Any Yamaha M-50 Experts Out There???

bhuskins

Super Member
I have an M-50 that just won't cooperate.

It came to me in protection mode and upon initial inspection the right channel had been pretty badly fried. No real damage to the actual boards but many burnt resistors and such. I had already planed a full recap and decided to completely go through it in the process. In addition to all the resistors that I replaced a full recap on the unit was completed. Also on the right channel several transistors were open so basically all the right channel transistors got replaced. The outputs on the right were blown so all the outputs (both channels) were changed as well. I likely will get into listing all the details and specific transistors used etc but it still has issues that I'm not easily tracking down. Power up stays in protection and a couple resistors will light up (poof) as well...hard to troubleshoot because I can't safely power it up. DBT no help in that regard.

Also replaced the safety cap for safe measure.

Any ideas how to approach this...too much time in it to shelf it.
 
DBT no help in that regard
Can't/don't use a DBT on these X-power supply models.
Which resistors are you burning up now? If your still burning resistor then you still have shorted transistors and/or diodes. Have you replaced any of the diodes and zener diodes yet? Guarantee you have got some bad ones.
 
Where did you acquire the replacement transistors ?



for reference..
M-50 outputs


2SA1146 ------ Sanken 2SA1186
2SC2706 ------ Sanken 2SC2837

BD Enterprise stocks them.
 
My M-50 worked fine after cap. job...

I would try a dim bulb tester, as mentioned above, before any more damage occurs.
I did and checked some stuff to see if all was the same per channel before going to full volt.
Great to have a channel working correctly to get sample voltages on the other.
I didn't replace transistors but did apply gel to old ones.
I did the cap job due to a badly burned small input voltage cap. It took half the board with it. I used a small hand grinder for making traces and made a new board. It was a challenge but a lot easier than buying a new board.
It has been working fine for 2 1/2 years now.
I also added a top plywood board with 2 fans for cooling. No big steel box here.
Sides totally open.
I never finished the alignment as I don't have the loading resistors for final check.
I did do part of the adjustments.

Paul
 
Where did you acquire the replacement transistors ?



for reference..
M-50 outputs


2SA1146 ------ Sanken 2SA1186
2SC2706 ------ Sanken 2SC2837

BD Enterprise stocks them.

Mouser

using...

NJW0302G
NJW0281G

Any issues there?
 
Which resistors are you burning up now? If your still burning resistor then you still have shorted transistors and/or diodes. Have you replaced any of the diodes and zener diodes yet? Guarantee you have got some bad ones.

Some Diodes and Zeners have also been replaced. I'll post the complete details later tonight on what's still burning and what all has been replaced so we can narrow this down.
 
So I remembered why I shelved it a couple weeks ago out of frustration...when I engage power at this point it's dead. I'd think it's the 12A fuse on the power supply board but it tests fine with a meter. Power switch is good. Where else would the power be stopped?
 
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Interestingly if I take an AC voltage reading at point AC1 and AC2 on the power feed board it reads about 6VAC when switched off and plugged in...unplugged the voltage immediately goes away. Switched on it reads 123VAC.

What should TT T1 and GY read that feed the transformer? Could I have blown my new safety cap?
 
I used mouser

667-ECQ-U2A224ML
.22uf 275v Panasonic film safety cap class X2 rated

I also replaced the 2 electrolytics on this board as well
 
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Looks like T1 (Brown) and GY (Grey) feed each side of the transformer and white that also feeds the transformer is the ground where the schematic shows a fuse inside the transformer on white as well. Power off T1 and GY read 0VAC and Power on they read 2.7VAC...hence the problem. So for starters, I need to determine what went haywire on the power supply board...any suggestions?
 
Have you/can you do the power supply /photo coupler alignment. To get it correct you need two meters.
 
Can that even be done with the amp in it's current state?

Is it blowing fuses or burning resistors ? If not,should be able to make those two alignments. Got to get the input AC power control working first.Or no power will get to the transformer.
 
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It's not but the transformer isn't receiving any voltage either is why I ask. There is a fundamental issue on the power supply board yet to be resolved and it was previously powering on (although in protection mode and smoking resistors.)
 
It's not but the transformer isn't receiving any voltage either is why I ask. There is a fundamental issue on the power supply board yet to be resolved and it was previously powering on (although in protection mode and smoking resistors.)

Look at the schematic of the input power control circuit.
 
So, -B reads about 32vac when taking the readig from -B and the ground flange from the secondary board. Making adjustment to 601 doesn't have much affect...maybe ½ a volt AC but it's fluctuating about that much without adjustment as well.

Photo Coupler reads about 0.003 vac (powered or not) when taking it from -B and TP1 and 602 has no affect on it
 
page 8 of the service manual has a troubleshooting guide for the power control boards.
 
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