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ROTEL ROTEL - Ground Hum From Milan to Minsk

JB8

JB8

Well-Known Member
Forum friends.

I have a little ROTEL RX-200A with the dreaded ground hum. It's 60 cycles, across both channels, and it's like a mouse in a garage - every time I think I have it cornered I'm foiled again.

Here's where we're at thus far:
  • Replaced the main amp transistors and four of it's caps (really didn't want to shotgun the main amp but several other threads indicated transistors may be the issue...) No change (all pinouts double checked)
  • Tightened/cleaned all of the ground screws on the unique boards
  • Thoroughly cleaned pots and switches
  • Tried reversing the plug to switch polarity
  • Does not react to moving hand around the top or bottom side of the board or from touching the chassis
  • Was going to replace the main filter cap but when I disconnected for testing it wasn't terribly far off, so I left it
  • Hum is across all functions and is present at zero volume - turning up volume eventually drowns it out
  • When engaging the speaker button I'm getting a fairly loud burst/pop, consistently
  • Lastly, the hum was there when I started replacing components, not a result of the changes
Some may ask, "JB8, why are you investing good components into such a low-powered unit?" Well, I'm a fan of the look, reminds me of the 330's (A, B and C) and I really need to understand how to chase down this issue. Aaand I'm on a bit of a ROTEL kick lately, just for fun.

I imagine since it's across both channels I should be looking at the power supply, I have a scope with one good channel and happy to start carefully probing, so fire away with suggestions as you have time!

IMG_0692.jpg IMG_0693.jpg IMG_0694.jpg IMG_0695.jpg
 
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Hey @Binkman , was there any text with your reply?

Looks like you reposted one of the pics I put up but there's no content, would love to know if you have some suggestions.
 
:lurk:

I have an RX-800 with Hummmmm too, so hoping to find some clues here. The way the wire bundles are run, I'm thinking of trying a full conversion to DC voltage for lighting to see if that solves anything.
 
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JB8 said:
  • When engaging the speaker button I'm getting a fairly loud burst/pop, consistently
Click to expand...
This will always happen when you engage the speaker switch on a capacitor coupled unit when it is switched on. Have the speaker on when you turn the unit on.
 
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As a person also dealing with a hum situation, mine stops when I disconnect the cable from the source/sources. I was suggested to use a Toslink cable setup. I am still reading about Toslink to up my knowledge on a new thing to me. Give them a pull and see what, if anything, happens.
 
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Thanks, All, really appreciate the support here!

@jheu02 , I'm in and happy to go the distance with this hum issue, hold on to your hat, could be a wild ride, ha! @cademan , you're a legend, always stoked to have your advice, I'll get some caps ordered or maybe do the "I don't have any axial's so I'll retrofit with the radial caps I have" move, appreciate your guidance! @Raccoon1400 , seems nuts that a unit would be approved for production even with such a loony, predictable, alarming sound associated with such an innocent action, but I get it and will check that off the list of things to be concerned about. @KingBubba , mine is a hummer with nothing hooked up to it other than speakers and/or headphones, but I'm intrigued by your Toslink cable idea - no idea what it is but it sounds cool.

I'll get to work and get the caps swapped and hopefully post pics of the finished product, sans the hum. Updates will follow ASAP!
 
I'm thinking mines a grounding issue somewhere, but the rats nest of wires with this early '70s unit makes getting at switches, boards, etc a real pain. I have already recapped many parts of it, but put it on the side burner for now
 
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JB8 said:
seems nuts that a unit would be approved for production even with such a loony, predictable, alarming sound associated with such an innocent action, but I get it and will check that off the list of things to be concerned about.
Click to expand...
Just the nature of the cap coupled design its self, not unique to this unit. It is an early design with a few non-idealities.
But you should only get a small thump if the speaker is on when you turn the unit on.
 
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All the big caps have been replaced, the hummmm remains the same. I have the aluminum and crafting a shield, hope it works!
 
No dice... 1/16' aluminum piece cut and dry fit to see if it would make any difference, zero change, hum is just as strong. Any other ideas, this hum is no bueno. Also, I set the idle, no problem and DC at speaker terminals is essentially zero.

I'm open to suggestions, there's got to be a way to kill this hum!
 
Both channels has to be a power supply issue. Have you tried replacing the diodes? They could be noisy.

Does the bottom cover make a difference?
 
Thanks, cademan, appreciate the support here. The case is one of those fake wood surrounds, so no metal bottom pan on this dude.

I'll get cracking on the diode replacement and update this thread with great news, hopefully, ha!
 
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