MTX 280, no output, TAKE 2

[ECM]

This is the second 280 with no output. PS works fine, opamps getting power, no shorted outputs. This amp turns on, just no output.

LM339 reads as follows:
1- 3.7
2- 5
3-11.2
4- 0.6
5- 3.3
6- 0.8
7- 3.3
8- 3.6
9- 5
10- 5
11- 7
12- 0
13- 3.7
14- 3.7

[Perry Babin]

1, 13 and 14 should be closer to 5v (~4.8 or so). It appears that they've gone high enough to turn off the transistor that drives pin 4 of the TL494 high (this allowed the supply to produce voltage).

You may have a defective capacitor connected to pin 4 of the comparator. Even though 1, 13 and 14 aren't at nearly 5v, pin 4 should be higher than 0.6v. It has to be higher than pin 5 for the muting to be released.

[ECM]

I replaced the capacitor tied to pin 4. Pin 4 still sitting at 0.6V. A second defective comparator maybe?

Since pin 4 is lower than pin 5, we have 5V on the output of pin 2. If I cut pin 2, should that cause the amp to un-mute since it's no longer high?

EDIT: I measured pins 1,13, and 14. 4.8V out. This goes thru a 100Kohm resistor to pin 4. At pin 4, I read 0.6V. The resistor measured 97.9Kohms. Any reason the voltage should drop that much? Maybe the input to pin 4 is open.

[ECM]

Pins 1,13, and 14 connect to pin 4 thru a 100k resistor. A capacitor resides between pin 4 and ground (pin12). The capacitor was replaced, no change. I next removed the capacitor and pin 4 went high, pin 2 went low (I did not check output). I installed ANOTHER capacitor and pin 4 went low again, pin 2 high.

What's going on here?

[Perry Babin]

Measure the resistance from the solder pad of the cap to pin 14. That way it will show if you have a bad connection on the 100k.

If that's OK, you may want to try a large film capacitor. The electrolytic you used may have had too much leakage.

You can't simply cut pin 2 loose. It's likely needed to drive a transistor's base low.

[ECM]

Checked continuity from pin 4 to pin 1 thru resistor, 100K. Checked from capacitor pad to pin 1, 100K.

Installed second new capacitor from different vendor, no dice. I'm reading 3.8V on pins 1,13,14 and all the way up to the resistor. After the resistor, it's 0.6V. The resistor seems to be dropping 3.2V.

If there is any current flowing into pin4, that would cause a voltage drop correct?

I have an extra LM339. I may just install it and see what happens.

ECM

[Perry Babin]

Earlier, you stated that the voltage on pin 4 went high without the capacitor. Does the amp have output without the capacitor?

[ECM]

With the capacitor removed, pin 4 went high, pin2 went low. Still no output.

This is with new LM339 installed.

[Perry Babin]

I'm not familiar with this amp but generally, in the MTX full range amps, the muting drive transistor (driven by pin 2 in this case) drives 5v to the constant current source for the differential amplifier in the audio amp. With pin 2 low, the PNP transistor should have 5v on the emitter and ~5v on the collector. The transistor is likely near the 339 but could be anywhere in the amp.

Is there a PNP transistor near the 339 that has 5v on the emitter (there may be 2 or 3) that is driven by pin 2 (via a resistor) and switches 5v to its collector when pin 2 goes low?

[ECM]

The output from pin2 does indeed go to two MPSA56 transistors.

The collector of one and the base of the other.

I'm beginning to think my power supply has something to do with the erratic behavior of the amps. I have one on order and will hold off until it arrives.