Subwoofer amplifier MIK

[Nite]

Hi there,
I have an amp that is built into a sobwoofer case that I haven't been able to fix. Maybe you guys can help me on you on this one? I have fixed a few amps before, but this particular amp is giving me a hard time. When I received it, both power supply mosfets (IRFZ44N) and four output transistors with one preamplifier transistor were shorted. So, I replaced them, but were unable to find the exact match for the preamplifier transistor B631K, for wich I used BD140 as a temporary replacement.

The amp powers up and everything seems to be fine: I get +-35V on the rectifiers and the led is green. However, as soon as I connect it to the speaker, the output transistors and preamps begin to get warm gradually to the point where they are so hot that I have to disconnect the power. I'm using a power supply with a weak current output of about 2A, so I have time to measure voltages before things get critical. I have checked other transistors in the audio section, but couldn't find any more shorts. What do you think? Thanks in advance for you time. Please see the attached pictures for reference.

 

[djQUAN]

is it attached to a heatsink during testing?

 

[Nite]

is it attached to a heatsink during testing?

No, it's sitting on my table without heatsink. The limited current output from the power supply doesn't let it get hot quickly. But maybe I should afterall attach the heatsink in order to protect the transistors I installed.

 

[Perry Babin]

What is the DC voltage across the wires that go to the speaker with the amp on but with no input signal?

Have you checked the other driver for leakage (would have to be pulled out of the board)?

 

[Nite]

Hi Perry,
DC voltage across the wires is 0.39 V. I checked the other driver for shorts in circuit. Do you think I should remove it?

 

[Perry Babin]

A leaky driver could cause problems and you can't check it for leakage in the circuit.

Do you have a scope?

Do the output transistors heat up if the amp is on with no speakers connected?

 

[Nite]

The amp doesn't heat up when the speaker is not connected. The trouble start as soon as I connect it. Unfortunatelly I don't have a scope, so I can't diagnose the amp as thouroughly as I would like to.

 

[Perry Babin]

Do all of the outputs heat up or do only 2 (of the same part number) heat up?

Check the driver.

 

[Nite]

I removed the driver that I didn't replace before, but it isn't shorted when checked out of circuit. I turned the amp on briefly with the speaker connected, and the outputs are still heating up. Two A1694 transistors dissipate most heat.

 

[Perry Babin]

Is it leaking (not shorted but conducting in a way that it should not)?

The heating is because those transistors are passing the DC to the speaker.

 

[Nite]

The amp is still heating up with the leftmost driver (the original part that I didn't replace before) out of the circuit, so I guess the fault must be somewhere else? How do I check transistors for leaking? Also, the trasistors at the bottom Q22 (TIP42C) and Q23 (TIP24C) get warm as well, what purpose do they serve in the amp? Could it be that the diodes next to the drivers are faulty? Thanks for all the help.

 

[Nite]

The leftmost driver kd600K doesn't appear to be leaking. B->E is 0.6V, the other way around is OL, B-> C is 0.6 and the other way around is OL as well.

 

[Perry Babin]

When you check for leakage, you also need to check with the meter set to ohms/resistance. The diode-check function isn't sensitive enough to see minor leakage.

 

[Nite]

OK, B->E reads 23 M ohms(red probe on Base and black on Emitter), the other way around is OL. B->C reads 22 M ohms(red probe on Base and black on Emitter), the other way around is OL. Do you suggest I should check other transistors in the audio section the same way?

 

[Perry Babin]

That transistor appears to be OK. Since no schematic is available and there aren't many transistors in the audio circuit, checking each transistor may be the best way to proceed. Confirm that each transistor has two good junctions and that there is no leakage.

 

[Nite]

That's fine, I will remove the transistors one by one for checking. Do you have any good tips to get that yellow glue off? It really slows down the whole process.

 

[Perry Babin]

I'm not sure for that particular fixative but for many fixatives, heating with a heatgun will soften it and allow you to peel it off of the board.

 

[Nite]

I replaced 2sa1023 and 2sc1027 with new ones and the DC voltage on the speaker connections dropped from 0.39V to 0.1V, so this means that at least one transistor is still passin DC? I have removed and checked most of them and they seem fine, but maybe I missed something. I'll test the rest of them tomorrow, but if this won't lead me anywhere, is it okay to replace 2sa1266 with 2sa1015 and 2sc3198 with 2sc1815 in the audio section. I have another amp for parts.

 

[Perry Babin]

Those should be good subs but only replace parts that you know to be defective.

The offset could mean that you have a leaky transistor but it can also be caused by mismatched transistors in the differential amplifier stage of the power amplifier.

 

[Nite]

After a long break from this amp I'm back. It appears to me that there is something wrong with TIP41C and TIP42C (located at the lower section of the pcb in the first picture). TIP41C has the voltages> B: 15.35V, C: 26V, E: 14.74V. TIP42C has the voltages> B: -16.5, C: -29.5V, E:-15,9V. The NPN transistor must have a negative emitter, right? How come all pins of either transistor have the same polarity. I removed them and got the same reading on the pads. Both transistors get quite hot as well. Do you have any ideas what I should check, rectifiers, maybe?