1965 Fisher 200-T one channel not working

[Helmholtz]

Caps have some non-linear properties and can introduce distortion in the signal path, though typically below 0.1%.
Foil caps are better than Al ecaps by maybe a factor of 10. Tantalums can cause distortion in the 1% range.
Measurements can be found in Merlin's hifi book.

[gui_tarzan]

Ran audio through the aux high and low inputs, same results. Tried headphones, same results. Trying to get the scope to cooperate to trace the signal but it's being stubborn.

[g1]

Still at the condition where mono gives output to both speakers, but stereo just gives one side working?

[g1]

Super-cleaned and tested the switch, no issues there. so it's between the switch and the power transistors

Assuming you mean the input selector switch here. The monitor switch S5 is after the input selector switch and is a dual switch. It could cause your issue.

[gui_tarzan]

Still at the condition where mono gives output to both speakers, but stereo just gives one side working?

Yep.

[gui_tarzan]

Assuming you mean the input selector switch here. The monitor switch S5 is after the input selector switch and is a dual switch. It could cause your issue.

No, the stereo/mono switch. I also cleaned and tested the input selector, the monitor switch, the pots, etc. Everything checks good with continuity. The balance control shows equal ohmage from wiper to left and right so I don't believe that is the problem. I was kind of hoping it was that simple.

My scope is cooperating now that it warmed up so I checked the input and output signal on each of the pre-amp, audio control amp, pre-driver and amplifier boards, all show sine wave equal to its counterpart (L & R). I have not yet tested voltages on those connections but I did on the test points the schematic shows and all voltages are close to the document. This testing was looking for a missing or highly degraded signal to try to pinpoint the location of the problem.

[g1]

I checked the input and output signal on each of the pre-amp, audio control amp, pre-driver and amplifier boards, all show sine wave equal to its counterpart (L & R).

This doesn't make sense if both amps are working in mono mode. The signal must be missing at amp board input (8B) when in stereo mode.
Look at point 8B with scope, in mono mode, then in stereo mode.

[gui_tarzan]

Thought I had it solved. When I put a signal on each side of the mono/stereo jack I got nothing on the right side out. Took the switch out and put in a brand new one, same thing. I will try more tomorrow.

I think my scope is f'd, what I thought was a good signal on the screen might have been just a fluke (no pun intended). It's acting very oddly.

[gui_tarzan]

Ok, I finally narrowed it down to the first board off the input selector. I had to go old school since my equipment isn't working right. I used a cable out of my phone playing audio to touch the left & right inputs of every board, starting at the output amp. Both sides play through until I get back to the preamp board. If I touch the left input with my audio probe I get a buzzing sound. If I touch the right input I get nothing. If I touch my finger on the top of transistor Q101 (BC147B circled in green) I get a buzzing sound. If I touch the top of Q103 I get nothing. I swapped the transistors and the problem stays with the channel. So there must be something burned out on this board on the right channel because when I have a signal going into the RCA jacks I get sound on the output side of the selector switch where it goes into the preamp board.

Q102 and Q104 don't do anything when I touch the tops of those. I took all four out and reseated them, no change. Unfortunately unless I missed it, nothing in the documentation says what these two are or what they do. I have to look up the numbers that are printed on them.

It feels pretty good finally isolating the area so I can figure out what broke on it.

Now I just need to find a new scope that I can get on a seriously low budget.

[gui_tarzan]

BC107B and BC109C. Both are "silicon planarepitaxial NPN transistors in a TO-18 metal case".

[g1]

Not sure if by 'input selector' you mean the S1 input selector or the S5 monitor selector.
Coming off the S1 switch is the preamp board P2003-2.
But Q201 and Q202 are on the 'audio control amp' board P2009. That is coming off the S5 monitor switch.

When I put a signal on each side of the mono/stereo jack I got nothing on the right side out.

Do you mean the monitor input jacks? Then yes I think the problem would be on the P2009 board. If you haven't tried going into the 'monitor input' jacks please do so.

[gui_tarzan]

Not sure if by 'input selector' you mean the S1 input selector or the S5 monitor selector.
Coming off the S1 switch is the preamp board P2003-2.

Yes, S1, Audio is present on all RCA input jacks when I plug them in in the back. It's also present on the output side of S1. It's present on the P2003-2 preamp board left and right inputs - 1A and 1J. The audio is not present on the R output.

But Q201 and Q202 are on the 'audio control amp' board P2009. That is coming off the S5 monitor switch.

Yes, you responded while I was updating my typo. They are Q101 and Q103 on the preamp board.

Do you mean the monitor input jacks? Then yes I think the problem would be on the P2009 board. If you haven't tried going into the 'monitor input' jacks please do so.

See my first response. I put the audio on all inputs - including monitor inputs.

[g1]

Ok, I missed the typo as we were posting at the same time.
The monitor inputs are after the preamp board. And you have to select them with the 'monitor' switch on the front panel. If you got no sound from the right using the 'monitor in' jacks, then you must also have another problem on the P2009 'audio control' board.
But it sounds like you worked your way from back to front and had no issues there, so can you recheck with signal into the 'monitor in' jacks?
Don't mean to be a pain, just trying to be thorough.

[The Dude]

Well, if the problem is on that board, I'd compare transistor voltages from working side to nonworking side. I also see some small value electrolytic coupling caps there. Maybe one of those has dried out. There isn't much on that board.

[gui_tarzan]

No, it's ok, I appreciate all the help. As strange as it sounds, this is my first solid state receiver repair in all the years I've been working on amps. I have only ever worked on tube amps prior to this.

I will check tomorrow. My brain is fried after work today and on this tonight. I figured if I have sound at the input stage on both sides but not after that in stereo, I'll start where I know I have audio out both sides in mono. Since mono plays both speakers I'll start with the power amp and work backward. That made sense and it's good all the way back in stereo to the preamp board. One of you guys made a comment on an amp thread about doing the same thing - make sure the power amp works and go backward from there to find out where the input signal stops.

The monitor inputs are switched right in front of the tone controls so I'm sure I tested there to but I'll doublecheck tomorrow.