Heath GR-64 Audio Distoration Problem

[Mark V. Johnson]

Heath team;
Very snowy day here so I thought I would  share my latest repair
adventure. A very humbling experience when you consider a GR-64 SW
receiver is really nothing more than  a souped up "All American Five" radio.

I go this cheap at a swap meet in the fall and grabbed it as the front
plastic dial piece was not cracked or scratched. I have an affection for
GR-64's as I got on as a kid at Christmas in 1969. It was my first real
SW receiver and one of my first Heathkits

This radio was very well built. In fact a sticker attached inside showed
it belonged to a Radio TV Repair guy. I did my usual check of solder
connections, tubes, all voltages and screw tightness and aligned it.
 From the first moment I turned it on, I was never satisfied with the
audio. A slight, but noticeable distortion of the audio was present. No
tweaking of alignment or tuning would cure it. Drove me nuts. At one
point  I actually convinced myself it was working as designed and that
was that. I then decided it was just not right and got down to trying to
find it. I checked all the bias voltages very carefully as it seemed
like a bias problem. I replaced all the electrolytics as I thought a 60
hz hum may be the cause. No change. I then abandoned logic, my scope, my
extensive collection of test gear and pulled out the manual and started
checking the wiring.

Sure enough, found it. Capacitor C24 a 500pf cap had a .005mfd cap in
its place. From hindsight, its easy to see what happened. The builder
likely ignored the step by step instructions and placed components on
the circuit board by using the printed values. 500 upside down can be
quickly read as .005. This cap is part of the AGC filter circuit.
Changed it out (to 500 pf as per spec) and now the radio plays
perfectly. This radio never worked right from day one!!!

The lesson is that one again, we (I in this case) need to be reminded
that Heathkits are kits and you can't assume they were built right in
the first place. I've fixed many tough bugs over the years but this
simple one almost got me!

Best of the season everyone!

Mark V. Johnson VE3DJU/VE3DDI

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