DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

I've racked up a PM1000 using the PSU-4448. Everything worked great when I first powered it up. Used it with a condenser mic - no problem. Later that day, nothing. The output cap (C37) had blown, so I replaced it. Everything works great, then that blows in under an hour. All voltages appear correct. I've used a 50V and 35V cap. Maybe it's the voltage leading up to the output cap?

Any ideas?

Thanks!
Chris

So is it your PM1000 board having this problem? C37 is a 33uf/25V according to the Yamaha schematic.

It's a DC blocking cap for the output stage of the PM1000. My guess is you may have a short somewhere on your PM1000 board that's impressing a voltage across the capacitor more than it's rated 25Volts.

Check your PM1000 board.

Have you recapped this PM1000 board? Maybe the capacitors on this board are leaking or shorted internally... being 1974 era, it's probably a good idea for a recap.

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Thanks for responding, Owel.

Yes, all sections being used have been recapped according to specifications of other folks who have racked PM1000s. The only thing I did differently is change the output cap from 35V to 50V, (and back again after blowing it).

The power supply seems to be working fine.

Would the board work if there was a short? It works perfectly for a while then output dies once that cap has blown.

Thanks again!

Are you inserting the capacitor in the proper polarity? A backwards capacitor will blow up you know.

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Yup - polarity's good. Would it pass audio if it was backwar - even for a little while? My experience with reversed caps is that they don't work at all or make a terrible noise, (not that I've done it that often!)

Thanks again,
Chris

Audio will pass even if it's backwards. (Audio is AC). But any DC voltage across a reversed cap will cause the cap to fail or blow up.

Did you check the schematic diagram of the PM1000 instead of relying on the board's silkscreen?

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