Yamaha CR-220 Noise and popping after unplugged

[DoctorDoug]

Think you are talking about R746, 4.7k, redo the measurement, 3.94k is too low, don't see alternate path to explain.

Yes it is R746 (terrible copy) and rechecked again— 7.45k ohms. I checked it several times earlier and got the lower value. Strange.

Scope results are interesting. Left channel is quiet. Right channel not good:
TR704 base: clean and original TR
TR706 base: clean and original TR
TR708 base: clean and original TR
TR710 base: noisy and is a new replacement TR
TR712 base; noisy and is new replacement TR
TR716 base: lots of noise and original TR
TR718 base: less noise but spikes that could be pops and original TR
TR720 and TR722 base: noise with some spikes that could be pops and are original transistors.
Diodes D702 and D704 are noisy with spikes—both ends.

What is your assessment?

 

[mbz]

Yeah, the spikes are more likely to cause the pops so I'd focus
on them. The noise might be an oscilloscope measurement/grounding... issue.
TR718b receiving spikes and passing them onto the outputs. The spikes
appear to come from TR710/712 but already replaced, I'd replace R746, it
"feeds" these 2 transistors. Would expect spikes on TR716b, maybe swamped
by the noise. Trying to implicate TR708, TR706 but no luck, maybe Pete can
make better sense of it...
EDIT: Good info, thanks...

 

[DoctorDoug]

Yeah, the spikes are more likely to cause the pops so I'd focus
on them. The noise might be an oscilloscope measurement/grounding... issue.
TR718b receiving spikes and passing them onto the outputs. The spikes
appear to come from TR710/712 but already replaced, I'd replace R746, it
"feeds" these 2 transistors. Would expect spikes on TR716b, maybe swamped
by the noise. Trying to implicate TR708, TR706 but no luck, maybe Pete can
make better sense of it...
EDIT: Good info, thanks...

Thanks. The noise is not from scope grounding because there was no noise on the left channel. I was thinking that TR708 was generating the noise output and sending it downstream. I did notice that I replaced TR707 on the left channel, which corresponds to TR708 on the right channel. I will see if I can find a 4.7k ohm resistor locally if I can’t put a Mouser order together. I will keep working through it. Pete may have an idea.

Thanks.

 

[DoctorDoug]

Thanks. The noise is not from scope grounding because there was no noise on the left channel. I was thinking that TR708 was generating the noise output and sending it downstream. I did notice that I replaced TR707 on the left channel, which corresponds to TR708 on the right channel. I will see if I can find a 4.7k ohm resistor locally if I can’t put a Mouser order together. I will keep working through it. Pete may have an idea.

Thanks.

MBZ, here is the latest on the CR-220. We have a winner.

I purchased a pack of 4.7k ohm resistors and never opened it. I pressed the DMM probes against the solder points at the back of the board. Got a solid 4.70k ohms.

I made the decision to replace TR708 with a KSC1815. The trace on the scope with power up and down was strange; there was no visible TR base noise. Bias voltage at TP3-TP4 was fairly stable and I turned up VR704 quite a bit to stabilize voltage between 33.1 mV and 33.2 mV. With headphones on, I could hear a slight brief whoosh sound at power on. Power off cycling and warming eventually had a series of small right channel pops on power off that were affected by the volume pot.

Next, I replaced TR716 with a KSC1815. Scope did not show any TR base noise at power on or power off. There was no noise heard on headphones or speakers at power on and power off. Bias voltage at TP3-TP4 was close to being locked between 33.3 mV and 33.4 mV with minor tweak to VR704.

I have been running it for about 2 hours with intermittent power on and power off test. No noise. It sounds like a new CR-220. Funny thing is that the original TR708 and TR716 tested fine but obviously failed at stopping power on and power off noise.

Looking back at this, it started with leaking caps that led to a catastrophic failure of most transistors in the left and right amp section. The left channel had blown power transistors and low output. The right channel sounded fine with even with transistor problems. Right channel power transistors were spared. This is when I started the repair process.

Thank you for your and Avionics help and everybody else who stepped in.