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Yamaha M-50 Power Supply Issue

tinkerer

tinkerer

AK Subscriber
Subscriber
I picked up a Yamaha M-50 Amp at an estate sale knowing it wouldn't power up. When I raised the hood it only took a few seconds to see why the unit would not power up. Capacitor C607 was fried up very nicely. Looks like it's a .22uF x 250v Metalized Paper cap. Mouser has a KEMET # PME271M622KR04 which appears to be a good match. There are three 6800pF caps near by and at least one could have also been damaged and I will probably replace those also with #PME271MA4680MR30.IMG_3311.JPG

Question 1: What would cause this capacitor to go up in smoke and what else should I check?

Question 2: Is there a better replacement than the Mouser # I have chosen?
 
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tinkerer said:
1: What would cause this capacitor to go up in smoke and what else should I check?
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A known and documented OEM part failure/recall. Usually reserved to the 220v models. But I have also seen this in the 110v models as well with the X-power supplies.
 
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tinkerer said:
here are three 6800pF caps near by and at least one could have also been damaged and I will probably
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Most likely not damaged. Just covered with soot and smoke from the larger caps demise.
 
All that sprayed resin should clean-up with alcohol pretty easily.
 
Thanks avionic. Yeah the fuse still test ok, so not a very good design for protecting the other components.

Thanks for the part recommendation.

What does Yamaha say is the solution when the Service Center is contacted?
 
Yup 120v only. I'll replace the cap and power up with DBT. We'll see, I'm usually not that lucky!! Got to order the cap so gonna be a few days. Thanks
 
You can use a variac..But not a DBT.

s-l300.jpg
 
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I replaced the .22uf cap that was burn up on the Control Circuit Board and still have an issue. I did power up on DBT, because I hadn't seen your message, and it showed a dead short. No relay click. Is it possible to damage the Control Circuit Boards using a DBT?

I then powered up using Variac and ran the SM check for Control Circuit Board 1.
@50vac = .1vac, target = .45vac
@120vac = 4.3vac, target = 5.5vac
So it appears I still have a problem with the power supply, and possibly another issue elsewhere.

I've looked pretty closely at all the circuit boards and do not see any other obviously burned components.

Looking for suggestions as these Yamahas are a bit different than the other vintage equipment I have worked on.
 
I've replaced:
TR601: 2SA949 with KSA1013
TR602,TR603: 2SA970 with KSA992
TR604: 2SC1815 with KSC1815
TR605: 2SC2240 with KSC1845

No help. Still same issues.
 
See if you have any luck with step 1 and step 2 adjustment on page 6 of the service manual..You will need two multimeters.
 
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