M-50 help needed

R

Rohol

Active Member
Hello, new to the forum. I am pleased to find such a wealth of information on the Yamaha models.

Acquired an M-50 after repairing my Carver MX-130 I've had since new.

I will change C607 which is original. On one channel of the Pre Drive board I found discoloration / damage to the following, beginning with the worst: R964 (previously changed to 1 watt/burned/unidentifiable), C920, C916, R954, C928

I am having trouble finding a R964, a 220 ohm flameproof carbon resistor that is as close as possible to the original. Any other P/N suggestions on any of these components are greatly appreciated. I will likely use Digikey.

Regards, Lee
 
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I couldn't find a 1/4W 220ohm fusible resistor last week, but i did find 1/3W. I know you're not supposed to move up in Wattage with these, as it defeats the purpose since the resistor won't "pop" and protect the circuit when its supposed to. But i've read of many cases here on AK where when the fusible resistors are not available, people are swapping in regular 1/4W Metal Film resistors.

So i figured a 1/3W fusible will at least offer more protection that the metal film. Anyhow, here's the link to what i used:
https://www.mouser.ca/ProductDetail/594-NFR2500002200JR5

I look forward to seeing what others suggest as well, so i can see what my options are.

EDIT: are you looking for a fusible resistor or just a regular resistor with a flameproof coating?
 
Hi,

Thank you for the rapid response and the link.

I believe your idea of using a 1/3 watt has merit, especially if whatever took out the substituted 1 watt is still a problem. The factory spec is "'flameproof carbon resistor". It's possible that something is lost in translation...

I suspect for test purposes it may be the better way to go. The 3 Pre Drive board resistors may not be subjected to very high current under normal operation.
 
I see your location is Toronto. That's where the M-50 just arrived from. I can't wait until we can cross borders again.
 
All the electrolytics on the Pre Driver board are Gloria, Nippon Chem-Com?? and Lelon manufacture. Is this factory spec? While it's out I might as well replace them. What are the best caps to install?
 
I mainly used Nichicon FG for replacement.

Symbol Type Spec Size OEM Replacement
C923-926 EL 22uF 100V (BOM 80V) 85C 10x12.5 Nippon Chemi-Con UFG2A220MPM
C927, C928 EL 47uF 6.3V 85C 5x11 Matsushita UFG1E470MEM1TD


upload_2021-4-1_9-14-49.png
 
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Rohol said:
On one channel of the Pre Drive board I found discoloration / damage to the following, beginning with the worst: R964 (
Click to expand...
I would be checking TR904,908,916 and 924 as well as zener diodes D902,906 and 914 along with general purpose diode D910. For starters...;)

C928 is also a remote possibility..

Resistors don't just decide to burn up-- Something else shorted causing the fusible to burn up.

Yamaha's are known for a lot of collateral damage when active devices short out.

Welcome aboard Rohol
 
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Thank you for finding the proper resistor spec. I will check mouser for any other parts. The only other part that looks damaged is C916. The others were sprayed by debris from the 1 watt decomposition. I have no doubt an active part failed to take that out. They will be checked asap. I'm down to one meter, missing a signal gen and the scope is intermittent so it may take some time. I also need to relearn a great deal.

Can the unit be powered up without the Pre Drive board once C607 is replaced?

I appreciate the help and the welcome. I look forward to reviving it. I will easily remember your forum name. I attended ERAU for avionics.
 
Excellent!!

Too bad I've forgotten a lot. Last I used it was failure analysis on the Tomahawk cruise missile in the mid 80's. My son will be winged as an NFO. He wants the F-18 SH Growlers.

It's a real pleasure to be here.
 
I've checked statically, every component on the Pre Driver board for shorts and there are none. I also carried over some cursory checks on the main board and input but found none.

Removed the power board and noted the three 6800pf caps are all cracked like C607. I will need suggestions on those as well. After looking up a few of the transistors on the Pre Driver board, I realize how difficult it is to locate good substitutes.
 
Any 6800pF safety capacitor of at least X1 rating and 15mm lead should work. I used Kemet R413I168050T0K.
 
Thanks!
That really helps to speed up a lengthy process. Time to walk away before I'm seeing these things in my sleep.
 
Rohol said:
I've checked statically, every component on the Pre Driver board for shorts and there are none. I also carried over some cursory checks on the main board and input but found none.

Removed the power board and noted the three 6800pf caps are all cracked like C607. I will need suggestions on those as well. After looking up a few of the transistors on the Pre Driver board, I realize how difficult it is to locate good substitutes.
Click to expand...

The 6800pF safety caps that I last bought are NLA. The Kemet that vinnysj mentioned will be good, it's Y2/X1 rated. I am dimly thinking that what I used had a 12.5mm lead spacing for a drop-in fit but 15mm should work.
https://www.mouser.de/ProductDetail/KEMET/R413I168050T0K?qs=wnTfsH77Xs5pFemiGBgaDg==

You will absolutely want to replace C607, it is (probably, in the US model) a 0.22µF, paper-in-oil and is a safety issue, replace with X2 film.
https://www.mouser.de/ProductDetail...iMZZMvQAn3cSZoTr%2bBQwT%2bBXvKiZm9Af%2b8Z400=

Cheers,
James
 
Rohol said:
Thanks!
That really helps to speed up a lengthy process. Time to walk away before I'm seeing these things in my sleep.
Click to expand...

Note that few of us have the same skill level as Avionic, I certainly don't. My point is that now that he has stepped in you should follow his help/instructions first and foremost and vet anything that anyone else says with him before proceeding or deviating from his instructions (and that includes anything that I might write). Too many cooks makes for a burned up amp.

Cheers,
James
 
Bratwurst7s said:
Note that few of us have the same skill level as Avionic, I certainly don't. My point is that now that he has stepped in you should follow his help/instructions first and foremost and vet anything that anyone else says with him before proceeding or deviating from his instructions (and that includes anything that I might write). Too many cooks makes for a burned up amp.
Click to expand...
I concur. Always take my input as suggestion and double check. I am still learning the ropes :)
Follow Avionic instructions and you are good.
 
Thank both of you for the 6800pf cap substitute. You are on the same page there. I have read much of what Avionic and both of you have written and have learned a great deal. I will continue to listen and have patience with this process.

Once all 4 are replaced, I'm wondering if the Pre Drive board can be left out to power up the rest of the amp to do some cursory checks. If so, what should be checked in order of importance?
 
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