asks. Please feel free to post this email to any forums you wish. The one thing I will tell everyone out there is - if you
have ANY kind of dual-slug inductors out there, hang onto them! They are getting hard to find.
When I get it on the air, I'll see if we can schedule a QSO on 40m. While I was waiting on this part to arrive, I put up a new 40m dipole. Last week
my Tuna Tins hit a reverse beacon in Toronto with 250 mA, so it's a fair bet you'll heard me loud and clear with my "power" HW-8 ;)
Cheery regards and 73,
John KC**9T (identity protected
|Below: the email I received back from the fellow about this repair/coil kit
|Above left...I found a potential base section for the new "cloned" coil form. It is plastic and just about the right size with the 4 pins
spaced about right.
The customer may choose to clone a coil or have a go at trying to remove the top slug (with plenty of hand pressure) and maybe he
can rebuild/repair the old coil form with parts that I have sent him.
|Here are some hanks of enamelled wire I sent
back to him with the coils.
The test coil winding is visible near the top of
(I did a test winding on the coil I had here to
see if it was "ballpark"similar inductance to
the HW8 coil.
All this was done for customer for a total of
$15 US dollars for parts and labour and it cost
about $7 US dollars for return postage.
The customer wrote back a good
recommendaton for my little service -->
I will happily endorse your service to anyone
-- to read the full email he sent -- look at the
end of this article.
So if you need a small job done like this..I can
possibly help.. my email is
firstname.lastname@example.org 73 earl ve3ab
|OK John. Im glad it all worked fine. I left the finishing touches to you because it would take a bit of time
to make things "look good". Im never quite pleased with the way my projects sometimes look. They work fine
electrically but I have to work on making things look better.
Ive used your email in an article I just wrote http://www.nettyelectronics.com/coilrepair
and I appreciate your kind words of recommendation. These are the kind of jobs I enjoy. They are short and
sweet and turn out well. I used to take in entire older rigs for repair ..but the postage is very expensive and also sometimes the older rigs
develop new problems when Im working on them-- so I really only work on entire large pieces of equipment that I buy to fix up on my own time
or stuff around here that breaks down. (Much less stressful
and no pressure type of jobs).
Sure..if you want to schedule a qso on 40 meters sometimes just let me know. My antenna is an LNR quad band end fed wire off the tower. I
have been getting some good signal reports with 100 watts lately and the radio Im presently using (IC-718) can be turned down to 20 watts or
so to conserve on energy. I often turn down the wick when Im on cw.
73 and talk to u agn earl ve3ab
|My reply to John as follows:
a small basement shop and
licenced small business in Elliot
Lake, ON. Canada
|This article is about a small repair I did in February 2016. I dont generally
fix or do repairs to entire radios now. Im too busy with my own rig repairs but I
do take on small jobs like this one. -- I do buy old broken down rigs for part out
and I take a try at fixing them first.
When I sell older "pulled" radio parts they are good tested parts and
guaranteed working. Im always in the market for parts radio projects and I buy
old collections of homebrew/radio parts (even used parts for testing.
MY EMAIL ADDRESS LINK IS HERE: email@example.com 73 Earl VE3AB