how to test a SS transformer?

[methodofcontrol]

a friend asked me to look at his cyber twin. power comes on but there's no sound. i checked the speakers, they're fine.
i have the chasis removed and the speaker wires hooked up to an external speaker cab. when i look inside the amp i see that obviously the mains fuse is fine because the amp is powering up, but i notice two blown fuses on the board (F2 and F3, both T1A 250v). everything else seems to work , so i figured these fuses were the problem. i was in kind of a rush, so i decided it would be ok if i just jumpered the blown fuses w/ a thin strand of wire just so i could i confirm if the amp would make sound after that. the plan was to turn the amp on strum the guitar a couple of times and turn it off. if the amp made sound, i would go buy the appropriate fuses, install and be done.
anyway...this was a display of lack of sleep leading to poor judgement. so within seconds of turning the amp on w/ the now jumpered fuses, a "pfft" of smoke appears and i realize i just zapped a voltage regulator. yeah, i know...i should've put the correct fuses in there instead of making more problems for myself. oh, and the amp still didn't make any sound.
i've read that voltage regulators are pretty robust and the only way to really destroy one is to reverse the polarity to the input. obviously, i didn't do that by replacing jumpering a fuse...is it likely the regulator is still ok? it was warm to the touch after my experiment, but that's normal for feeding it too much voltage. (the regulator in question is a 7915, it is labeled U32 on the board.)
so now...that leads me to the transformer. the only thing before those fuses
is the tranny. i've got actual fuses in there now, and i've powered the amp up w/ no ill effects so far. however, i still have no sound. and that voltage regulator gets very warm to the touch. i feel like if i left the amp on for another couple of seconds i may have got another "pfft" of smoke from the regulator.
i have to check out the diodes in the rectifier after the fuses, perhaps they're messed up and feeding the regulator AC instead of DC? i don't know...any suggestions for me?

[methodofcontrol]

i tested the rectifier diodes and all the caps between the fuses and the regulator. all seems well.
am i wrong in assuming the following things?
A)i broke the regulator w/ my fuse jumpering, and i simply need a new one before the amp will work or i can continue trouble shooting. (how cani test the regulator to be sure of it's condition?)
B) the tranny somehow broke and is feeding way too much current into the amp. hence blown fuses.

what do you guys think? i could use some advice.

[SWEINC]

It is unlikely that you have a problem with the transformer. You are looking at your problem from the wrong direction. The power supply is not putting too much current into the regulator. It is getting hot because something it is feeding down the line is shorted and causing it to overheat. The first thing to do is disconnect the output pin of the regulator from the circuit and see if it puts out the correct voltage when it is unloaded. Then measure between the point in the circuit it hooks up to and ground to see if there is a short.

[methodofcontrol]

i had that thought about something shorted. i will check it out, thanks for the tip on measuring the regulator...that makes sense. i'll check it and post my findings.

[Steve Conner]

Sounds like some variation on the old solid-state death theme:

1) User hooks up too many speakers in parallel, using a speaker lead with an intermittent short circuit too, and cranks amp really loud.
2) The manufacturer's cheap ass protection circuit can't cope with this abuse, so the output transistors/ICs blow out.
3) Transistors etc. always fail short circuit, so huge amounts of current now rush through from the power supply, and those two fuses blow. That's what they're for: to stop the transformer going on fire once the transistors have blown out.
4) Well meaning but inexperienced repair guy shorts out the fuses and fries even more things.
5) Optionally you can now sell the amp with no warranty on eBay and the process will repeat from step 3 indefinitely with a string of new owners. Or take it to someone with experience in solid-state gear to have it fixed, and return to step 1.

[R.G.]

There are many byways in your question.

First, you've been given good advice. Chances are that the amplifier is killing fuses, not the transformer. But let's answer several of the questions.

1. You test a transformer by first seeing if it puts out the right AC voltage when the primary is driven. If that does not happen, you disconnect the whole thing and do resistance testing and short circuit testing. Resistance testing means using an ohmmeter to check for whether windings are connected through wire resistance as they should be and not where they should not be, including not shorted to the core. Short circuit testing checks for whether there is an internal shorted turn. You hook up a 100K resistor and neon bulb across any winding. Then you connect any winding across a 6V lantern battery momentarily and open it, having made sure that all the external leads are open. If the transformer is OK, the neon bulb will flash because you've stored energy in the winding inductance and it's released across the neon bulb. No flash equals shorted turn internally.

2. Any amp which is blowing fuses needs to be tested with a light bulb limiter. This is a power cord setup where an ordinary line voltage light bulb is in series with the incoming AC. The light bulb will let through small amounts of current practically unchanged, but will limit the total current to less than the bulb's current. A 100W bulb, for instance, will keep the total current below 0.8A on USA 120V power. A 25W bulb will keep it under 0.2A. This will now keep the circuit from blowing fuses while you dink around looking for the problem.

3. In a SS amp that blows fuses, Steven is correct - the outputs are probably fried, as are the drivers, predrivers, maybe some resistors and possibly the traces on the PCB. Step one is to test every single semiconductor in the amp for being shorted with an ohmmeter. Sometimes you get lucky and only the outputs blow. Sometimes you get to remanufacture the amp.

As an aside, Steven is correct that such amps get recycled through ebay. I love this. I get to buy an enclosure, power supply and heatsink setup for dead cheap and I can trash the amplifier junk that was in there in favor of my own stuff.

[Enzo]

i was in kind of a rush, so i decided it would be ok if i just jumpered the blown fuses w/ a thin strand of wire

We never have time to do it right, but we always have time to do it twice.


