SWR SM-900 Popping

[rf7]

This amp pops pretty loudly when turned on, and it happens with both channels. It has the green power amp (SWR2000) pcb's for the power amp module. There is a timeout circuit (D7, D8, Q13, & Q14) that is supposed to stop this from happening. All the components measure OK in circuit. The owner says it has popped since he first got it.

If the rails come up too slow, would that cause popping?

Any ideas what else I can check?

Thanks in advance.

[Jazz P Bass]

Can you post a schematic?

[Enzo]

I never worry about power up/down pops.

Try replacing the 2.2uf timing cap. That cap is what makes it take a moment before coming on.

[rf7]

The guy says he's taken speakers out with it, on power up. I tried replacing the cap, it did not help.

[Enzo]

Really? The transient is limited to the available power supply voltage. And that voltage is what the amp clips at normally, so the pop is no louder than the loudest sound the amp can make.

Use the jacks, and see if the pop is in the power amp or the preamp.

[rf7]

Ah, the divide and conquer!

The preamp definitely pops on power up, it looks like a peak at about 1V on my scope.

[Enzo]

SO apologize to Q13 and Q14 and then we move on. Go ahead, we'll wait.

SO the pop is preamp. OK, set the power switch on but leave it unplugged from the wall. get the amp as far as possible from a wall outlet, then plug the amp in. Does it pop less than using the panel swich on the amp? Sparking power switches make lots of noise. Does it matter if a guitar is plugged into it or not? If so does the volume control on the guitar make any difference? SO let's find out if the probem is even in the amp at all.

Turn all your knobs to zero, flip it on, still pop? Turn just the master up, still pop? Now turn up the EQ too, still pop? Now turn up the pre or gain, still pop?

And I bet you didn;t think of this: Unplug the power to the power amp module and just listen to the preamp. ANy difference? SCope the preamp power supplies. I know it is hard to look for turn-on transients on powr supply rails, but give it a shot. We want to eliminate the possibility it is the preamp being affected by a transient from the power amp.

And back to basics. Make sure the input jaqck grounding contact is working, then nothing plugged in the input, flip the thing off and on, and scope stage by stage looking to see where this transient pop shows up on the signal path.

[rf7]

SO apologize to Q13 and Q14 and then we move on. Go ahead, we'll wait.

SO the pop is preamp. OK, set the power switch on but leave it unplugged from the wall. get the amp as far as possible from a wall outlet, then plug the amp in. Does it pop less than using the panel swich on the amp?

Not really.

Sparking power switches make lots of noise. Does it matter if a guitar is plugged into it or not?

Nope.

If so does the volume control on the guitar make any difference?

No.

Turn all your knobs to zero, flip it on, still pop?

Yes

Turn just the master up, still pop?

Yes, crazy loud.

Now turn up the EQ too, still pop?

Oh yeah.

Now turn up the pre or gain, still pop?

Yes.

And I bet you didn;t think of this: Unplug the power to the power amp module and just listen to the preamp. ANy difference?

No.
'

SCope the preamp power supplies. I know it is hard to look for turn-on transients on powr supply rails, but give it a shot. We want to eliminate the possibility it is the preamp being affected by a transient from the power amp.

That was it, one of the paralleled 1.1K ohm resistors on the feed to the -15V rail was not connected. They did a miserable job and had just tack soldered those together, and this one was not really attached at all. Must have been built on a Friday afternoon after a liquid lunch. It still pops a little with the Master cranked, but nothing like before. With the Master on zero, there is no pop!

Thanks Enzo, for the thought process tour! Much appreciated.