Yamaha M-50 protection not going off

vfr800fiman

AK Subscriber
Subscriber
Hi all-
My M-50 seems to have developed an issue with the protection relay not clicking off(on?) it used to switch off if I toggle the speaker switch on and off a few times, but now it's not even doing that. I can get it to go off eventually if it power it on and off a few times, and play with the speaker switch.
Any ideas where to start with this? Are the relays a common failure with these, or is it a DC offset issue. I'm not even sure what the conditions have to be for the protection to go off.
Any suggestions would be helpful if someone has worked on this amp before. I bought it new in the early 80's, and it's worked fine up until now.

Thanks in advance.
Glenn
 
Hi all-
My M-50 seems to have developed an issue with the protection relay not clicking off(on?) it used to switch off if I toggle the speaker switch on and off a few times, but now it's not even doing that. I can get it to go off eventually if it power it on and off a few times, and play with the speaker switch.
Any ideas where to start with this? Are the relays a common failure with these, or is it a DC offset issue. I'm not even sure what the conditions have to be for the protection to go off.
Any suggestions would be helpful if someone has worked on this amp before. I bought it new in the early 80's, and it's worked fine up until now.

Thanks in advance.
Glenn
The relay is not the problem. Its the circuit that controls the relay that has issues.Most likely a power supply/protect circuit problem..Got a meter ?
 
I've got the best meter Fluke! Plus two scopes, what's the deal with the control board? Thanks for the quick reply, I'd like to get this fixed!

Glenn
 
First. Measure the DC offset for both channels per the service manual. Just to make sure we don't have a shorted output dumping raw rail on the output.
 
Thanks, I found the SM here and it's a very nice scan. Thanks to whoever uploaded that!
I also see you need a variac for some of the checks on the protection board. Luckily we have several at work I can borrow.

I probably won't get it to the bench until this weekend, stay tuned.
Thanks for the help.
Glenn
 
M50 problem...

My M-50 went bad and I found the small cap mounted on the vertical circuit board above the power supply area was blown taking 1/2 of the vertical circuit board with it. I was able to make a new board with a hand grinder and removed the burnt half and replaced with the newly made part. I inserted new components into the board and it worked. Been running daily for over 2 years and no problem.

Paul
 
Thanks, I found the SM here and it's a very nice scan. Thanks to whoever uploaded that!
I also see you need a variac for some of the checks on the protection board. Luckily we have several at work I can borrow.

I probably won't get it to the bench until this weekend, stay tuned.
Thanks for the help.
Glenn

Have you checked the voltage at the amplifiers outputs yet. BTW when adjusting the photocoupler and B- --use two meters! per the service manual.
VR601 and 602 inter-react with each other.

Right now we just need to figure out whats up with the protect circuit.
 
Of course I go to measure for any DC on the outputs and the protection clicks off after about 4-5 seconds. I did this twice and it still worked fine.
I figured I'd measure it anyway, and I get .004vdc left and .009vdc right. This probably is meaningless since the protection clicked off anyway.
Stay tuned
Glenn
 
Of course I go to measure for any DC on the outputs and the protection clicks off after about 4-5 seconds. I did this twice and it still worked fine.
I figured I'd measure it anyway, and I get .004vdc left and .009vdc right. This probably is meaningless since the protection clicked off anyway.
Stay tuned
Glenn

Use the test points as designated in the manual for measuring DC offset. Testing there is before the relay. So it doesn't matter what the relay does , your still measuring the DC voltage at the amplifier(s) outputs.
Millivolts would be the expected voltage in a good amplifier assembly.So if 4mv and 9mv still apply, that is perfectly OK. It means the problem is in the protect circuit or protect circuits power source.
 
I thought I'd post an update on my protection not going off. I've been using this amp since I first posted, and haven't had an issue with the protection not going off. Is this a result of the relay being dirty, or maybe the caps in the circuit being cycled through charge/discharge cycles?
I'm just curious how this could self cure.
I did have this amp in storage in my dry basement for about 8-10 years.

Glenn
 
I thought I'd post an update on my protection not going off. I've been using this amp since I first posted, and haven't had an issue with the protection not going off. Is this a result of the relay being dirty, or maybe the caps in the circuit being cycled through charge/discharge cycles?
I'm just curious how this could self cure.
I did have this amp in storage in my dry basement for about 8-10 years.

Glenn

Who knows . FM..
 
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