/ audio

Power amp repair

The setup:

Attempting to set the sub limiter on the driverack to keep the clip lights on the sub amp from blinking. The sub amp is unfortunately underpowered in this application, so there’s no headroom – the amp needs to run to redline to get any decent output.

So we disconnect the subs and run a 60 Hz sine wave through, pushing it until the amp clips, tweak the limiter threshold on the driverack until the lights go out, rinse, repeat. This is very standard stuff, until…

Poof.

The sub amp lets the magic smoke out.

While this particular amp has seen some rough days on the road, it was a hefty piece and there was no reason to put it to pasture. It was built with all through-hole components as well, which should make it easy to troubleshoot (even though I was unable to find a schematic). After opening the unit up, I was not presented with the signs of burnt transistors or resistors that I would have expected given the amount of smoke that came out of the vents. Although it was clear that this unit could run hot, evidenced by some discoloration on the top lid.

After scouring all the individual PCBs carefully, the only suspect parts were the filter capacitors on the power supply. There were matched pairs for each side of the amp, and many of them were bulging at the top a considerable amount. I decided to replace all the electrolytic caps in the area even though some showed no signs of distress, for two reasons: 1. better safe than sorry, and 2. they were all mostly attached together with some type of adhesive to prevent damage from transport and vibration.

This amounted to a parts order for some 16 rather large electrolytic caps, and when they arrived a healthy amount of cutting adhesive and desoldering. I was hopeful this would breathe new life into this amplifier, since there were no other visibly damaged components from the limiter-threshold-setting shenanigans. Sure enough, powering up the amp now included the “PROTECT” LEDs shutting off after a short delay, and the amp seemed ready to work again. Better still, after re-racking and connecting the subs my ears told a story – that the amp prior to this incident was anemic, and muddy. I would guess that the power supply caps had been in poor shape for quite some time before they gave up the ghost. Now the amp was snappier, and more confident.

So I suppose the moral of this story is –

Mind the cap.