[Heathkit] GR-78 modulation hum

[Mike McCarty]

SX-25 wrote:
> Hello all,
> 
> Has anyone any experience dealing with modulation hum on the mid-70s

I have experience dealing with modulation hum in a number of
vintage radios, not this one in particular.

> 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

The first place to look is the "line bypass" capacitors. That is
their main function. Modulation hum is usually caused either by
too small filter caps, or missing or inadequate line bypass caps.
I can't read the value on the blurry schematic I have, nor the
component numbers, but the two line bypass caps need to by replaced
with Y2 rated caps.

> 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

Heater/grid bypassing problems causing hum is usually a result
either of a heater-cathode short (in unipotential cathode tubes)
or inadequate filament supply filtering (in directly heated
cathode tubes). Neither of those can cause modulation hum.

> wire bundle may be the culprit, although it is tightly dressed and
> routed precisely as Heath described in their assembly manual.

That also would not cause modulation hum.

> Anyone have any other ideas?

The cause of modulation hum is excess power being absorbed in
the power supply when the power rectifiers "top off" the reservoir
(input side) filter cap. If the cap stores insufficient energy,
this can cause it. The purpose of "line filter" caps is to provide
a constant, low impedance short at RF across the AC mains power line.
This mitigates the problem of the RF impedance across the AC line
suddenly dropping down when the filter cap get switched to the
AC line for "topping off".

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

Likewise, OM.

Mike
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