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ROTEL ROTEL - Ground Hum From Milan to Minsk

Thanks, @mbz , really appreciate all the counsel here, not a fun problem to attack but still a worthy puzzle.

Since we've essentially bled the stone dry I'm just curious, could there still be a component issue somewhere in the power amp that could be causing the hum? I've replaced entirely too many parts thus far, but, if there's a chance that staying the course may produce success I'm happy to keep plodding ahead with replacing potentially problematic components. The output transistors are still original along with a couple of the small transistors on the power amp section so if they could be causing the issue I may have some replacements to pop in.

In the meantime, I'll get after the testing you suggested and see what I find and also start looking for a new scope too, ha! Would be nice to have one I can count on.
 
A quick check of the base of the output transistors (2SC789's) revealed something interesting, I hope. TR606 (working left to right based on pic in post #59) had a fairly flat "wave", the two center output transistors showed the curly, non-consistent wave shown in the first pic, but the far right transistor actually presented the rectified wave. This was with the V/Div at 20mV and the Time/Div at 2ms, no tweaking the setting, just moving from base to base so I could establish a baseline. Helpful?


IMG_1196.jpg IMG_1195.jpg
 
This would make more sense to me if it was main rectified 120Hz hum that you are hearing. There is a mobile phone app that can "listen to and decode" the hum, alternatively you could record it and load it into "audacity" or similiar SW.

You could try checking at TR612b or wiper of VR604, also try TR609b.
 
I've been using a frequency generator on my phone and matched it to the 60 Hz tone, but I'll double check to confirm it's not 120Hz.

Also doing some random voltage checks throughout the power supply and seem to be getting some significantly higher numbers than shown on the schematic. I'll record my findings and update tomorrow. If I'm getting some wonky numbers voltage-wise perhaps the overages could be contributing to the issue, hopefully. More soon!
 
How does this end! I have the same issue on an RX-150a except I have hum on only the left channel. This thread had me glued! Is there any update or solution?
 
Hey there, @Fdski , welcome to Audiokarma!

Sadly the old girl had to be sold off to recoup some of my losses, never nailed down the cause of the grounding issue and certainly couldn't live with it. There were some excellent solutions explored in this thread that you may want to look into, unfortunately my little Rotel could not be saved, by me at least.

Good luck, this forum is gold Jerry, gold!
 
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