Yamaha Receiver (RX-500) in protection - DC on Right Channel Output

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Electoneman

Electoneman

Member
Joined 2016
#1
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 +20vDC 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.

I have worked backwards measuring the various transistors on the right channel and the incorrect voltage tracks back quite a ways. But I'll be damned if I can find the culprit. These are the voltages as per the service manual and what I'm measuring:

Manual: B / C / E // Measured: B / C / E
Q202: -11 / 0 / 0 // -11 / 0 / 0
Q204: 44 / 43 / 44 // 45 / 45 / 45
Q206: 0 / 43 / 0.5 // 0 / 45 / 6.0
Q208: 0 / 45 / 0.5 // 6.0 / 45 / 6.0
Q210: 44 / 18 / 44 // 45 / 0 / 45
Q212: 44 / 1.1 / 44 // 45 / 21 / 45
Q214: 0.5 / 1.1 / -1.1 // 19 / 21 / 19
Q216: 1.1 / 45 / 0.5 // 22 / 45 / 22
Q218: -1.1 / -45 / -0.6 // 19 / -45 / 20
Q220: 0.5 / 45 / 0 // 21 / 45 / 21
Q222: 0.6 / -45 / 0 // 21 / -46 / 21

As you can see, things start to go off at the emitter of Q206. Any help would be appreciated. Thanks.
 

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Last edited:
#2
Electoneman said:
Manual: B / C / E // Measured: B / C / E
Q206: 0 / 43 / 0.5 // 0 / 45 / 6.0
Q208: 0 / 45 / 0.5 // 6.0 / 45 / 6.0
Click to expand...
Well first Q206 and Q208 are NPN therefore I expect in normal operation to see for manual (base +0.5V positive relative to emitter)
Q206: 0 / 43 / -0.5
Q208: 0 / 45 / -0.5
So at this point I am not sure if you have measured 6V or -6V instead of -0.5 or if measurements are not relative to ground
If what you wrote is correct (6V relative to ground)
  • Q206 and Q208 are off, no current (Vbe should be > 0.5V in normal operation) no current trough R250 while the voltage on R250 is 6.0 V on one pin (what you have measured in emitter), minus -13V on the other pin (from D203), therefore 18V on R250 with no current means R250 could be interrupted or
  • cold soldering somewhere in Q206, Q208, R250 area
If what you wrote is incorrect (instead you have -6V relative to ground)
  • Vbe for Q206 is 6V , a silicon junction can not handle more than let say 0.8V in direct conduction, Q206 is gone, or
  • cold soldering Q206 base or emitter
In any case check
  • Q208 shorted base emitter, could explain the same voltage 6V on base and emitter
Waiting for your reply with some clarifications.
 
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#3
floriant said:
Well first Q206 and Q208 are NPN therefore I expect in normal operation to see for manual (base +0.5V positive relative to emitter)
Q206: 0 / 43 / -0.5
Q208: 0 / 45 / -0.5
So at this point I am not sure if you have measured 6V or -6V instead of -0.5 or if measurements are not relative to ground
If what you wrote is correct (6V relative to ground)
  • Q206 and Q208 are off, no current (Vbe should be > 0.5V in normal operation) no current trough R250 while the voltage on R250 is 6.0 V on one pin (what you have measured in emitter), minus -13V on the other pin (from D203), therefore 18V on R250 with no current means R250 could be interrupted or
  • cold soldering somewhere in Q206, Q208, R250 area
If what you wrote is incorrect (instead you have -6V relative to ground)
  • Vbe for Q206 is 6V , a silicon junction can not handle more than let say 0.8V in direct conduction, Q206 is gone, or
  • cold soldering Q206 base or emitter
In any case check
  • Q208 shorted base emitter, could explain the same voltage 6V on base and emitter
Waiting for your reply with some clarifications.
Click to expand...
Thank-you for your reply. I eventually found the issue - Q212 (2SA1145). It was not shorted or open, must have been leaky under load. Replaced it with a 2SA1370 which I had on hand (and seems like a good substitute) and CLICK. Set idle current and we have audio on both channels.

Unfortunately, the Left channel is quite a bit louder than the Right channel through both the speakers and headphones. All pots were thoroughly cleaned with De-Oxit.
 
#4
Electoneman said:
Q212 (2SA1145) must have been leaky under load
Click to expand...
If Q212 was leaky, your Q208: 0 / 45 / 0.5 // 6.0 / 45 / 6.0 must have been something like Q208: 0 / 45 / 0.5 // 6.6 / 45 / 6.0.

Electoneman said:
Unfortunately, the Left channel is quite a bit louder than the Right channel
Click to expand...
Short + of C235 with + of C236 to see if the problem is in the power amp or before.
Also C242 received ~ 20V trough R258 (from Q220: 0.5 / 45 / 0 // 21 / 45 / 21) and since the capacitor has a max voltage of 6.3 V , that is how you got ~ 6V at Q208. One possible scenario is that the 20V affected C242 and now R258 + R254 negative feedback (that was supposed to be DC only thanks to C242) is now negative feedback for audio signal too, which would cause a lower gain for the Right channel.
 
#5
floriant said:
If Q212 was leaky, your Q208: 0 / 45 / 0.5 // 6.0 / 45 / 6.0 must have been something like Q208: 0 / 45 / 0.5 // 6.6 / 45 / 6.0.


Short + of C235 with + of C236 to see if the problem is in the power amp or before.
Also C242 received ~ 20V trough R258 (from Q220: 0.5 / 45 / 0 // 21 / 45 / 21) and since the capacitor has a max voltage of 6.3 V , that is how you got ~ 6V at Q208. One possible scenario is that the 20V affected C242 and now R258 + R254 negative feedback (that was supposed to be DC only thanks to C242) is now negative feedback for audio signal too, which would cause a lower gain for the Right channel.
Click to expand...
Shorted the +'s of C235 and C236. Left channel still louder. Put the balance knob all the way over to the Right and shorted again, Left louder.
 
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