AMPEG SVT-CL-no pilot light, no output

[nevetslab]

Last week an SVT-CL came in with the note No Power. Plugged it in, No Pilot Light, but did have heater voltage. When I pulled the amp apart, first thing I looked at was the LV bipolar system (from bottom of main PCB). All four rectifier diodes D22 thru D25 were reading like 50 ohms in both directions, the two 15V zeners D27 & D28 were shorted. And, applying mains voltage thru the smaller power xfmr would feed AC voltage to the four filter caps C24, C25, C26 & C27. So, I replaced all those and looked for further damage, still pondering what caused that failure. I never did find anything else abnormal, went back thru the system to check the power tubes in pairs, just to see if any of them failed. All came up nominal.

This morning I learned that the client had plugged the amp into the lighting system mains, which was 240VAC. Throw the power switch there, and the small power xfmr lights up, takes out the low voltage bipolar supply, while turning on the bias supply up to near double voltage. I had already buttoned up the amp, with everything working 'normal'. Though that may have shortened the life of the 100uF/250V bias supply cap C23. Nothing else powered up, since the LV bipolar supplies were down.

I hadn't run into this one before. AC mains fuse was still intact, as the abnormal voltage still wouldn't pull enough current to pop that fuse. No idea how long the amp was left on in that partially failed state. The heater supply would have been twice the voltage. So, one amp to keep my eyes on for further failure modes.

[Jazz P Bass]

"Stupid is as Stupid does".

[xtian]

240vAC was made available on a "standard" 120vAC style outlet? Wow, that would be a nasty honey pot for musicians...

[The Dude]

240vAC was made available on a "standard" 120vAC style outlet? Wow, that would be a nasty honey pot for musicians...

I wondered that myself. Can you elaborate, nevetslab? Regardless of lighting rig or something else, why is there 240V on a standard 120V outlet?

[nevetslab]

I wondered that myself. Can you elaborate, nevetslab? Regardless of lighting rig or something else, why is there 240V on a standard 120V outlet?

I don't have any of those details yet. I was busy at the time gathering more gear to take back to the shop for preventative maintenance, and didn't even stop to ponder how the dickens someone DID manage to plug one into 240V. Easiest way, of course would be with a power cable having it's appropriate power plug formated for that....but....as Jazz put it ......stupid is as stupid does. Scary! I will push on that some now. Until I heard that today at the equipment depot, I was under the impression this came back from Village Recorders in Santa Monica, presumed to have been used at a session.

I hadn't come up with an idea of just what took down the LV bipolar supply...both sides and all four bridge diodes. I pulled and tossed the related electrolytics, but they're still in the trash box to be rescued to see. The two zeners were shorted. I'd expect more damage from powering it up off 240V, wired for 120V. How long would all of the tubes last, with double the heater voltage being applied? You'd get (attempt to get) +/- 40VDC into the first stage of the supplies, then you've got much higher current flowing into the zeners, doing their best to still regulate to +/- 15VDC. And, the bias supply, nominally -180VDC, it would have doubled, and stress the 100uF/250V filter cap.

I'll see what I can find out. From our general manager, it sounded like it was maybe an outdoor gig. I'll see what I can find out and add to it, and....if that IS indeed what caused it, get the amp back to the shop to dig deeper. All measured/sounded fine after only finding the LV bipolar supplies being down.

[Enzo]

You may never have worked with CRTs, but in my past industrial experience I worked with CRT video monitors for the arcade industry. Part of CRT monitor service involves "rejuvenating" CRTs. After applying high voltage discharges to blow away flakes of stuff between elements, we bake the cathodes by turning the heater voltage way up. The heater glows white like a light bulb.

Could be lit up like that for a couple minutes. Point being it really doesn't hurt the heaters. It might shorten the life of a tube a hour or two, but since the tubes can last decades...

In fact I have seen tube testers that had a rejuve feature, and that is how they do it, crank up the heaters to bake the cathodes.


Just a thought. maybe a generator wired up for 120 but someone had a switch set to 240?

[nevetslab]

You may never have worked with CRTs, but in my past industrial experience I worked with CRT video monitors for the arcade industry. Part of CRT monitor service involves "rejuvenating" CRTs. After applying high voltage discharges to blow away flakes of stuff between elements, we bake the cathodes by turning the heater voltage way up. The heater glows white like a light bulb.

Could be lit up like that for a couple minutes. Point being it really doesn't hurt the heaters. It might shorten the life of a tube a hour or two, but since the tubes can last decades...

In fact I have seen tube testers that had a rejuve feature, and that is how they do it, crank up the heaters to bake the cathodes.


Just a thought. maybe a generator wired up for 120 but someone had a switch set to 240?

That's a relief......your wealth of knowledge from a lifetime of experience never fails to amaze and humble me. Every day in the shop and on this forum is an education. I'll know more on the facts of this being subjected to 240V and how/why, if anyone at our end knows. And, will pass that along. I know we have some 2kVA 120-240V step-up xfmrs to accomodate bands who are in from outside the country and need to run their 240V gear in our studios. Those outputs are Shuko style. But that's here at CenterStaging's facility. I do at least have the facility to test the waters, as long as I'm willing to pay for the damages.