Those are 1 amp fuses, so it is not SS output devices. 1a is for a +/-15VDC LV supply for op amps and stuff.

This is a Cyber Twin, so it is not a simple amp. it could have many causes of this symptom. SInce these fuses blow, I'd bet on the op amps first, but a bad Host board (the DSP section) can also kill your sound.

As was said above, the transformer is not going to pop fuses or blow things up. Check the four rectifier diodes for shorted condition, absolutely. They cost less than a dime. If ONE is shorted, REPLACE ALL FOUR. With power OFF, check all three legs of BOTH regulators for shorted to ground condition. None should be. REmember the pinout of the 7815 and 7915 are different.

If an op amp is bad out in the amp somewhere, it may not short either supply to ground, but it might short them together. SO check +15 to -15. They should not be shorted together.

Often a bad op amp - one that blows fuses - will be cracked. Look at them closely.

-15v is involved in the muting circuit, so problems there can keep the power amp from making sound.

Just to help narrow down the problem, if you find something loading down a 15v rail, there are chips on the other boards than the main one. SO disconnect the ribbon cables to the other boards and see if the proble, still exists. If so it is on the main board, if not, it is on one we disconnected.

[methodofcontrol]

so...
the rectifier diodes check out ok.
the 2 voltage regulators immediately after the rectifier are not shorted to ground.
the op amps, (Q312, Q313, Q412, and Q413) short out when i first touch the meter to the middle and right legs on each. but i only get a short beep from the meter, then they read open. is that normal?
enzo, when you said "check +15 to -15, they should not be shorted together", i wasn't following you. what do you mean?
thanks for helping me with the amp guys. i know i'm an amatuer w/ SS amps, but i appreciate the learning experience. also a bit of background on the amp i just recieved.
my friend played the amp at a bar, when his set was done he packed his things into a corner and socialized a bit. when he came back to retrieve his things and leave, the amp was missing. apparently a drunk guy thought it would be funny to steal the amp. the owners of the bar were able to get the amp back for my friend and a fewdays later it was back in his hands. the first time he plugged it in to play it made noise but the volume was all "messed up" and there was some squeally noises. he quickly turned the amp off and when he got home he tried it again, at which point it didn't work at all and he contacted me.
i don't know if this info helps any of the troubleshooting. but there it is. thanks again everyone for the advice!

[Enzo]

Uh... The Q numbers are transistors. Q312 and 313 etc are the output transistors. They should not be involved in these fuses blowing. If legs 2 and 3 are not dead short together, they are probably fine. What your meter saw was the power supply filter caps charging.

When I ask if the + and - 15v supplies are shorted together I mean exactly that. Is the +15VDC and the -15VDC shorted one to the other. In other words, is there a low resistance from one to the other.

Op amps - operational amplifiers - are the little 8 leg ICs all over the place. They have numbers like U3, and the chips themselves are 4558 or similar types. They run on the 15v supplies. +15 goes to pin 8 and the -15 goes to pin 4. If the chip fails it can short out internally, and that winds up shorting the +15 and -15 together insie the chip. This of course loads down your power supply.

However it happened, something is wrong, and it is affecting those power supplies. That is where we are in the process.

SWEINC's idea was a good one, that was to lift the output pin of each regulator from the circuit and see if the proper 15v is coming out of it.

You need to isolate the problem.

[methodofcontrol]

ok as per SWEINC's advice, i checked the voltage regulators output. the 7815 is putting out +15vdc as it should be. the 7915 regulator is putting out -23vdc, perhaps i should replace this one. (this was also the one that smoked earlier during my examination)
enzo, i didn't see any cracked op amps. (i was looking at the 8 pin chips this time, sorry about the mix up before...long story). however, i checked them all for shorts between pins 4+8 as you suggested. i also checked continuity between the regulator outputs and pins 4 or 8(respectively) in an attempt to make sure the op amps were getting power, and i measured the resistance between pins 4 and 8 on all of the 8 pin chips.
here's some info, not sure what to do with it (of course) what do you think?
main board:
U1-measured open w/ continuity and resistance settings on multi meter. i also could not get continuity between regulator outputs and pins 4 or 8.
U2- open w/ continuity test. 82 ohms w/ resistance test. and i got continuity between regulator outputs and pins 4 and 8.(respectively)
U3- open. 82 ohms. continuity between regulators and pins 4+8.
U4- has some goop on it from where a ribbon cable was gooped down on the board. so, i'm not sure how good these results are. it measured open, 382 ohms, and i didn't get continuity from regulators.
U5- open. 82 ohms. continuity from regulators.
U6- open. 82 ohms. continuity from regulators.

the headphone jack/ effects loop board:
U1- open. 175 ohms. no continuity from regulators.
U2- same as U1. (U1 and U2 are next to eachother directly next to the headphone jack.)
U3 through U6- open.82 ohms. continuity from regulators.

the board w/ the midi jack on it:
U34-open w/ both tests. no continuity from regulators.

thanks again for the help. hopefully this info will help narrow down the problem.

[Enzo]

Yes, please replace the 15v regulator that does not make 15v at its output. Then we can move further.

[methodofcontrol]

well...now that the holidays are over and i have the day off, the faulty voltage regulator has been replaced. i had to take out two smaller boards and the transformer to remove the main board so i could replace the regulator neatly. i'm gonna wait on the next piece of advice before i put anything back together. i checked for cracked traces since the board was out, didnt find anything there.
so what's my next step? thanks again for all of the help.

[methodofcontrol]

just a bump. i'm getting sick of having this amp on my bench.