Need your help repairing an amplifier

I don't know where to put this (or if it's even a on topic thing for these boards), but yesterday I blew one of the channels on my amplifier by accidently crossing the output wires while it was on

Please Register or Log in to view the hidden image!

Now I have to take it apart and find the part that's been shorted out then replace it. My question is, how do I go about finding which of the 200 or so transistors, capacitors and so on are broken and which are still working?

 

Log in or Sign up to hide all adverts.

post the schematic

 

Log in or Sign up to hide all adverts.

One of the hardest things to find are emitter resistors. They usually go out when the output transistor goes out. If you are not qualified to do such work, I would suggest you not attempt it. It is far easier to create more damage than you realize by puttering around with it.

 

Log in or Sign up to hide all adverts.

spookz: I don't have any schematics, but I could post pictures of the circuitry if that would help.

wet1: I know, personally I don't like fiddling with it either, especially with the damage I might cause, but I brought it in to a couple places for an estimate and for the money they want I may as well buy a new amp. Before I do that I thought I'd at least try to fix it. I'll be going into town today to buy a meter to test the transistors and such, then I'll take a look at it tonight. Any tips on avoiding damaging the components?

 

A lot of amps have fuses on the output check that first!

 

Already have =) My amp has a single fuse, I guess it doesn't help much because it's still intact. One channel still works, it's just the left one that's shot. Could there be another set of fuses inside the amp? I haven't opened it up to check yet.

 

I haven't opened it up to check yet.
Well open it,there are sometimes fuses inside,mind you they are milliamp ones,so you have to go to a specialist for them.

 

Some do have fuses on the circut board. Sometimes in clips and sometimes soldered. Most do not.

Emitter resistors are usually black in color rather than color coded. The repair shop I worked at would usually replace the transistor and cut out the emitter. They were to hard to get hold of. 90% of fixing stuff is just looking at it. It is the other 10% that the repair shops earn their money.

Normally the output resistors have a heat sink. The preamps is one other place to look for the emitters. They will be near by.

You could go to an electronics warehouse (where the repair shops get their parts) and ask for a SAMS for your particular equiptment. Make sure and take your model # and serial # of your equipment. SAMS sells schematics for almost all equipment. That is where the repair shops get their schematics from.

As for tips, be careful not to scratch the printed side of the board. Careful not to lay things on it as they will sometimes crack but not break the board, creating yet another pain to hunt down and fix.

If you need to solder transistors, MAKE SURE AND USE A HEAT SINK when doing the soldering as they are very sensitive to heat. Don't lose the solder drops into the board do not lose screws while reinstalling the board.

Most of this is common sense but you would be suprised...

 

Thanks for the tips wet1. You say the emitter resistors are black and not color marked? Because I do see 4 black resistors that with a single silver color band around them and some writing that looks like "4004 DC", and they are numbered 209, 247, 109, 147. Could these be the L/R outputs of both channels? Here's some pictures I took of it =P :

http://www.geocities.com/swrwookie/amp.html

Sorry about the poor quality, they were taken with a digital video camera =/

Are those the emitter resistors? The second shot is of the output terminals.

 

I think they are diodes.

 

Hmm, those are the only things that look anything remotely like black resistors, are you sure they weren't output transistors wet1?

 

Transistors are ear marked by having 3 leads out the bottom of the piece.

Output transistors can not be readily identified that way as they are usually mounted in such a manner that you can not see them till they are removed.

IC's, which I see you have, are usually a combination of circuts combined into one unit. Most of these and the transistors have other numbers than what you will have when you buy replacements. You need the number off of the part for cross referrence purposes.

Your photos are to dark for me to recognise the pieces that would be the emitters. They are normally smaller than a typical resistor. The diodes are normally marked with one band and usually have a triangle indicating which way the current flows. It can well be that they are incorporated within an IC. It is not typical but is possible.

Do a careful look of the board for any "discoloration" indicating excessive heat. Heat is the death knell of electrical componets. Look at both sides for this as it is easier to see from the backside than from the componet side.

 

Well wet1, I went and took the whole board out, there are 6 large transistors around the outside of the board and 4 even larger ones as well. They were all attached directly to the frame of the amp with thermal compound and had a heat sink screwed on top of them. The contacts on the underside of the board for these transistors all have what looks like soldering flux residue on them, but the flux on one of the transistors looks more burnt than any of the others.

As for the emitter resistors, I'm looking for a small black resistor (two legs) with no color bands right? Because I don't see anything like that on the board. Someone told me the output or power supply transistors are usually the first to go when a channel blows.

There's 4 large transistors on the edge that are all labled with 'Q' numbers on the board (Q7, Q8, Q9, Q10), they are completely identical, right down to the numbering and other markings on the transistors themselves. I read that output transistors are usually marked with 'Q' labels on the board, could these be them? I attached a picture of them. There are 4 more 'Q' transistors around the outside, but they do not have identical numbering on them, so unless each channel uses a different kind of transistor I don't think they are output.

Thanks again for your help wet1.

 

Forgot to attach the picture, here it is =P

 

Well The bit in the pic looks in good condition!

 

Everything looks in good condition =/ There's next to nothing out of the ordinary on the board itself, which makes it very difficult to find the problem, and not having a schematic to go with this amp isn't helping =P I can't even get a SAMS for my amp because there are no pure electronics repair stores here, maybe in Edmonton but that's an hour and a half away.

 

You could try getting a multi meter & tracing back to where the volts start.ie there is 0 volts coming out of one of the outputs,because its blown.
so trace back from there dont know if that will help.

 

The transistors that you have found with the heat sinks and compound are the outputs. They are usually the largest on the board but just as often are found off the board.

Because you have not seen the emitters they are most likely internal to the IC's. Do not worry about them until you find that output replacement does not cure the problem. If you find that to be the case then you will have to find exactly what circuts are within the ICs. These are usually common types and diagrams of them are contained within crossreference manuals. (You will usually not find the exact replacements by the numbers on your ICs as they are normally manufacturers numbers.) Hence the need for crossreferencing.

Sometimes it is easier to trace a problem with a signal injector and probe. They can be had rather cheaply. You inject the signal into the componet and look for either no signal on the other side or for a bad distortion on the other side.

Using a meter will inject it's own capacitance into the circut at times and so will working under a florscent light (injects 60 cycle hum in the US). If using an ohm meter, make sure that no power is in the circut. Also be aware if it is not isolated for the rest of the circut that you could be reading values of other componets in addition to the componet you are testing.

 

Well i've burnt up quite a few amps in my time so here's what i've found out

(on the JPEG)
the green ones i've burnt up due to the output crossover like you

the heat sync pads i burnt up due to letting the posite main wire hit the ground so you shouldn't worry about them

hope that helps some

 

amp repair

hi,
i just join sciform. i was wonder if anyone knows how to repair or knows where i can go to find info on repairing a non-tube guitar amp?