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Project: Yamaha CR-820 (Rebuild)

I am not sure, need to test more but I think the loudness issue might have been the relay somehow. I swapped that and it sounds fine now? Welp I guess I'll continue with transistors and diodes.
 
The is the initial symptom I received the unit in, TR720, TR722, TR726 and TR727 were all shorted and R766 was smoked. I replaced them and the unit seemed ok so I proceeded with the total rebuild.

Today I started replacing the rest of the main board transistors and after about 12, I checked the unit and it seemed ok but then I pushed it a little bit and the right channel got really distorted and R766 started smoking. I replaced R766 and borrowed a thermal camera and R722 gets really hot, around 170F before I decided to shut off the unit. It's still playing fine but any clues what I should look for? I have since replaced the remaining transistors and diodes on the amp board with the exception of the Zener diodes and it's still doing it.

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So I have everything about wrapped up but had one concern. Using a thermal camera I see this metal oxide resistor getting up to 200 degrees F. Looking at the board and schematic there seems to be a disconnect. I've circled the resistor getting hot and no surprise it's dropping the voltage from -74V to -14V. The schematic shows that it should be -45 on it's initial leg but how would that be possible when it's in parallel with the 2.2 k resistors and not in series with the one the schematic shows. If the schematic is wrong, that's fine but should I be concerned about a 200 degree resistor?

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The schematic is wrong in that the voltages for R821 are reversed. The left lead is-74.2 and the right lead is -45.8. The power dissipation in R820 is 0.93 Watt in a resistor rated for 2 Watts. 200F = 93C, hot but not unexpected since the resistor is running at about half its power rating.
 
bill_p said:
The schematic is wrong in that the voltages for R821 are reversed. The left lead is-74.2 and the right lead is -45.8. The power dissipation in R820 is 0.93 Watt in a resistor rated for 2 Watts. 200F = 93C, hot but not unexpected since the resistor is running at about half its power rating.
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Oh ok wow that makes a whole lot more sense and I'm kind of embarrased I didn't realize they just had reversed the numbers. Only other concern now is I replaced TR808 and TR809 with Onsemi KSC2690AYSTU and 808 is running around 130F vs 809 at around 100F. I'll probably put it back on the bench and check some of these voltages again. This receiver is weird to me in that you can have a bad component yet it might still operate seemingly ok.

*EDIT* Looking at the schematic, TR808 and TR809 are the +12 and -12v feeds to the tuner board respectively. There's seems to be more load on the +12v side so I guess that all makes sense as well.

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Ok, so long story short, that distortion is still there when presence is turned up past like 2 clicks. I've now reverted all of the capacitors where I had installed bipolars vs. the original polarized caps in the amp board and tone board with the exception of 4 1uF wima film caps that aren't in the signal path (C743,C744,C745 & C746). The only bi-polar electros in these boards are the original two chemi-con 1uF Bipolars at C751 and C752. One thing maybe I didn't mention before is that I replaced all four of the driver transistors with new onsemi MJ21195G & MJ21196G. Everything seems to be working normally aside from this screechy sibilance that literally starts my ears ringing within a couple minutes. With headphones I can play it more loudly than I am comfortable with before the distortion sets in but it starts at a fairly low volume over my speakers (New Large Advent Loudspeakers). I just can't imagine what could cause this. I don't see how it could be any caps I put on the tuner board because I don't see signal passing through them unless it's radio or phono.
 
All I can see is that there seems to be some ringing and right channel has a lower amplitude than the left. This is a 10k and 1k square wave with everything set flat at about 10 Vrms.

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I think I fixed that ringing but it didn't help. Is it odd that I can hear my signal with no speakers attached? Like the transformer is singing my signal? It's not the same pitch but if I change the frequency so does the sound I hear coming out of the receiver. Like a harmonic or something.
 
Super Noob said:
Check your grounds…
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I think the ringing was just coming from using the all-in-one fnrisi scope and signal generator at the same time. I discovered later on that I could touch the connectors and make that ringing come and go. I actually think everything is fine, the right channel has a little less amplitude than the left but not enough to really notice listening. I did find a group of four resistors that have drifted or were just off a few ohms that explains the difference there.

This is really going to sound stupid but I think the harshness I was hearing was kind of a mental/physical thing combined with the music I was listening to for testing. I think when I got the caps and driver/output transistors all done I went and tested the receiver and I simply was playing it too loudly because of the way it works. You don't really get a strong bass response with loudness set flat and I think I turned it up too high to compensate to the level where I was doing actual hearing damage and my ears rang for two days. I was then super sensitive to that shrill sound and never went back and tested the same song with my sansui 2000 which I had been using primarily. I finally did and could reproduce those harsh tones with it. I think this is all simply a case of A) not listening to the amp enough before doing the re-cap and B) having my head in a place where this thing is surely going to sound perfect after doing "all that work."

I really appreciate everyone's help here on this and I apologize for wasting some of your brain cycles. Thank you so much @Super Noob for this guide. I am not sure if I would have tackled this without it.
 
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