2238B noise on power amp (both channels)

ocdecio

ocdecio

The proof of the restoring is in the listening.
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Here's something that is puzzling me. It is a spiky wave 20mV / about 520Hz with the amp running in idle and shorting plugs at Main In. The noise was there before replacing capacitors and diodes at the power amp and power supply.

I doubt it is anything at the amp itself since both channels exhibit exactly the same behavior. Just wanted to ask if anyone has seen this before.

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No! That's weird alright. I assume you've looked at the power supply outputs and they're clean. I'd start by switching every switch, one at a time and returning it to where it was to see if anything affects it. I'd also look at the preamp outputs, even though they're not in play to see if it has the noise and is getting coupled into the amps somehow. Maybe even short the preamp output jacks to see if it affects it.

If somebody else has worked on it, they may have had wires loose and rerouted. Also, any number of wrong components put in.
 
rBuckner said:
No! That's weird alright. I assume you've looked at the power supply outputs and they're clean. I'd start by switching every switch, one at a time and returning it to where it was to see if anything affects it. I'd also look at the preamp outputs, even though they're not in play to see if it has the noise and is getting coupled into the amps somehow. Maybe even short the preamp output jacks to see if it affects it.

If somebody else has worked on it, they may have had wires loose and rerouted. Also, any number of wrong components put in.
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Hi Randy, this wave is with shorting plugs at Main In so hopefully no interference from switches. I also don't see this at the amp input. Shorting the preamp outputs is something I never did before. Wonder if having the covers off can cause that? The 2252B is very similar and it has some extra grounding straps if I remember.
 
This signal is looking at the speaker outputs using 8ohm speaker load resistors? Could this be noise on the 2238B chassis ground that is affecting the scope signal ground.
 
dankik said:
This signal is looking at the speaker outputs using 8ohm speaker load resistors? Could this be noise on the 2238B chassis ground that is affecting the scope signal ground.
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Yes, and the noise is audible (high pitch ~ 500Hz) when you turn the volume all the way down. Otherwise it sounds fine. I noticed the 2252B which shares a lot with the 2238B seems to have some extra ground straps including one that connects to the left of the heatsink. I'll see about installing the top and bottom cover and find out if it makes a difference. Just thought it strange to have noise at this frequency.
 
Hmm, maybe some LED light is oscillating at that frequency. Glad it's really okay!

Congrats on hitting the 1000 post milestone!
 
rBuckner said:
Hmm, maybe some LED light is oscillating at that frequency. Glad it's really okay!

Congrats on hitting the 1000 post milestone!
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Hey thanks!! Likely true, it's all we have in the house nowadays.
 
Ha! I once was trying to figure out why my AM was only picking up static across the dial, only 1 very strong station. I turned off my LED bench light and magnifier lamp light and the noise went away.
 
dankik said:
Ha! I once was trying to figure out why my AM was only picking up static across the dial, only 1 very strong station. I turned off my LED bench light and magnifier lamp light and the noise went away.
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One of my first units is a 1120 integrated (no tuner). It was picking up AM radio from a nearby gospel station and it turned out to be a low quality capacitor in the preamp.
 
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