Sony XM-SD51X

[RiLoWa]

This amplifier appears to use the MTX style of power supply. With the rectifiers removed here is what I measure

IC 6 = 494

1: 5.4mV
2: 4.79V
3: 62.8mV
4: 4.96V
5: 1.677V
6: 3.511V
7: .4mV
8: 14.35V
9: 24.7mV
10: 19mV
11: 14.35V
12: 14.35V
13: 4.99V
14: 4.99V
15: 4.99V
16: 3.1mV

IC7 = LM339

1: .628V
2: .628V
3: 14.35V
4: 4.99V
5: 8.62V
6: 4.03V
7: 4.95V
8: 3.395V
9: .434V
10: .600V
11: 3.395V
12: 2.6mV
13: 4.99V
14: 78.1mV

The voltages on pins 4-7 of the LM339 are close to the expected ranges according to the schematic, but the output is low. Pin 10 appears to be driven by Pins 1 & 2.

I lifted the collector of Q44, which is connected to Pin 4 of the 494 and the power supply operates however it draws ~ 6A and the clamped down power supply FETs begin to get very warm after a few minutes of operation. I measure +/- 17V on the op amps and measure ~ +/- 57VAC on the rectifier pads.

Is the LM339 defective or do I still need to find something else?

 

[Perry Babin]

With the black probe on the emitter and the red probe on the base, what is the DC voltage on Q16?

 

[RiLoWa]

~70mV

 

[Perry Babin]

Did the amp produce clean audio when you had Q44 out of the circuit?

If so, reconnect Q44 and lift Q16. Does the amp power up?

 

[RiLoWa]

No, with Q44 lifted, there is no audio reaching the output terminal.

 

[Perry Babin]

Try lifting Q16 (you can leave Q44 out). If that doesn't allow the amp to power up, lift legs 1 and 2 of the LM339 to see if the amp will power up.