Smoked M-50...in protect, where to start?

[herbd2]

Hello,
I recently picked up an M-50 which was sold as "in protect mode"....the seller told me he caused the issue by running too many speakers at once. It only cost me $30 so I took my chances. After opening the case, I found a nest of burnt resistors on the right side, that got hot enough to melt the solder and cook the board a bit. I started by removing and testing all the resistors in that area, they all still tested OK, so for short term action,I put them back in, and resoldered them, had to add some jumpers and such on the back where the traces had been broken.
Confirmed all looked OK and matched other side. However, the unit is still in protect mode. When I power it up, it is making some kind of click/static type noise....I haven't had the nerve to keep it on long enough to figure out where the noise is coming from. Anyway I'm assuming too many speakers means too much current and hence the cooked resistors. Was hoping someone could give me some pointers on where to begin this troubleshooting journey? I did test the output transistors for any opens between base and E, but nothing showed open. Fuses are ok, unit powers up, power light and meter lights come up, but with the funky sound I've never left it on for more than a few seconds....

 

[llwhtt]

The cooked resistors are a byproduct of shorted transistors and if the M50 is like other Yamaha amplifiers it has probably Chernobyled, the common name for the all of the destruction. The last M60 I rebuilt took over 100 parts to get it back to normal. Just about every transistor and it's associated emitter resistor were shot in both channels. No blown fuses is common as transistors are commonly called three-legged fuse protectors. I'm sure Avionic will chime in soon.

Craig

 

[bberkom]

Yeah those output transistors can't be okay, and I'd bet the drivers got hit too.

 

[herbd2]

So is it even worth the time or sell for parts?

 

[whoaru99]

So is it even worth the time or sell for parts?

That's sorta your call. It's a decent amp when working but what is your time worth to diagnose and fix it?

 

[avionic]

C-H-E-R-N-O-B-Y-L'D

 

[avionic]

Need to start at the outputs and work your way back to front end of the amplifier circuit.

 

[herbd2]

Based on the feedback, I think that may be far too long of a path for an M-50 revive.
The world needs parts units too I suppose.

 

[llwhtt]

Is the board just discolored or charcoal? That may be the deciding factor.

Craig

 

[petehall347]

rebuild it then decide

 

[herbd2]

Well, I'm not sure how far this is going, but I did some disassembly. I put on my radioactive head gear and gloves and began to step into Chernobyl land...Removed the output transistors and checked them. The larger output transistors on both channels were indeed toast.
Two of the smaller ones which plug in were bad also, but the other matching pair (l/r) checked OK. I have suitable replacement part numbers from other threads here, so that task is behind me.
I checked all the resistors that got hot and they are still reading OK.

So now I have the board out and getting ready to start checking my other components. Is is safe to assume anything here? Like just replace the transistors on this board....or should I be checking for failures component by component? How about the smaller PCB, what are the odds it has been affected? Just wondering if there is a history of what usually is toasted in these situations with M amps?

 

[avionic]

Print out a copy of the schematic. High-lite the bad parts you have found so far. This helps me establish the various possible paths of destruction.

Similar to what I've done in this B-6 repair.

 

[herbd2]

I started testing transistors and I am finding that not all are bad, but the ones that are, are consistent / matching failures in both channels. I also checked the emitter resistors and they seem fine. The small 21/22 transistors which propped on the heat sink by copper, both checked OK. Do I need to check any other resistors? Do you think the smaller board would be affected? (do I need test all those transistors as well?
The attached shows green OK, red BAD, so far. Any suggestions for next steps?

 

[avionic]

I started testing transistors and I am finding that not all are bad, but the ones that are, are consistent / matching failures in both channels. I also checked the emitter resistors and they seem fine. The small 21/22 transistors which propped on the heat sink by copper, both checked OK. Do I need to check any other resistors? Do you think the smaller board would be affected? (do I need test all those transistors as well?
The attached shows green OK, red BAD, so far. Any suggestions for next steps?

I need to see a circuit schematic not a parts locator diagram.

 

[avionic]

Also it has to be larger and legible.

 

[llwhtt]

Avionic what do you think was the path of destruction on your B6? Isn't that the pyramid amp? If so it's a definite capacitor cooker.

Craig
ex Black Knight

 

[avionic]

Multiple paths until all paths went "open". Look at the schematic. And it all probably happened in microseconds.

 

[avionic]

Isn't that the pyramid amp?

Yes.Schematic was one of the two I'm currently using. Both of them had a burnt channel. I probably have less than $300 in the pair. Bought'm broken.

 

[llwhtt]

I repaired one probably about 20 years ago. IIRC I replaced every electrolytic capacitor in it and it fired right up. Somewhere I still have the hard copy SM I got from Yamaha. If you get stuff broken and repair it yourself it can be really cheap. If you're like me I don't charge myself much.

Craig