Stage Line 500W amp repair project

Haha, that's great Pete! I would cover all of it, apart from the fusible resistor, in hot melt glue or similar, to give it some structural strength and prevent it from touching other parts. But you've got it, and you even replaced the bicolour LED with two red ones in anti-parallel. I thought you said you were a novice!

OK, do you have a multimeter? What make and model?

You can start by measuring the output transistors. The basic test is collector-emitter resistance. Transistors should measure open circuit from collector to emitter. So set your multimeter to the highest resistance range (probably 20 MΩ) (or if it's an auto-ranging meter, just set it to the resistance range) and connect the probes between the collector (case) and emitter of a transistor. Try the multimeter both ways round. If the multimeter reading changes from "0L" (or whatever the multimeter shows on resistance range when the probes aren't connected to anything), the transistor is probably faulty. Although a very high resistance may be acceptable for a power transistor.

Don't touch both multimeter probes while you're making this measurement, otherwise the multimeter will measure the resistance of your body. This isn't dangerous at all, but it will affect the measurement. You can touch ONE probe - the one you're holding to the transistor's metal can - but only hold the plastic part of the other probe.

Here is the pinout for this package, which is called a TO-3.



Let me know the results, and also the make and model of your multimeter.

Also, keep track of whether there are any plastic or mica insulating pieces used with the transistors or the bolts that hold them. You will need to get new ones, and fit them the same as the old ones were fitted. Also keep track of where the white heatsink compound was previously spread.

 

Getting a low reading from B to E or C is okay.

Do these readings again with the diode check range on your multimeter. They should read about 0.6V. If you get a reading with the black lead on the B, it is a PNP, with a read lead on the B is an NPN. You should have 2 of each.

Bob

 

The diode range is marked with the red circle, but I'm interested in measuring leakage. Your results in post #24 are good, but it's better if you use the 2000k range, marked with the green circle. Use the black lead in the COM socket and the red lead in the VΩmA socket, as normal.

The meter should read 0L (or something like that). Touch the probes together. It should read 0000 or some low number. If that's OK, you're set.



The MJ15003s are NPNs. For them, put the red probe on the collector (case) and the black probe to the base, and then to the emitter. The meter should continue to read 0L in both cases.

The MJ15004s are PNPs. For them, do the same but with the red and black probes reversed.

There's no need to measure any other combinations of pins.

 

That's right. The MJ15003s are NPNs and the MJ15004s are PNPs. There should be two of each. Each channel has one NPN and one PNP.

 

When you say "no reading", do you mean that the meter shows 0L, just like it does when nothing is connected to the probes? If so, then those transistors are probably OK.

The second transistor, MJ15003, that reads 0000 to the emitter and to the base, is definitely dead.

When you tested the first MJ15004, in post #33, you say the number was around 1200 with some variation. Is it possible you were touching the red probe with your finger? So your body would have been connected to both probes? This will affect the reading. You can have your fingers in contact with one probe, but not both. Try the measurement again.

Next, can you upload some fairly close-up photos of the part of the circuit board close to the heatsinks. Take the photos from directly above the board, in natural light if possible, no flash, no direct sunlight. See https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/

 

Hi again. Sorry for the delay.

Yes, those pictures are very good, thanks.

I'm asking a lot of questions in this post. Can you go through it point by point and respond to all of them please.

When you said in post #18 that you had accidentally connected positive and negative voltages backwards, what exactly did you do? And did you make the same mistake on both channels?

And the four fuses in a row that blew, that you replaced with the fusible resistor things, they're F1, F2, F3 and F4, right?

And one of the output transistors is short, but the other three are OK, right? The meter on the 2000 kΩ range shows "1" (same as when probes aren't touching anything) when you measure from collector to emitter?

Yes, the transistor you linked in post #35 is the MJ15004. I'm not sure whether to suggest that you replace all four of the output transistors, or just the faulty one. The other ones may have been stressed, and that multimeter test is good but doesn't guarantee that they're OK. They're not very expensive. How do you feel about replacing them all?

Can you please unclip the four metal clips that are holding the driver transistors onto the sides of the heatsinks, and tell me the markings on those transistors. There's an error in the schematic and I need to know the actual part numbers.