Yamaha RX-371/3064 Amplifier Problem

[Denver Don]

Hello all, I have a strange amplifier problem I hope someone can help with.
My RX-V371 right front amplifier output doesn't provide output for as long as 15 minutes after turn on. After that it works fine. I have eliminated any connection load to that output that could be the problem.
I have the service manual and the Protection has not been triggered, the voltages appear to be OK and the receiver functions perfectly in all other respects immediately after turn on.
I can't guess what could cause the RF channel delay and was hoping someone here has seen this and could advise on what to look for.
Thanks for any help in advance

 

[dave9]

I would suspect that there is a break in the circuit, bad solder joint, PCB crack, IC socket with oxidized contacts and thermal expansion improves the electrical connection.

What exactly does "voltages appear to be okay" mean? You have verified the voltage input to the output transistors for the right front channel, not just coming out of the power supply subcircuit (not just at the bulk capacitors)? Generally (not always) a bad solder joint will develop on components that get hot. Larger resistors' solder joints are another likely culprit.

Either way, you can probe for DC power from the beginning to the end, or start at the end and work backwards, or pick a spot in the middle and work towards the direction it isn't working (voltage too low or not there), and the same is true for the low voltage AC signal path while you have power and a source audio signal.

Since it does not work cold, you might be able to measure resistant point to point without a live circuit which is inherently safer, but remember to measure the component leads rather than only solder joints, as measuring just the solder won't tell you if the lead connection to the joint is bad.

 

[Denver Don]

Thanks for your reply Dave9, what you deduced was exactly on the mark. I took off the cover and pressed on the Main PCB just behind where the rear panel front spkr output terminals are located and I could make the RF channel come and go, obviously a marginal connection..... but getting at that PCB would be a nightmare.
If you've been inside a Yamaha A/V Receiver you would understand. The board that would have to be removed is at the very bottom of the chassis and would require virtually total disassembly of the receiver which has multiple interconnected PCBs not using connectors but soldered flex cables,.... as i said a total nightmare.
Being and engineer myself I know what I'm about to say is not what I should do and certainly not a fix, but I'm hoping the gods might give me a pass just this once, ;-)
I found that when I pressed on the relay that enables the RF spkr, I could toggle the output. Since I couldn't get at the underside of the PCB, I rocked the relay back and forth until it no longer toggled the output. In fact I couldn't get it to quit again. Now I realize I probably just cleaned off an oxidized cold solder joint and probably in the hopefully distant future the problem will return, but at that point I will probably want a new setup requiring features this receiver doesn't have and I will just scrap it.
Forgive this lengthy report, but I wanted to thank you for your timely response that stimulated my action and understanding of the problem and at least I know exactly what to do if the problem returns.