MTX Audio 600XD

[Dr Zeus]

I've got this amp on the bench with blown power fets which doesnt seem to want to power up. I removed all fets to measure signal on the gate pads, but I'm not measuring anything at all before or after the 100 ohm gate resistors.

The power supply fets are 1H12SL (Fairchild) and measure:
Leg 1: 0
Leg 2: 13.7
Leg 3: 0

TL494C:
Pin 1: 0
Pin 2: 4.76
Pin 3: 0.064
Pin 4: 0.828
Pin 5: 1.482
Pin 6: 3.341
Pin 7: 0.004
Pin 8: 12.94
Pin 9: 0
Pin 10: 0
pin 11: 12.94
pin 12: 12.42
Pin 13: 4.95
Pin 14: 4.95
Pin 15: 4.95
pin 16: 0

This amp also suffered from a small explosion due to this coil closest the spearker terminal outs having what looks like a weak solder joint on the bottom of the PCB. Nothing in this area measuring incorrectly with the DVM at this point.

 

[joesch1999]

you need to go from pin 11 and twelve they power the fets and figure out what is bad.

 

[Perry Babin]

If this power supply drives its FETs with a transformer, you won't see the same voltages as a normal amp.

Do you see pulses (any kind of pulses) on pins 8 and 11 or is it straight DC?

 

[Dr Zeus]

I do see +20v waveform. 10 volts/div at 5uS: There is a similar waveform at the fet gate resistors.

 

[Dr Zeus]

Interesting thing is, I wasnt getting anything at all at the gates before just now. Let me try putting this amp back together.

 

[Dr Zeus]

What kind of ps fets can I use in this amp It came with Fairchild 1H12SL and I think legs 2&3 are stuck and shorting. What is 1H12SL? When I do a google search I get a link to this thread!

 

[Dr Zeus]

OH Im such a dummy. i guess thats a 75339P. A 55v 75A transistor. I guess I've got IRFZ48N and IRF3205 fets I could use.

 

[Perry Babin]

I've never had to repair one of these but I've used the 3205s in other MTX amps.

 

[Dr Zeus]

Cool. I replaced the surface mount gate resistors with new 68ohm ones and put some IRF3205s in place. The amp powers, but the PS fets get hot after a short time. The amp produced +55 rail voltage. The waveform on the PS Fets is kind of not good at 10v/Div:

 

[Perry Babin]

Does this amp have PZTA56s driving a transformer?

If so, have you checked them?

Are the 10 ohm resistors between the base and emitter within tolerance?

 

[Dr Zeus]

I just had the PZTA56s off the PCB and they check out OK. The 10 ohm resistors between base and emitter are also good. With the fets installed, the degraded signal doesnt appear until after the small transformer next to the A56s.

 

[Perry Babin]

That drive signal isn't going to be like in other amps. It really doesn't look that bad for an MTX amp using this type of drive circuit.

How much current was it drawing when the FETs were heating up?

Did you replace both gate resistors coming off of the small transformer?

Do they also have individual gate resistors for each of the power supply FETs?

 

[Dr Zeus]

With 4 fets, this amp only has 2 gate resistors. The originals were 100 ohm which I replaced with 68ohm smd. I got the amp to power up, but the ps fets get hot after just a few seconds. For temporary purposes, I placed one SamWin sw50n06 fet into each bank.

I removed the rectifiers (REC102 FEP16DTA) and the amp powers up still with the fets getting hot.

I did not replace the 10 ohm resistors near the PZTA56s since they are measuring just fine.

 

[Perry Babin]

Look at the waveform on the source leg of the power supply FET and VERY carefully note where the top half deflects to on the top of the waveform. Does the gate waveform (on the gate leg) go about 10v above that point?