[Heathkit] GR-78 modulation hum

[Singley, Rodger]

The GR-78 is an odd unit since effectively it is always running from the nicad battery pack although they are being trickle charged from the line if it is plugged in.  As I recall, the receiver could not be run 24/7 because the current draw exceeds the recharge rate which is an interesting design.

Since you are getting modulation hum even with the AC line disconnected I wonder if your problem is actually due to your nicads approaching the end of their life and exhibiting higher internal resistance.  If that is the case, then your "B+" isn't as stable as it should be and the impact of one part of the receiver circuit on other circuits is possible.

Unless your batteries are nearly new, I would consider replacing them.  At the least, check all the cells separately under a moderate load to see how they behave.  It is possible your conversion oscillator is being lightly modulated by swings in the battery supply caused by the varying current draw of the audio stages.  Does the problem become more pronounced as you increase the volume?

Also, since your GR-78 does run on batteries and has a built in whip take it out away from your other radios and antennas to make sure that the modulation hum isn't being created in another member of your receiving stable (or a preamp or active multicoupler).

Good luck with it.  I have a GR-78 and have never noticed a problem with modulation hum.  But it isn't one of my favorite receivers and I really like its older brother (the Mohican) better.

Rodger WQ9E


-----Original Message-----
From: heathkit-bounces at mailman.qth.net on behalf of SX-25
Sent: Thu 12/17/2009 9:47 AM
To: heathkit at mailman.qth.net
Subject: [Heathkit] GR-78 modulation hum
 
Hello all,

Has anyone any experience dealing with modulation hum on the mid-70s vintage Heathkit GR-78 portable shortwave receiver? As with most modulation hum it is prevalent on stronger stations and the greater the signal strength, the greater the hum. I have checked out the usual things, bonding of the tuning capacitor to ground...voltage being supplied to the front end and removal of AC charging and allowing it to run just from batteries. In my tube type radios modulation hum is often cured by arrangements of filament to grid bypassing, but obviously such is not the case here. Of course, my wire bundle may be the culprit, although it is tightly dressed and routed precisely as Heath described in their assembly manual.

Anyone have any other ideas?

Thanks and hope all on this reflector have a safe, healthy and happy Christmas in peace.

WPE9GHF
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