Ampeg SVT4 Pro--blem

[drewl]

Odd,
this amp starts getting an oscillation/farting out at about 4watts output, output signal has spikes on it and the limiter starts kicking BUT - only when speakers are hooked up to both Ch A and Ch B speaker outputs.

Each amp works fine when there's a load on only one of the channels, as soon as you hook a load up to the 2nd channel output, it starts cutting out.

I think it might be a problem with the Bridge/Mono switch, because I can still get a signal output on the Ch A and B 1/4" jacks in Bridge/Mono mode- according to the manual in this mode only the one Speakon jack should have an amplified output signal.

Whatever the problem, I have to pull out the main power amp board, I've already had to repair sveral bad pc board connections on some of the other boards.

I've tried every mode of hooking this thing up, biamp mode- running one channel output into the others, all with the same result.

Just wondering if anyone encountered this problem before.
Thanks.

[Enzo]

I may be wrong, but I think the manual means to say you should only USE the bridge speakon jack in that mode, not that the other outputs go dead.


I see nothing in the circuit that would disable the 1/4" jacks.

Does this happen in both mono and stereo modes? Or only when in bridge/mono?

[drewl]

I read that from users that own the amp that only the one speakon jack would be functional in the Bridge/Mono mode.

Anyway, the amp had a few blown parts in the output power amp section which must have caused the oscillation/noise spikes when both output channels were used.

Both channels had a 3.3ohm 1w resistor on their output (R168&269) open.
R101 which passes channel A's signal was fried, but still read 1kohm
-V feed to Q105 ch A lo drive resistor R126 fried
And the bridge/mono switch was intermittant.