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Yamaha RX-500U In Protect

Electone

Electone

Well-Known Member
Working on a nice old Yammy receiver. Relay won't click - it's in protect. Measured normal DC offset on Left channel (10mV). Right channel is showing +17vDC which is obviously why it's in protect. Pulled the two output trannys on the R channel (2SC2579 & 2SA1104) and it now comes out of protect. But, both test fine out of the unit - no shorts. The drivers also test fine.

Kinda stumped now. Any help would be appreciated.
 
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Electone said:
Kinda stumped now. Any help would be appreciated.
Click to expand...
Keep walking back in the circuit. Either a transistor , diode or capacitor is loading down the negative side of the circuit by -17vdc. Check voltages on the bases of the output and driver transistors. Should only be around 1 - 1.5vdc.
 
Yep, getting 17-18 volts on the bases of both outputs and drivers. Will continue backwards and see what I find. Not sure if this is related, but the aluminum heatsink is getting hot fast...
 
Do you have a good schematic scan ? The HFE scan is worthless..
 
I found a better scan elsewhere. At least you can see the voltages. I'm going to upload it to HFE.
 
Yeah, this is a nightmare. The board does not mark the resistors or capacitors and the board schematic is illegible...
 
I've been working around, checking voltages, wondering where this excess 17-20 volts is coming from. Schematic shows D202 should have 45V on the cathode and 1V on the anode. I'm getting 45V on the cathode and 20 on the anode. Diode checks out fine out of circuit. Changed it out, no difference.
 
These are the measurements on the transistors in the right channel amp circuit.

Transistor: Manual: B / C / E // Measured: B / C / E
C202: -11 / 0 / 0 // -11 / 0 / 0
C204: 44 / 43 / 44 // 45 / 45 / 45
C206: 0 / 43 / 0.5 // 0 / 45 / 5
C208: 0 / 45 / 0.5 // 6 / 45 / 6
C210: 44 / 18 / 44 // 45 / 0 / 45
C212: 44 / 1.1 / 44 // 45 / 21 / 45
C214: 0.5 / 1.1 / -1.1 // 19 / 21 / 19
C216: 1.1 / 45 / 0.5 // 22 / 45 / 22
C218: -1.1 / -45 / -0.6 // 19 / -45 / 20
C220: 0.5 / 45 / 0 // 21 / 45 / 21
C222: 0.6 / -45 / 0 // 21 / -46 / 21
 
-------- B -- C -- E
Q208: 6 / 45 / 6
Q210: 45 / 0 / 45
Q212: 45 / 21 / 45
Q214: 9 / 21 / 19
Q216: 22 / 45 / 22
Q218: 19 / -45 / 20

Sanity check-- Remove Q204 --make sure its not shorted all the way around.
 
Check Zener D203 for a shorted condition. Check Q212 out of circuit.
 
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I have pulled Q206 (the initial weird voltages) and tested it in diode mode on my DMM. I get .635 b-c and .638 b-e. I also swapped it on the board with its counterpart Q205. For Q204, I get .631 b-c and .644 b-e. Is the schematic incorrect as it shows there should be 44v base, 44.6v collector and 43.4v emitter? That's also what I measure on its counterpart Q203 (which I also swapped with Q204). Problem remains on right channel. Stumped...
 

Attachments

Seemingly getting nowhere in the amp stage, I decided to poke around in the protection stage.

Q406: b -1.2, c -.07, e -.07
Q406: b -.07, c -.07, e 0

I'm getting:

Q406: b 2.9, c -46, e 0
Q407: b 0, c -46, e 0
 
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Replaced Q212 2SA1145 with a substitute 2SA1370 - CLICK! The transistor I pulled continues to measure good on my DMM but when I put it back into the circuit, the voltages go crazy on the right channel again. So, my guess is that it is probably leaky.

I've got audio on both channels. LEFT channel is definitely louder than the recently repaired RIGHT channel. Have to look into if that new 2SA1370 is not a great match.
 
Put a 2SA1370 on the left channel as well, no difference in volume. Left channel still louder. All pots cleaned with De-Oxit.
 
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