Yamaha M-50 output issue!

Ren Ren

New Member
I'm losing output power intermittently...

The M-50 has been purring right along with no real problems after adjusting the DC offset and idling current were adjusted to within specs.
Then, this morning, I get no output to either set of speakers. The protection indicator lights at this time.
I fiddled with the speaker select and speaker on/off switches and it came back on. It dropped out again maybe 15 minutes later. I shot some DeOxit into the switches and it snapped to life. Again, after a time it has dropped out. I have double checked the speaker wires for shorts assuring none were present.
Are those relays a weak point as well? I had tapped on them and it came to life from it...once. Could not duplicate any of these "fixes" to a certainty of the problem. It's boggling me.

What else would trigger the protection circuit. No burnt parts smelled or eyeballed.

Any direction here would be cool. I have the service manual here in front of me.

Rennie
 
Last edited:
Have you checked alignment steps 1 and 2 using two multimeters? Could also be a cold solder connection(s). Tap around on the various boards with something non-metallic and listen for the protect relay clicking (toggling).
 
Last edited:
Two meters to test GY & VI terminals on the Cap board measured against voltage at control board...? Is this the "steps 1 & 2" of which you speak?
I do not have a variarc. Where does this put me?
 
FYI, I have done plenty of tapping to no avail. With the speaker switch off I get a visual signal on the display but when I choose one speaker or the others it goes straight into protect no matter my volume level.
I also double checked the DC offset voltage and it is within +/- 1mV.
I think I can borrow an variac. As I assume this test will find or rule out SCR's or other part...
 
FYI, I have done plenty of tapping to no avail. With the speaker switch off I get a visual signal on the display but when I choose one speaker or the others it goes straight into protect no matter my volume level.
I also double checked the DC offset voltage and it is within +/- 1mV.
I think I can borrow an variac. As I assume this test will find or rule out SCR's or other part...

Does the protect activate without speakers attached ?
 
Yes, it activates with or without speakers attached. Two different pairs tested.
I did perform the alignments on page 6 but I had overlooked page 7.
I'll perform those adjustments as needed after a nice nap.
I will report back.

FYI: Strange, but I have been able to get it to come on at times while jacking with the PJ - VR board. It will eventually shut back off. I re-soldered the connections here in these boards.
 
Disconnect the small connector on the top center of the pre-drive board.See if it comes out of protect mode.
 
Adjustments #1 - #5

Removing the connector from the pre-drive board made no change in the protection lamp.

Adjustments read as follows:

Steps #1 & 2: Before adjustments readings were 58.3v at -B & E, and 1.42v at -B & TP1
I brought the power supply down to 56.5v. Then, after adjusting the photo coupler down to 1.0v I noticed that the PS had risen. I can only bring that down to 57.5v when the pot bottoms out!

Step #3 and have 0.1mV on each channel.

Step #4 is set 10.1 left & 10.0 right.

I wire it all up. Protection light is on with no output.
Five minutes later the stereo starts working fine again. Note that the Protection Lamp is still lit.
It has only been running for 15 -20 minutes, clean / no distortion.

I will get back with my report and or new findings...

Thanks for your guidance so far!
 
Last edited:
The protection lamp went off within an hour. Listening to Dark Side of the Moon at ample volume at present...
I am hoping this is the end of this thread. I leave the chassis open for several hours and seal it all up later after I build some confidence.

Avionic, you have assisted me in the past and you have lead me to the solution each time. I appreciate your time. Thank you.
 
The protection lamp went off within an hour. Listening to Dark Side of the Moon at ample volume at present...
I am hoping this is the end of this thread. I leave the chassis open for several hours and seal it all up later after I build some confidence.

Avionic, you have assisted me in the past and you have lead me to the solution each time. I appreciate your time. Thank you.

Have you replaced any caps yet. Sounds like possibly you have a bad cap in the PS/protect circuit. I wager the protect circuits timing cap. Have a look at C143 a small violet colored e-cap with red text.(if its still OEM) See if it maybe has shrunken plastic sleeving. 33µf/16vdc.
 
Last edited:
Yes, I'm still having intermittent outages. for about 5 - 20 minutes at a time.
It sort of cuts a good chunk of the excitement in my listening enjoyment.
I had to replace C-607 the first day after it went up in smoke.

I found the little bugger. It shows some corrosion on the leads and as soon as I touched it to get a better look at the values the sound shut down.

Now to find a Rat Shack that hasn't closed it's doors. First, a quick check with Mouser up the street.

Thank you.
 
Yes, I'm still having intermittent outages. for about 5 - 20 minutes at a time.
It sort of cuts a good chunk of the excitement in my listening enjoyment.
I had to replace C-607 the first day after it went up in smoke.

I found the little bugger. It shows some corrosion on the leads and as soon as I touched it to get a better look at the values the sound shut down.

Now to find a Rat Shack that hasn't closed it's doors. First, a quick check with Mouser up the street.

Thank you.

Mouser..:thmbsp: Ratshack :thumbsdn: I've always hated RS.:D
 
Mouser Part# UKA1C33OMDD1TD In stock 69¢.
The old one came apart upon removal. It had a lead come loose which caused part to fail.
I fired it up and I expect no more problems.

Again, thank you for walking me through this. It really feels good to fix the unit myself assisted by good guidance!!
 
Back
Top Bottom