Please help: trying to make one working MTX 1501d from 2 broken

[valterdaw]

I am merging two broken MTX 1501D amps, one of them is badly burned; and I almost succeeded - amp started playing, everything seemed to be normal. I left the amp on my table for 7 hours, not powered, and now it is not working
Power supply works fine (+/-72 V), secondary supply (?) works fine (+/- 17 V). Huge power LED463 lit, LED399 lit, but the darn LED238 close to driver IC's is off (it was on 7 hours ago). One of the A3120 is getting hot. No sound... Seems to be in mute mode or something... I really really don't understand this amp.
It so frustrating to work on this model without schematic Please help!!!!

[valterdaw]

I think the reason LED238 is off, that I have 3.2V on speaker terminals...

[Perry Babin]

If it has 3.2v on the speaker terminals with a speaker connected, you probably have shorted or leaking output transistors. It's also possible that the 3120 failed.

Do you have ~18v from pin 5 to pin 8 on both 3120s?

Is the voltage from pin 2-3 on the 3102 ~0v?

Is the voltage from pin 1-3 on ALL of the outputs ~0v?

[valterdaw]

Originally posted by Perry Babin
If it has 3.2v on the speaker terminals with a speaker connected, you probably have shorted or leaking output transistors. It's also possible that the 3120 failed.

I just replaced two A3120's and they are both running cool now. So the hot one was probably defective

Do you have ~18v from pin 5 to pin 8 on both 3120s?

19V

Is the voltage from pin 2-3 on the 3102 ~0v?

0.4V

Is the voltage from pin 1-3 on ALL of the outputs ~0v?

it's 0


I am sorry, I measured voltage on speaker terminals without speaker. With sub connected it's 0.007V. LED238 is still off

Thank you!!!

[Perry Babin]

Post the DC voltages on the pins of the LM339.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:

[valterdaw]

Originally posted by Perry Babin
Post the DC voltages on the pins of the LM339.

Pin 1: 4.6V
Pin 2: 4.6V
Pin 3: 13.3V
Pin 4: 4.9V
Pin 5: 8.7V
Pin 6: 4.2V
Pin 7: 4.9V
Pin 8: 4.6V
Pin 9: 3.4V
Pin 10: 0.8V
Pin 11: 3.4V
Pin 12: 0V
Pin 13: 4.6V
Pin 14: 0V

[Perry Babin]

Those look OK.

Measure the DC voltage on the collector of Q274 (between the 3120s).

Also measure the DC voltage on pins 4 and 8 of the NE5532 near LED238.

[valterdaw]

Originally posted by Perry Babin
Those look OK.

Measure the DC voltage on the collector of Q274 (between the 3120s).

Also measure the DC voltage on pins 4 and 8 of the NE5532 near LED238.

Collector of Q274 : -12V
NE5532 pin 4: -14V
NE5532 pin 8: 17V

Thank you!

[Perry Babin]

You may have 2 problems. The collector should be at the same voltage as the negative 17v. The negative 17v isn't at -17v.

VERY carefully measure the DC voltage on the 8 pin IC (LM311) near the outputs. Don't touch the metal on the probes and don't let the probes short to anything in the area. The LM311 is sitting on the negative rail. Do this with no signal and no speaker connected.

Confirm that the 3rd leg of FET256/257 is at negative rail. If so, place your black meter lead there when measuring the voltage on the LM311. Post the voltage on all 8 pins.

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

[valterdaw]

Originally posted by Perry Babin
You may have 2 problems. The collector should be at the same voltage as the negative 17v. The negative 17v isn't at -17v.

VERY carefully measure the DC voltage on the 8 pin IC (LM311) near the outputs. Don't touch the metal on the probes and don't let the probes short to anything in the area. The LM311 is sitting on the negative rail. Do this with no signal and no speaker connected.

Slowly rising to 6.1V (in 20 sec after turn-on) and then rising even slower....

Confirm that the 3rd leg of FET256/257 is at negative rail. If so, place your black meter lead there when measuring the voltage on the LM311. Post the voltage on all 8 pins.

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

FET 256/257 third leg at -77.6V

Relative to it, LM 311 pins read

Pin 1: 74.6V
Pin 2: 74.6V
Pin 3: 74.8V
Pin 4:74.6V
Pin 5: 83.9V
Pin 6: 83.9V
Pin 7: 74.7V
Pin 8: 84.0V

Thank you for your help!