HUM - GR-54 & SB-301

[Bob]

A clue...if the hum is 60 Hz then suspect pick up or heater-cathode
short/leakage. If it's 120 Hz, suspect filters.

Bob

"Mark V. Johnson VE3DJU" wrote:
>
> 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.
>
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