Yamaha B-1/UC-1 & C1 Recap

got me a b-1 a few weeks ago. not sure if all four 2sk77 are still alive. the amp hasn't been turned on for decades and the previous owner seem to have fiddled with the power supply, all boards look fine though, at least to me. how can I measure the vfets to be sure they are ok?
 
got me a b-1 a few weeks ago. not sure if all four 2sk77 are still alive. the amp hasn't been turned on for decades and the previous owner seem to have fiddled with the power supply, all boards look fine though, at least to me. how can I measure the vfets to be sure they are ok?

Attached is basic testing and is a good start. Sometimes they may fail under load though.
 

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Hello kentlabs

got me a b-1 a few weeks ago. not sure if all four 2sk77 are still alive. the amp hasn't been turned on for decades

Since decades ...So don't switch the power on with V-FET in place without having new capacitors replaced ! you may damage them
 
Has anyone questioned the original and replacement main caps voltage of 100V????

I see 75v AC at the transformer secondaries. Post rectifier, DC voltage would be in the 106v neigborhood. Am I missing something?

Edit:
one more observation: the 75v secondaies voltage is based on a 117v primary. In US most areas, the mains voltage is 120v, thus ~78v secondaries

that would place the DC in the ~110v range...for caps rated at 100v continuous duty.

I am hopeing I am missing something and I am wrong on this.

...even if that voltage drops under load...it will not drop that much:idea:
 
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The schematic shows that ...
All the ones I did measured in at about 87.5 V DC ± 200 mV. The main rails are unregulated and as such will vary with the AC mains powering the amp. The mains here can be anywhere from 112 to 117 V AC depending on the time of day ...

As for the B-1 sch, it has to be the worst sch I've seen for accuracy. :thumbsdown:
 
All the ones I did measured in at about 87.5 V DC ± 200 mV. The main rails are unregulated and as such will vary with the AC mains powering the amp. The mains here can be anywhere from 112 to 117 V AC depending on the time of day ...

As for the B-1 sch, it has to be the worst sch I've seen for accuracy. :thumbsdown:
so measure first, look at the schematic later...got it

Thank you, now it starts to make sense.
 
thanks audio-clinic! I won't do anything until I gathered all necessary info! I really don't want to kill those rare Vfet's!
i have the SM and will look into it at the weekend, and check every board one by one.
 
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