MTX TA7801 repair

[Perry Babin]

You can't use a zener that's the same voltage as the regulator. For a 15v reg, you need to use a 18v zener. I'm assuming that the 3w zener is across the output of the regulator.

If the op-amps are running too hot to touch and the voltage from pin 4 to pin 8 (for 8 pin op-amps) is less than ~32v, they are likely defective.

[intensem1rider]

Well I am getting 30.8V on pin 4-8 on both op amps, is that close enough to say they are good? I just replaced them today.

I have done some poking around looking for high voltage drops across resistors and the only one I have found so far that's a big drop is R421 near the 3120's with a 20+ voltage drop, however, the resistor is still good at 2k.

I'm going to keep poking around to see what I can find wrong.

[Perry Babin]

After replacing the op-amps, are they still getting too hot to touch?


I have R421 as a 10k resistor. 20v across a 10k ohm resistor that size is OK.

[intensem1rider]

Yeah, the Servo op amp is too hot to touch, I replaced it again with another one, same thing, still running too hot. I checked R421, it is 10k, I made a mistake, I thought it was a 2k I replaced.

I have the following readings on the opamps with the x-over board disconnected and the jumpers in place.

Servo
1= -7.08
2= -0.001
3= -0.001
4= -15.12
5= -6.92
6= -6.88
7= -0.018
8= +15.90

Input
1= 0
2= 0
3= -0.001
4= -15.12
5= -0.001
6= -0.000
7= -0.001
8= +15.90

The pin 1 output on the input opamp has my mind a little boggled. But on the servo opamp, with the inputs on 2-3 and 5-6 the outputs are the right polarity, but not +-Vcc like I would expect on an out of circuit test.

[Perry Babin]

The op-amp seems to be OK. Do you have any DC on the speaker terminals?

[intensem1rider]

No, theres no significant DC on the outputs, it's in the 10 milli-volts range

[Perry Babin]

You stated that the amp was up and running. Is it producing output? Sometimes, if the amp is muted, that op-amp will run hot. If it's producing audio, it's not being muted.

The voltages you posted are very close to what I'd expect.

[intensem1rider]

When I took those numbers the amp was muted, but when playing audio through it to a 4ohm load the servo op-amp was reaching 160-170 degrees, that concerned me.

Would it be safe to say that I can run the amp through some high output tests? I bought the amp off the customer for relatively cheap and gave him a deal on a new amp so that way I can hopefully get this up and running properly this time and sell it.

[intensem1rider]

What would you consider excessive idle current?

With no input to the amp, but the amp turned on it is pulling 5 Amps. Is that excessive? Or normal for an amp this size?

[Perry Babin]

I would consider that excessive. I would expect 2 amps or less. If you power it up and let it idle, do you find that one particular area of the heatsink gets hot faster than the rest?