transformer

[andyjazz]

This may sound like a dumb question but if you do a simple continuity check on a power transformer and the two leads on the primary are "open" the transformer is bad - right? This transformer is in a Peavey Delta Blues. There is no power to the tubes so I checked the leads from the transformer secondary (With the amp turned on and the pilot light lit) and could get no voltage. I also got no voltage on either side of the fuses that are in line. I've checked and checked and checked and checked.

I'm just skeptical because it seems to me that the chances of a power transformer going bad are fairly small and I don't want to order a new one just to find out that the amp still don't work.

[52 Bill]

There are no dumb questions, other than the unasked ones.

If the primary winding shows no continuity, then the transformer is bad. It is possible that you could open up the case and check to see if the lead wires are still attached to the winding wire or to see if there is a thermal fuse. Many transformers have a thermal fuse wired in the primary that will open up when the transformer gets too hot or is stressed. If yours has one and it has opened up, you could bypass it to see if the amp will work and once you are satisfied that the transformer is bad and that the amp is work fixing, order a new transformer from Peavey.

[R.G.]

Bill's right - if the primary is open, the transformer *can't* work. Also right in that some of them use a thermal fuse, and that it can be tested.

Be very, very, (about fifteen times) careful about opening a transformer up and messing with power on it then. You're exposing yourself to electrical hazards.

If you open it up, find a thermal fuse, and bypass it to find the transformer then works, your next thought will be "Hey! I can just solder a wire across that fuse and it'll be fine." Resist this idea firmly. It's like putting a metal short across the incoming AC power fuse. If it was the internal fuse, it died for a reason. Shorting across it just removes its ability to protect you from the other failure you haven't found yet.

[Enzo]

Transformer failures ARE rare. But that doesn;t mean they don;t happen.

[olddawg]

Yeah, I guess it isn't a great idea to jump a thermal fuse. Especially if a replacement is easily available at a reasonable price and the unit is worth more than that. Truthfully though, I've seen many thermal fuses fail for no apparent reason. I have a couple of units that I jumped the internal fuse that have been running 10 years. Of course you have to insulate the jumper and I usually add a fuse at the primary if there isn't already one there. If you can fry bacon afterward or have leakage to the chassis then it's a big no-no.

[andyjazz]

Got the new transformer today. Installed it, turned on the amp and the tubes light up (Good sign) and then I get the high pitched squealing that sounds like when I hooked up the output transformer on my old Gibson the wrong way!

Just as a reminder this amp is a Peavey Delta Blues 210 combo. I finally figured out how to wrestle the folded circuit board out of the chassis without wrecking anything (Those of you who have worked on Classic 30's will know what I mean) I'm poking around in the filter caps, looking at connections, etc. also one of the el84's seemed to be glowing way too red, thought maybe a screen cap or resistor might be suspect. Anybody got some ideas? Am I heading in the right direction? Any help is appreciated.

[acorkos]

swap the el84s and see if the overheating follows the tube or stays with the socket. If it follows the tube, pop in a new set and you're done.

[52 Bill]

When you pulled the board, did you have to remove the output transformer primary wires?

[andyjazz]

The output transformer connections are a plug that I marked with a sharpie to make sure I re-connected it in the right position. I had a bunch of other stuff to do today and I've got a gig tonight. I'll be back at this thing tomorrow afternoon. I checked all the tubes on an old tube checker I have - seems to be pretty reliable. It's a real old-timer with the "english" knob and everything- all the tubes checked out good so I suspected something in the highV DC chain like the filter caps - i dunno. I took the whole thing apart so I could get at the filter caps and some of the big resistors - just so I could check for continuity, or solder breaks, and what have you. I'll check back tomorrow afternoon.

[andyjazz]

I'm sure most of you can relate.

Whenever I take an amp apart I meticulously mark every connection so that I know when I re-assemble I'm not scratching my head trying to remember which plug goes on which pin etc... I have often taken pictures with my phone camera and brought them up on a laptop just so I could see if I had re-connected anything bass-ackwards. With this amp I am relatively certain that I simply marked the goddamned plug connecting the output transformer THE WRONG WAY!!! I turn the connecting plug around, presto! guitaro ampo!

Just goes to show you. Anything can happen even if you try to take every precaution. BTW this is a great sounding amp. Easier to work on than the Classic 30. Thanks for everyone for your ideas. I really value this forum for the chance present problems and get ideas. Occams razor was the application here: If it sounds like an output transformer hooked up backwards-then it's an output transformer hooked up backwards!

[52 Bill]

Been There, Done That.

I believe that it was Master Enzo that said, "Don't look for reasons not to check something."

[Prairie Dawg]

My digital camera is probably the most valuable tool in my shop. All you have to do is screw something up one time. Before I started regularly doing this I screwed up the connections on a Bassman reissue and spent an entire flight from Milwaukee to Boston figuring it out. Much easier if you take good pictures. They're also helpful when the next one comes in, too.

A lesson well learned. Good detailed photos are a must, especially when the repair job doesn't go as planned.

EDIT. I didn't have the amp with me, mind you, just the termination diagram, schematic, and my hand written notes.

[R.G.]

My digital camera is probably the most valuable tool in my shop. All you have to do is screw something up one time. ...
A lesson well learned. Good detailed photos are a must, especially when the repair job doesn't go as planned.

I love my digital camera too!

I ran into something that may help more. The Yeho(?) MUVI pocked digital video camera can clip to your pocket and record up to two hours of digital video on a flash card. Costs about US$60-80.

[Prairie Dawg]

For what they cost that sounds like a great deal, Amazon has them for $55.
. It would be a really good adjunct to a camera. I archive my photos and that helps as well.

[andyjazz]

Well just go figure. Now the guy calls me up and says the amp is toasted again. He says he played through it for about a half hour then it made some noises and just quit! I asked him and he confirmed that now, upon fire up, the pilot light turns on but the tubes don't light up - exactly what was going on before. The transformer blew! The only thing I can figure is a diode in the rectifier section going flaky? They are on the other side of a fuse but I guess anything can happen. Anybody seen something like this before?