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!!!!
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!!!!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?
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?
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:
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:
It appears that you still have DC on the output. Did you have a speaker or dummy load connected?
I stated that the LM311 was on the negative rail. That's not correct. It's riding the carrier. You still have to be careful when probing the pins on it. The voltage on it is swinging ~150v.
It doesn't appear that the LM311 is driving the protection circuit on. Something between it and Q274 may be causing the problem. For now, let's concentrate on the regulated power supply voltage.
With chassis ground as the reference, measure the DC voltage on the anode (non-striped end) of rectifier D472.
Also measure the voltage across the inductor next to it (marked 221, most likely). Post the voltage.
Do the same for the inductor next to D474.
If the voltage is different across the diodes. turn the amp off and measure the resistance across the inductors. They should be essentially the same (~10 ohms).
I stated that the LM311 was on the negative rail. That's not correct. It's riding the carrier. You still have to be careful when probing the pins on it. The voltage on it is swinging ~150v.
It doesn't appear that the LM311 is driving the protection circuit on. Something between it and Q274 may be causing the problem. For now, let's concentrate on the regulated power supply voltage.
With chassis ground as the reference, measure the DC voltage on the anode (non-striped end) of rectifier D472.
Also measure the voltage across the inductor next to it (marked 221, most likely). Post the voltage.
Do the same for the inductor next to D474.
If the voltage is different across the diodes. turn the amp off and measure the resistance across the inductors. They should be essentially the same (~10 ohms).
Perry Babin said:
No, no load connected
it's -18.2V on anode of D472 and 17.9 on cathode of D474
It's 0.3V for the first one and 0.6V for the second
They were 9.1 and 8.5 ohms. I replaced them both with inductors from parts amp (it had both inductors at 10 ohms)
after that voltages:
D472 anode: -18.4V
D474 cathode: 18.1V
voltages across them stays the same: 0.3 and 0.6 V
Thank you!
Something's wrong. If you draw a line between the inductors and the closest rectifiers, the line will pass through 2 capacitors. The outside terminals of the capacitors are connected together and connected to the chassis ground. Confirm this on your amp.
No matter whether the woofer is connected or not, there should never be any difference in the voltage from the common points on those capacitors and the ground (always 0v). Confirm this on your amp.
No matter whether the woofer is connected or not, there should never be any difference in the voltage from the common points on those capacitors and the ground (always 0v). Confirm this on your amp.
Outside terminals of these caps connected together and to ground.
Voltage on inner terminals behaves somehow strangely:
On cap, which is closer to RCA it is different for connected/disconnected load.
with load: -16.6V, without: -17.9
Cap, which is farther from RCA's has a constant voltage on it's inner terminal: +17.3
Voltage on inner terminals behaves somehow strangely:
On cap, which is closer to RCA it is different for connected/disconnected load.
with load: -16.6V, without: -17.9
Cap, which is farther from RCA's has a constant voltage on it's inner terminal: +17.3
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