MTX 4320-24vdc from speaker out

[DtotheD]

Hey everyone. I recently purchased an MTX 4320 from FeeBay. "Excellent condition" I was told. I installed it, and it powered up just fine. Within a few seconds, it released the magic smoke from my LF speaker.

I tested it, and it's putting out 24VDC out of the LF speaker output. All other channels seem to work, but there was a significant amount of alt. whine present.

I would love to repair this amp myself, but I don't have any experience repairing amps. I guess this will be a good opportunity to learn.

I checked out this thread, and it looks like a good place for me to start. :MTX 4320 wants a second chance

The only thing I noticed visually is 4 white block resistors labeled 102k100 appear to have gotten extremely hot. They're brown and melted at the bottom.

Time to go read up on amplifier repair.

[Perry Babin]

The 102k100 are capacitors, not resistors.

Have you checked the output transistors? It's likely that one is shorted. It's also likely that one of the 0.1 ohm source resistors is open or out of tolerance.

[DtotheD]

Thanks Perry.
Initially, the "102K" made me think they were resistors. After some searching online i realized they were capacitors. I'm hoping these will work as a replacement.

Capacitors - Polyester Film - C001UPF

Fet #10 is shorted (all legs), Fet 11 shows a short between legs 2&3.
Fets #5,6,7,& 8 (near the power supply) appear to be shorted as well.

These FETs were tested still attached to the board. Not sure if that may cause a false reading or not. I guess the next step would be to remove them and test again.

I'm not sure where the source resisters are located on this amp.

[Perry Babin]

Those caps should work.

The 'source' resistors are the large 0.1 ohm resistors near the output transistors. In most amps, the source resistors are connected to the source leg of the FET. In this amp, one is connected to a drain and the other to the source.

The power supply FETs are in two groups of parallel transistors. If one is shorted, it will make others appear shorted but generally, when one power supply FET fails, the rest fail also.

[DtotheD]

I'm compiling my parts list. so far i have:

.001uF Polyester Film caps (x6)
.0047uF caps (x6)
.22uF caps (x8)

IRF3205ZPBF-ND (x4) Power supply FETs
FQP50N06-ND Audio Output FETs

I tested the source resisters, and they appear to be OK. All showed .1 Ohms.

For the output FETs, should I order just 2? I'm wondering if the output between the two channels will be different with mismatched FETs. At 1.25$ a piece, I have no problem ordering 8 to keep it all matched. Overkill?
Thanks again Perry, you've been a great help!

[Perry Babin]

Also order the capacitor connected to pin 5 of the TL494. They sometimes drift in value and cause the capacitors to overheat. On the side of the transformer nearest the rectifiers, there is a capacitor and resistor. Check the resistor to see if it's within tolerance.

The output FETs don't need to be matched.

[DtotheD]

Ok, I'll only order the two FETs then and add that cap (10uF 63v) to the parts list. The resister reads 1 ohm which appears to be right, but it's tough to tell if the first band is red or brown.

it looks like the rectifiers have been swapped before (mismatched). Can i test these in the traditional form? i.e. checking for shorts between the legs?

[Perry Babin]

The cap connected to pin 5 is a small cap, not a 10uf (if that's the cap you're referring to).

Out of the circuit, the rectifiers check like regular diodes.

[DtotheD]

Ok I'll unsolder those rectifiers and test them. I was looking at the incorrect cap when i wrote that. It turns out the cap connected to pin 5 of the TL is labeled 2A682K, which is .0068uF, but I'm unsure of the voltage rating.

[Perry Babin]

The voltage rating isn't important for this application.