MTX TA7801 repair

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Perry Babin

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Joined 2003
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What is the DC voltage on the 2 large caps near the op-amps?

Place the black meter lead on the negative speaker terminal when measuring the voltage. Measure the voltage on both side of both caps. Do this with the jumpers in place (no preamp board).

Does it produce the same noise and DC without the jumpers in place (again, without the preamp board)?
 
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intensem1rider

Member
Joined 2007
Ok, I put in a 100 ohm resistor, the one that I removed was now reading 1k.

The new readings on that op-amp are:
Pin 1: 2.05V
Pin 2: 1.01V
Pin 3: 0V
Pin 4: -5.10V
Pin 5: -0.001V
Pin 6: -0.011V
Pin 7: -0.01V
Pin 8: 5.67V

So im about 10V short on pin 4 and pin 5, they should have -15V and +15 volts from what I figure.

There is still 98k hz noise on the output, but now, with the jumpers in place, I can input 50hz and it plays it as well as the noise. But with the crossover board connected, I get +20VDC and 98K hz noise.
 
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Perry Babin

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Joined 2003
bcae1.com
If the voltage across each 100 ohm resistor is 10v, the op-amp is drawing too much current (and is probably getting really hot). The voltage on pins 1 and 2 also seem to indicate that the op-amp is defective. Replace it.

After you get ~0.0v on pins 1 and 7, reinstall the jumpers and confirm that you still have essentially no DC voltage on the speaker outputs. If there's no DC, reconnect a speaker to see if it produces audio. If you still get the noise that you did before, the problem is likely in the area of the amp were we were originally working (2 op-amps and 2 3120s).
 
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Perry Babin

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Joined 2003
bcae1.com
You want the TL072CD. The N version is a DIP.

The X version of the 5532 is in a larger case (SOP) than the SOIC package and probably won't fit on the pads.

You should order plenty of the 100 ohm resistors. They are cheap and you'll eventually need them for another repair.

If you don't have any good desoldering braid, order a small roll.
 
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intensem1rider

Member
Joined 2007
Well I replaced that single op-amp by the inputs and yes the old one was defective, the new numbers are:

Pin 1: 0V
Pin 2: 0V
Pin 3: 0V
Pin 4: -14.79V
Pin 5: 0V
Pin 6: -0.002V
Pin 7: -0.002V
Pin 8: 15.63V

Those numbers were the same with the x-over board, and with the jumpers:

And now to the other op-amps:

With the jumpers:
Servo:
Pin 1: -7.73V
Pin 2: 0.015V
Pin 3: -0.003V
Pin 4: -15.18V
Pin 5: -7.23V
Pin 6: -7.13V
Pin 7: -0.39V
Pin 8: 16.02V

Input with jumpers:
Pin 1: -0.004V
Pin 2: -0.002V
Pin 3: -0.002V
Pin 4: -15.19V
Pin 5: -0.002V
Pin 6: -0.002V
Pin 7: -0.004V
Pin 8: 16.03V

Servo with x-over board
Pin 1: -5.17V
Pin 2: 0.012V
Pin 3: -0.003V
Pin 4: -15.2V
Pin 5: -5.05V
Pin 6: -4.95V
Pin 7: -0.018V
Pin 8: 16.03V

Input with x-over board
Pin 1: -1.67V
Pin 2: -0.86V
Pin 3: -0.85V
Pin 4: -15.21V
Pin 5: 0V
Pin 6: 0V
Pin 7: 0V
Pin 8: 16.03V

There is no longer DC across the 2 100uF caps

I guess I should replace the input and servo op-amps again?
 
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Perry Babin

Member
Joined 2003
bcae1.com
What is the DC voltage across the speaker terminals with the jumpers in place?

The op-amps on the main board seem to be OK. Leave the jumpers in place for now.


With the amp off, measure the resistance between pin1 of the input op-amp and pin 2 of the servo op-amp. Reverse the leads and measure again. They should be the same. What is the resistance?

Measure between pin7 of the input op-amp and pin3 of the servo op-amp as you did for pins1-2. What is the resistance?
 
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