Autotek MX 1300 2 channel amp

[barber02]

So this amp came to me with the complaint of low output and overheating. I found 4 swollen rail caps so I replaced them. I have checked the drives and followed the audio signal through the amp with my scope all the way to the speaker terminals and I can’t find any issues.

The power supply produces +57.2 vdc and -56.7 vdc. The 10 rail caps are rated at 63wv.

I checked the regulation on the power supply and I can see changes in the duty cycle with amp under load so I can assume the power regulation is working as it should.

What else should I check?

The amp is stone cold. Granted I haven’t driven it hard.

 

[twilliar]

I look for variations or anomalies in the P-P of the sign wave for 2 channel amps to locate any variations in the output assuming the output transistors are up to par.

 

[Perry Babin]

Is the audio actually low?

In some of the amps like this, the power supply inductors (near the caps that failed) intermittently shorted. Sometimes they ran so hot that they would partially desolder themselves.

When testing, as it heats up, confirm that the bias is holding.

 

[barber02]

Perry, It doesn’t sound low to me. I’ll check the inductor.

Twillar, I didn’t observe any anomalies.

Thank you for both of your inputs.

David

 

[Perry Babin]

The amp may have been driving a defective speaker or bad/shorted wiring that was causing both low output and overheating.

 

[Achim1409]

I had it on one of my amps, that a resistor in the pre amp circuit was damaged. Directly on the op amp for the input. So i had only 20% of Output.. It sounds normaly to me, but it was realy to low. I injected a 200Hz signal with a Generator. Measured the output and was able to compare. I needed 8V of Input to reach max output with gain full in.