MTX 6500D

[mike49504]

Amp powers up and idles perfectly..

On the power supply fets 1 get

Leg 1:0
Leg 2:13.8
Leg 3:0

I get all different readings on every op-amp


Amp produces no audio..


Any ideas?

[Perry Babin]

That's normal for this amp.

Do you have rail to rail oscillation on the outputs?

Do you have rail voltage?

Is the gain set to the maximum position?

Have you operated all switches and pots through their entire range with speakers connected to see if the amp produces any sound (noise or audio)?

What op-amp readings vary (AC, DC, include pin numbers)?

[mike49504]

I have both + - 59 volts of rail voltage..

I get oscillation from rail to rail ..


Gain is set to maxium posistion..

If i mess with the gain pot i can get the spear to thump back and fourth..

Im gonna try switching the pot out to see if that fixes the problem if not i will post back with voltages on op-amps

[Perry Babin]

Don't try to swap the pot. If you're using a used pot, it's likely going to be damaged when you remove it from the board.

What specifically is wrong with the readings from the op-amps?

[mike49504]

First op amp

Pin 1:17.16
Pin 2:15.14
Pin 3:15.79
Pin 4:1.44
Pin 5:11.46
Pin 6:10.61
Pin 7:17.26
Pin 8:18.04

2nd op amp

Pin 1:16.53
Pin 2:16.53
Pin 3:16.57
Pin 4:1.43
Pin 5:16.58
Pin 6:16.50
Pin 7:16.50
Pin 8:18.10

3rd op amp

Pin 1:16.59
Pin 2:1.18
Pin 3:0.57
Pin 4:1.43
Pin 5:16.59
Pin 6:15.96
Pin 7:17.24
Pin 8:18.12

4th op-amp

Pin 1:2.67
Pin 2:2.67
Pin 3:2.04
Pin 4:1.42
Pin 5:2.04
Pin 6:2.66
Pin 7:2.66
Pin 8:18.13

5th op amp

Pin 1:17.46
Pin 2:1.44
Pin 3:0.77
Pin 4:1.42
Pin 5:1.44
Pin 6:2.19
Pin 7:2.66
Pin 8:18.14

6th op amp

Pin 1:-1.82
Pin 2:0
Pin 3:0
Pin 4:-18.22
Pin 5:0
Pin 6:0
Pin 7:0
Pin 8: 17.44

[Perry Babin]

You're missing the negative 15v on most of the op-amps.

Find the PZTA56 near the op-amps that has -rail on the collector. What is the voltage on the base and emitter?

There may be two PZTAs that have -rail on the collector so don't stop looking for the PZTA56 after you find one.

[mike49504]

Base:-59.6

Emitter:1.30

The other 1 has the following voltages..
Base:0.5
Collector:1.3
Emitter:1.3

[Perry Babin]

It could be an A92. Possibly next to D118.

[mike49504]

The A92 has

base:-17.95 volts

Emitter:-17.92 volts

[Perry Babin]

That one is OK. Which A56 did you find earlier with -rail on it?

If you can't see the circuit board designation, what are the designations for the components connected to it?