HUM - GR-54 & SB-301

[Mark V. Johnson VE3DJU]

Hello all. A month ago I posted a note regarding my ordeal with fixing a
HUM problem on my hw-16. Well its seems I can't buy anything Heath without
it having a hum problem. The latest is a GR-54 Gen Coverage receiver and an
SB-301. Thought I 'd share my work and invite comments.

The GR-54 worked great but in standby or no volume, there was a hum that
was very annoying. With this radio, standby removes B+ from the RF amp
stage...The audio circuit continues to run. I tried new electrolytics, no
change,  I also changed every bypass and coupling capacitor in the audio
stages, no change. I isolated the AF amp by removing this input lead to the
6HF8 and the hum reduced. BUT there was no hum from the previous circuits.
It was almost as if the input lead (shielded) was picking it up. I replaced
the input lead with new shielded wire..no change. Finally, I put my dual
trace scope on the audio output and the power supply to see if the hum was
60hz from the power supply. Looked like it was.  At that point, my wife
came in to speak to me and low and behold I could see her voice modulate
the waveform. Her voice was being picked up through the on board
speaker...Guess what..Connect an outboard speaker and almost zero hum. What
was happening was the speaker which is mounted right next to the power
transformer was acting like a mic, picking up the transformer hum  and
feeding back into the circuit. Don't they (experts from long ago) tell you,
never put a speaker inside a receiver...I now know why! Great that I just
invented the wheel eh? I also think because of poor high end (15 - 30 mhz)
sensativity, the audio stage had to be fairly high in gain to compensate.
This will also make it susceptable to hum problems..Just a guess.

The Sb-301. Got it in a trade. Brought it home, checked for any obvious
problems and checked for the right fuse. Plugged it in..Got signals and
HUM. Remember, everything I buy hums. Going back to my Mainframe Computer
repair days I started doing a pre-install screw audit. Sounds dumb but on
that stuff intermittent connections was a big problem. First duty on any
install was a screw audit..Thats is tighten ALL screw connections. Well, I
started with the 2 screws on the filter cap...One twist bam no hum..Solid
as a rock and QUIET. Talk about a quick fix!

Hope this helps..Welcome any feedback. Thanks for the space.

Sponsored by the City of Tempe 

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