Possible overvoltage. What is broken?

[Morocotopo]

Hi, I have a Ibanez digital delay, rackmount, from the 80´s,  given to me, it´s not working, doesn´t turn on. I suspect someone connected it to 220V (our voltage here) and the thing is 125V. I opened it and the power supply section has the mains transformer, a 7805, a 7812 / 7912 pair, the corresponding diode bridges and the corresponding smoothing caps for each power feed (one +5V, one +12V and one -12V I believe). The caps have a bit of brown goop at their base, but no exploded tops. No obvious burnt parts, no bad smell, no molten varnish at the transformer, no other obvious broken/burned parts.
The question is, if it was overvoltaged, what are the parts more likely to have failed? I think: open transformer winding, blown smoothing caps, blown regulators... what else?
I ask because if the IC´s are blown, it´s not worth the time to try to repair it, but if it´s something as simple as a regulator/cap change, it might be worth it.

Thanks.

[anchovie]

I'm not familiar with the particular unit, but I'd have though that something mains operated would have a fuse between the mains lead and the transformer. If you're lucky, that should have blown before anything else in the unit, meaning a cheap repair!

[Morocotopo]

Anchovie, I didn´t mention the fuse because that´s checked. Well, actually bridged for testing purposes, to discard its socket and wiring, as well as the fuse itself.

[davidallancole]

Do you have a meter to check the voltage on the output of the transformer?  Sometimes those little transformers have thermal fuses inside them and that could have gone.  Check out the voltages and go from there.

[Morocotopo]

 Well, the transformer has no voltage at its secondaries. David, good idea about the thermal fuse, I´ll see if it has one, problem is, the thing is enclosed, I´ll try to open it without breaking it.
So it seems like a blown transformer... that should be easy to repair, except for the fact that it´s a fat but low profile one, obviously desgned to fit into a 1U rackspace enclosure. Good luck trying to find a replacement!! 

In case that I replace the original transformer with another one, I have to figure out the specs. I think, if the thing has a 7805 and a pair of 7812/7912, the transformer should have secondaries of no more than 1A, because that´s the max current the regulators can handle, right? So it should have, let´s say, a 15-0-15 @1A and a 9@1A secondaries... am I correct?

What about the smoothing caps? Let´s say that they are blown, if I replace the transformer, is there danger in powering it with blown caps to the rest of the circuit?

I´ve searched for the schem all over the net without any luck. It´s an Ibanez DM1100 digital delay, by the way.

[twabelljr]

I had a stereo receiver that had the thermal fuse blow in the transformer. The fuse was not serviceable. Transformer required. I'd diode check each rectifying diode also for opens or shorts.

[Morocotopo]

Twabelljr (what a name!  ), good idea about the diodes, they are a bridge pack... I´ll see how to check them.

[R.G.]

Well, the transformer has no voltage at its secondaries. David, good idea about the thermal fuse, I´ll see if it has one, problem is, the thing is enclosed, I´ll try to open it without breaking it. 

Bad idea. The thermal fuse will be in the primary, and that means you're working on opening up and then having to re-close a primary power section. That means you have to get all the safety details correct when closing up or you're creating a safety hazard for yourself and whomever uses it in the future.

So it seems like a blown transformer... that should be easy to repair, except for the fact that it´s a fat but low profile one, obviously desgned to fit into a 1U rackspace enclosure. Good luck trying to find a replacement!! 

There are many toroids now that might be made to fit.

In case that I replace the original transformer with another one, I have to figure out the specs. I think, if the thing has a 7805 and a pair of 7812/7912, the transformer should have secondaries of no more than 1A, because that´s the max current the regulators can handle, right? So it should have, let´s say, a 15-0-15 @1A and a 9@1A secondaries... am I correct?

Maybe, maybe not. How many secondaries, rectifiers, and filter caps are there? You may be able to use a transformer with 12-0-12 volts on a single centertapped secondary. 15-0-15 might be better. You may not need another secondary for the 5V. At low power (5V under 2A is low power, by the way 8-)  )
often 5V is obtained from a secondary that's not perfect for it.

What about the smoothing caps? Let´s say that they are blown, if I replace the transformer, is there danger in powering it with blown caps to the rest of the circuit?

Any electro cap with brown goo or obvious heating or mechanical damage should be replaced even if it seems to be OK. "OK" is a temporary condition, even for new electrolytics.

[Morocotopo]

R.G., as the Englishmen would say, brilliant! The idea of a toroidal transformer never crossed my mind. Opening the original to try to fix it is beyond my abilities and as you say, a safety hazard.

The original transformer has 5 wires in the secondaries, two yellow wires that go directly to the 7805 via a diode bridge and two red and one blue wire that go to the 7812/7912 and ground, also via a diode bridge. Each respective path has its own smoothing caps (3 in total). No other regulators or power connections from the transformer.  Hope I´m clear. Maybe I´ll draw and post that part of the circuit to be more clear.

I think I´ll do this: connect temporary transformers and replace the electros and diode bridges. Not too much work. If it works then, I´ll try to find or have made a toroid that fits into the enclosure.

Thanks everyone.

[R.G.]

The original transformer has 5 wires in the secondaries, two yellow wires that go directly to the 7805 via a diode bridge and two red and one blue wire that go to the 7812/7912 and ground, also via a diode bridge. Each respective path has its own smoothing caps (3 in total). No other regulators or power connections from the transformer.  Hope I´m clear. Maybe I´ll draw and post that part of the circuit to be more clear.

Hmm. You had it right, I think. Separate secondary for the 5V. Oh, well.  You may still be able to get by with a single secondary.

I think I´ll do this: connect temporary transformers and replace the electros and diode bridges. Not too much work. If it works then, I´ll try to find or have made a toroid that fits into the enclosure.

And, as you say, brilliant. Good idea to find out if a transformer and some capacitors would fix it before spending a lot of time and money on it.