Help me fix my MTX 1000D, no sound, just static

[hakentt]

Bought this MTX 1000D monoblock from ebay broken. The amp powers on but there is no sound just static noise goes to speakers.
I checked all FETs they are all good, as well nothing looks burned on the board exept one diode that I checked and it is still good the D446 (MUR1540).
But looks like someone was fixing it, installed the capacitor 220uf on the U141 (THAT 2155) and also 50ohm resistor under it.

Does anybody know where I should start?

[Perry Babin]

Does LED238 light up?

Do you have a scope?

If that amp uses SSP45N20As in the audio section, clamp the power transistors down and never apply power without having them clamped down. If the output transistors fail, the amp may be VERY difficult to repair. These amps are VERY sensitive to changes and it's VERY difficult to find replacement FETs. Even if you find some NOS SSP45N20As, they may not work.

[hakentt]

LED does lite up. I don't have a scope. I just was testing it further, all FETs are clamped down, but buzzing noise was there, no sound, then all of a sudden all FETs from 436 to 441 labeled 75339P blew up, lit on fire! One of the ssp45N20a blew up too

Whats going on here

[Perry Babin]

The power supply FETs failed. Did the output FETs fail also (do any read near 0 ohms from leg 2-3)?

Whenever working on an amplifier, you should always have a relatively small (~10-15 amp) fuse in the B+ line. It helps protect the amp from significant damage (generally).

[hakentt]

Originally Posted by Perry Babin

The power supply FETs failed. Did the output FETs fail also (do any read near 0 ohms from leg 2-3)?

Whenever working on an amplifier, you should always have a relatively small (~10-15 amp) fuse in the B+ line. It helps protect the amp from significant damage (generally).

good idea with the fuse thing, next time I will use it. Yes two output ssp45N20a blew up too, as well as all power FETs. Subwoofer blew too, this may be the cause.
But why did I have static noise before things blew up?

[Perry Babin]

I don't think it's going to be practical to repair.

I can't determine the cause of the static with the limited information provided.

[hakentt]

Originally Posted by Perry Babin

I don't think it's going to be practical to repair.

I can't determine the cause of the static with the limited information provided.

I have extra power FETs but for ssp45N20a I have nothing, what would be OK replacement for them, I know you say they are hard to find

[Perry Babin]

These amps can be very difficult to repair when the outputs have to be replaced. Even when I have one on the bench, it can be very time consuming to repair. At one time, I considered making a clocked drive circuit board for them to make them easier to repair but I don't see enough of them to make it worth the time.

[hakentt]

With your help I have fixed the same version of MTX amps and they were totaly toasted too.
I fixed
MTX152
MTX302
MTX500D

So I don't know why are you giving up on me now.

[Perry Babin]

It's nothing personal. These amps can be very difficult to repair when the outputs need to be replaced. The other amps were easy to repair.

I learned the hard way that these aren't generally worth the time. I've tried exact replacements (NOS) as well as other FETs and generally had lots of problems. In the same amps, I tried a set of old, used outputs that were from another amp and the amps worked fine. I went back to the replacement/substitute outputs (including exact replacements) and the amps would not work properly.