Strange SWR Basic 350 problem

[Axtman]

Okay here is a really strange one for you.

I have an SWR Basic 350 red face amp that I am trying to repair. It is currently hooked up to a light bulb limiter. When I turn the amp on while it is hooked to a speaker cabinet (load) the bulb goes to full brightness. There is a slight hum and about 10 volts DC across the speaker terminals.

Now for the strange part. If I unplug the speaker cabinet and turn on the amp the light bulb glows dimly. If I plug the speaker into the amp while the amp is still on there is no change (no hum or change in bulb brightness). I can then play the amp and everything is fine. I can turn the amp off and turn it back on quickly (with the speaker load connected) and everything is fine. But if I turn the amp off and wait a bit (long enough for the red LED indicator light to fade to black) then turn the amp on (with the load connected) I get the hum and full brightness bulb just as before.

What the heck is going on?!

[Enzo]

The heck is you are using the amp on the bulb limiter.

The bulb is to prevent fuses blowing and damage to the amp. So you plug the amp into it and if it doesn;t light up bright, then a fuse would not have blown. So we remove the bulb and plug the amp straight into the all, no load. Fuse OK? CHeck the output, no DC on the speaker wires? Do not connect a load if there is any DC on the output. By "any" I mean say more than a quarter volt. Then connect a load. Still OK? Turn if off for a minute, then back on. OK?

The bulb limiter is not intended to be used when operating the amp. The voltage to the amp will be lowered, and not stable, in that as the amp tries to draw more current, the voltage it gets goes down. So the amp will be unstable, as you found.

[J M Fahey]

Many amps, and SWR is a classic example, "wake up stupid" when you turn them on.

Basically some electrolytic caps take some time to charge, and while they are doing so, output voltage is anywhere, until power amp input differential pair stabilizes and pulls it to 0V (or less than 100 mV in any case); thatīs why most amps thump on turn on and you see the speaker cone go in or out and then return to normal, clear example of DC applied to it.
No big deal, too short to damage anything, except on real BIG PA type amps, which include a speaker relay to connect it only after a couple seconds.

Of course, while you have DC on the speaker, the amp pulls a lot of current from the supply but again, only for a second or so.

Now what happens if you have a bulb limiter in line with mains and a speaker connected?

Some amps donīt care (Peavey and others), others pull supply down a lot ... and never get to charge the needed capacitors so they get stuck with DC on speakers ... thatīs whatīs happening there.

You have 3 possibilities:
* no bulb yes speaker: thump > normal
* yes bulb no speaker: thump (you donīt hear it, yet meter and scope show it) > normal. Adding a speaker: no problem.
* yes bulb yes speaker: thump pulls rails so low that it never recovers. Even if you disconnect and then reconnect speaker you are already blocked.
Common in most SWR.

IF you turn properly working amp off (with limiter) , wait for caps to discharge and turn it on again, you are back to square 0 .