SWR Silverado power-up problem

[Euthymia]

I have a SWR Silverado on the bench that has a failure mode I have not seen before.

When I opened the amp for repair, I found a metal buckle, like a d-ring from a cheap backpack or purse, had welded itself across the terminals of the IEC jack.

After I removed that and cleaned up the mess surrounding it (IEC jack and power switch were carbon-y), the amp powered up on the lightbulb limiter okay with no load on the speaker jack. If I powered it up connected to my dummy load, I'd get a bright bulb.

Then I noticed that if I first powered it up with no load, which worked okay, then connected a load before the PS caps discharged, it would power up with a load.

I pulled the PS caps and tested them on my power supply, and one was fine but the other one made the needle on the ammeter jerk wildly.

New caps, and now it's back to the same behavior: power it up with no load, dark bulb. Power it up with a load, bright bulb. Power it up with a load before the caps have a chance to discharge, dark bulb.

Ideas?

[Enzo]

If both sides of the output conduct, whether from shorted devices or from an open bias string, then the two rails condcut to each o0ther and fuses blow - or your bulb lights. No load required.

Without a load, the amp is free to put DC on the output. Without a load, no current flows, so your bulb stays dim. Of course as soon as you load it, current flows and the bulb lights up. So, is there DC on the output bus?

Why would the amp put DC on there someetimes and not others? GOod question. But the key here is voltages.

Is the input connection ground intact? You don;t have it unplugged from the module do you?

WHich power amp do you have?

[J M Fahey]

Hi euthymia.
Maybe you *don't* have a problem at all.
As I understand what you say (I may be mistaken, of course), what happens is:
1) Bulb limiter - amp on - no load= dark bulb
2) Bulb limiter - amp on - with load= bright bulb
3) Bulb limiter - amp on - no load -> then load= dark bulb
What you don't say, and I must guess, is if on condition (3) you apply some audio, you get sound on the speaker.
Even more: if you raise the volume, the bulb starts blinking in rhythm.
If that is so, you have no problem.
As a final test, wire a switch across your bulb , and after powering the amp as in (3), short the bulb without turning amp off or unplugging it.
Probably all will work fine.
What happens is that when you turn a power amplifier on, it is "stupid" for a second until it wakes up, that's what causes the typical turn-on thump.
That's because the output rail voltage, which is later corrected to "0" through DC feedback, can be any value for a second and the amplifier will pull high current from the supply (because it's loaded).
*If* you have a bulb limiter, that will cause a voltage loss.
Most amps power up normally anyway, but many don't, and stay locked , putting DC on the speaker terminal.
As you have already tested, if you let them wake up and then connect the load, no problem at all.
I think you can power that amp loaded , without bulb limiter, with no problems.
Just to play it safe, do it first with a power resistor instead of the speaker and check you don't have DC across it .
Good luck.

[52 Bill]

I agree with JM Fahey 100%.

For some reason some SWR amps will always light up my light bulb limiter and will not power up normally. I think that it only does this with their discrete power amps, and not the IC output amps.

I guess this is one of the cases where Enzo's variac start up would work better than the light bulb limiter.

I'd double check everything and then power up straight from the wall and see what happens.

[Enzo]

VAriac might eliminate the confused wake up, but at some point you still have to be able to flip the powr switch and have it work.

[Euthymia]

JM and Bill had it: I powered it up without the lightbulb limiter into a dummy load with no problem, then tried it with the speaker, no problem.

And so prayer returns as an integral part of the solid state amp repair process.

My takeaway from this is that powering up that SWR circuit through a lightbulb limiter can result in sticky rail slam.

I wonder if a variac slow-start would have the same result.

Oddly, the original problem the amp had, a leaky power supply cap, resulted in a very similar symptom. I tried powering it up, no lightbulb limiter, and it rail slammed and popped its fuse.

Of course, rail slam is a symptom of many SS power amp maladies....