400series mk3 bh won't power on?

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400series mk3 bh won't power on?

Post by MajorJohnny » Sat Jun 06, 2020 1:24 am

On to the reason i'm here, i recently came into possession of a 1980 400 series mk3 bass head that won't power on.

The previous owner stated that the amplifier was put away working ten years ago but when i wanted to check if it worked it didn't do anything. Must be a fuse right? Nope.... all of them were ok, even those on the inside yet not even the indicator light or any other light came on, it just hums a little, no sound, no lights ...well.... when i flip the switch the "Limiter" light flashes briefly and there's a very faint crackling noise coming trough the speaker that isn't affected by the volume pot but that's about it.

Busted out the multimeter to check if the fuses really were ok and also checked the switch for continuity, all checked out fine.

So i looked up the schematic and the emidiate suspects were either C34 and C35 (the big blue electrolytic caps) or C36 and C37 as they're the first caps right behind the rectifier stage and nothing beyond that point is seeming to get any power at all.

Upon opening the amplifier up i saw that the big blue caps looked ok, no bulging tops or sides, C36 looked like it might have been a little hot at one point in it's 38 year lifespan but no bulging here either and as i don't have an ESR meter all i can do is desolder the caps to check if they're ok.

Not really wanting to do that i was wondering if there are people here that might have an idea what else it could be so i can troubleshoot that before i start to tear this thing down and replace stuff that may not need replacing.

Also can a cap fail that bad that it won't allow an amplifier to power up without bulging?

Any help would be much appreciated,

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