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Yamaha M80 repair

Thanks for the reply!

I did readjust bias to 15 mv immediately after powering on with output transistors installed. It adjusts accurately and remains stable after warm up. The OTOvoltages above are with both channels adjusted to 15 mv after warm up.
 
Pull one leg or R152 and check the resistance. Should be 220Ω.
 
bobok7787 said:
Working on a friends M-80 that was re-capped and had the infamous glue completely cleaned and all local corroded components replaced. It was working fine until recently, when the left channel went out.

First thing I did was unsolder all 8x output transistors to test out of circuit, all tested okay using diode test function on DMM. I also tested the other 8x power transistors on power boards 3 and 4, all tested okay. I also tested the driver transistors on the power boards, and the bias current transistors on the main board, all tested okay.

Second thing I did was solder in 470 ohm 1/2 watt resistors across the bases and emitters of all output transistors per this awesome thread: http://audiokarma.org/forums/index....-if-you-are-servicing-a-yamaha-m80-85.591887/

I then powered on using a DBT with a 100 watt bulb, it powered on perfectly! The bulb dims and all is well. I tested base voltages at all output transistors, which measure around 1 volt with out any load. Collector voltages are +/- 39.5 volts.

Thinking I was clear for takeoff, I re-soldered all output transistors, powered on DBT and the bulb shows a dead short. If I remove the RP/LP power wires, there is no short and it powers on fine. I also tried removing one power feed at a time, both channels show a short. Should of left the working channel alone, I regret that decision!

Any idea on where to start? I've inspected my solder connections up and down under microscope, I've tested for shorts between the collectors and ground. Could my meter be giving me a false sense that the output transistors are okay?

Thanks in advance for your help, here are a few pictures to reference!
Click to expand...


You'll get there.

LOL @legendary. He was't even mitigating the DSBG until someone pointed it out to him. ;)
 
Prepare yourselves for a long-winded post. So as I was probing around trying to trace the low base voltages throughout the VAS stage I heard a POP, and watched my dim bulb tester light up like a Christmas tree. I can't say for certain whether I bridged a collector to emitter junction with my probe tip or not (I had it guarded up to the very very tip), but what I do know is that chaos ensued.

I re-checked every VAS and output transistor in the bad channel and ended up buying replacements for all 4x output transistors (2 of the 4 were shorted), TR124 + its base resistor, TR112, TR120 and TR126 (TR126 was blown right in half), and fuse resistors R136, R138, R146, and R148 were also all either way out of spec or open (all were replaced with same value fusibles). I installed everything except the output transistors (I have 470 ohm B-E resistors installed in there for testing).

After that I was back at square one. Powered up on DBT, I still had <100 mV for base voltage at the PNP output transistors.

Digging deeper, sweeping and comparing component values from working side to bad side I finally found zener diodes D114 and D118 both open. I didn't have any 0.5W zeners on hand so I ran to my nearest Ratshack and picked up 2x 1N4733A's. I installed those and wouldn't you know it, I now have ~0.6V at the bases of the PNP output transistors! BUT, now the NPN output transistors base voltages are only ~400 mV :(

Voltages at TR120 are:
B - 1.01 V
C - 30.82 V
E - 400.3 mV

My working channel NPN driver (TR119) is:
B - 1.102 V
C - 30.82 V
E - 0.495 V

Is that 0.09 V difference enough to not "turn on" TR120?

Thanks for your help!
 
Yes it is, I pulled it from the circuit yesterday and it measured 216 ohms.

I guess my auto-range DMM might be throwing me off a bit, 400 mV is still 0.4 V. Still a difference from the PNP's of the same channel; is it close enough?
 
Finally got it! Figuring my base voltages were okay I wired in C-E resistors to simulate the bias current, yet again my base voltages were all out of whack :( I double checked the PNP emitter resistors and sure enough, one was completely open but looked perfectly normal. I installed a new one, powered on, and everything measured okay. Then I installed the new output transistors, powered on, adjusted bias to 15 mv, and we're back in business! This amp sounds fantastic, gobs of power. Hope to never have to work on one again...
 
Hello everyone! repaired my m85, everything works, put a idling current of 15.5 mV in both channels, but in class A it is not clear, it is written in the manual that it should be above 31 mv, I have one channel 120 mv, the second 150 mv, is this normal? I measure according to the technical bulletin, not a service manual.
 
topchiy said:
Hello everyone! repaired my m85, everything works, put a idling current of 15.5 mV in both channels, but in class A it is not clear, it is written in the manual that it should be above 31 mv, I have one channel 120 mv, the second 150 mv, is this normal? I measure according to the technical bulletin, not a service manual.
Click to expand...
Welcome to AK..
 
I found the answer to my question
 

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