Bad hum right channel input Pioneer SA-7100

daveyh

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I have a bad hum in the right channel of my Pioneer sa-7100. All other inputs work. I have rebuilt the phono board and cleaned the contacts. I removed the selector switch and cleaned it while it was out. It is not the wire to the turntable. It is only the right side input. I have checked all the wires to see if they are broken and solder joints and all look good, In fact, it looks like new on the inside of the amplifier. Anyone have this same problem before?
 
So - it's just the right channel on the EQ?
Not your TT? You checked it?
And you rebuilt the EQ because was humming?
 
Yes I did rebuild the EQ/phono board because it was humming, I cleaned all the switches more than once with DEOXIT 5. I still hums like a bad RCA turntable cord. But the cord is good.
 
Caps and transistors?
A list from this forum?
Swapped the TT RCA cords or tried a different TT?
I'm asking because I don't want to assume anything.
You could swap inputs to the EQ by lifting the input side of the 4.7K resistors R1 and R2. Use a jumper to cross the inputs.
You could swap the outputs in a similar fashion and isolate the issue to the EQ or chassis circuits.
 
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It was a list from this forum. Replaced all caps and transistors. I have done this before with good results. Tried different cords and turntables, Still a bad hum right side phono 1 and 2 only. I do not see 7.7K resistors R1 and R2. I see 4.7K. Is this a typo?
 
Zebulon has more experience than me, so I'd go with his suggestions. That said, I've been working on a SA-7100 for a LOONG time, and what finally helped me locate the problem (still trying to fix) was having an audio probe (DC blocking cap in series with probe that goes to input of a different unit) to audibly trace the signal , and find out where it started going bad. I'd also check grounds on the Phono board, and RCA inputs, but given that it's only in one channel it seems like a long shot. Good luck!
 
I have been probing with my SG. I tried Zebulon's idea but to no avail. Sounds like a ground issue but not sure. Still working on it.
 
I have tried to locate this with my SG and I can't find the problem. I have recapped and added new transistors to the EQ recently. When I pull the pre-jumpers in the right side the sound stops. I have also cleaned relay and rebuilt the protection just for good measure. It is not a bad RCA cord or turntable problem. All other inputs work except the Phono 1 and 2 rIght side. It sounds like a ground wire is off. I have taken out the control switch and cleaned and cleaned it since the problem. Nothing seems to fix it. Any ideas where to look next??
 
I have also checked ALL the resistors in the EQ and protection board and compared them with the good channel. All are good.
 
I would of liked too hear more about my TS post.
You said it's only in the right channel.
Is it emanating from RCA's to the EQ or between the EQ and the switching to the tone board?
If the other inputs are good then its in the EQ, EQ out or between the RCA's and input to the EQ.
Reflow the EQ :dunno:

Check the routing of the signal wires. Check images on the web to see if signal wires are run correctly.

Do you know about the poor-mans signal tracer?
 
I do know about the poor-mans signal tracer. I have one. I have also tried to re-flow the EQ board.

I have been working on vintage electronics for over 10+ years and this has go me stumped. I just can not find the problem. Tracing it does not provide the same sound. I have tried the trace the wiring down. Perhaps a bad selector switch?

I have not had the boards removed from the harness. I used KSA992 to replace the 2SA725 and KSC1845FTA to replace the 2SC1345. I am going to work on it a little today and will keep you posted.

I am going to try to swap inputs to the EQ by lifting the input side of the 4.7K resistors R1 and R2. Use a jumper to cross the inputs.You could swap the outputs in a similar fashion and isolate the issue to the EQ or chassis circuits like you suggested today. If that does not work I am going to the tone board!
 
I think I may have found the problem. Upon using my homemade SG and probing the EQ I probed capacitor C1 which is a 1/25 uf small cap located next to the screen print of the board number printed on the EQ board it came back to working on both channels. I re-flowed the whole board earlier and it did not not work. Upon continuing to probe the same C1 cap it made a poping noise several times until it finally starting working in both channels, I tried to use a wooden probe to probe for loose components but there was nothing loose. Now no hum and plays fine. Hmmmmm,
 
Sounds almost like one of the leads in the cap has broken loose internally and it "welded" itself when it popped for you. If the cap hasn't been replaced, do so. If it has been replaced, do it again. Another possibility is a fractured trace on the board at the cap. Don't cut the leads, but fold them over the traces and solder the leads to the traces to minimize or alleviate fractured trace possibility.
 
I am going to work on it again this morning. I will replace the cap as suggested. I will keep you all posted.
 
Thought I got it fixed. A few days is all it lasted. Now it is doing it again! :crazy:
 
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