Mtx 421d Repair

[91h22a]

Hello again! I have a Mtx 421d that powers up and seemed to operate normally at idle.

When testing the amp using a dummy load and a 60hz 4Vpp sine wave I see the power led begin to flicker. This also happens when the amp is connected to a headunit.

When I scope the power supply fets (75339's) I begin to see oscillation in the waveforms when gain is anywhere above min and oscillations increase as gain increases. Also on pin 8 of the 494 I see what appears to be a normal waveform (per my bcae repair dvd ) but again when the gain is set at a higher than min level... oscillation occurs. I have checked my shield grounds and they are proper.

Any input on this problem would be appreciated.

Thanks in advance

 

[Perry Babin]

What are you using to power the amp?

 

[91h22a]

I have an AC to DC 12-15V 50A regulated power supply.


Pyramid PS52KX Power Supply 12-15 VDC 40A

 

[Perry Babin]

Can you post a photo of the 'oscillation in the waveforms'?

 

[91h22a]

It will take me a few minutes...

 

[91h22a]

I currently cannot post photos...I have in the past but nothing I click on when posting seems to work except typing...strange

I will try adding the photos to my album here on the forum

 

[91h22a]

Probe on source of 75339p Gain at Minimum





probe on source of 75339 gain midway output not clipped. Hard to see difference here




Probe on 494 pin 8 gain min



Probe on 494 pin 8 gain midway, amp output not clipped

 

[Perry Babin]

That's not out of the ordinary for MTX amps.

 

[91h22a]

I was playing with this amp further and as I increase gain to about 3/4 the led flickers rapidly and I hear a ticking...similar to an arcing sound. I have wiggled the transformers but I cannot seem to locate the sound. May be normal.

This is what the output looks like



Also when all of this occurs the ammeter on my power supply surges from 20A to 40+. I also had D3 near U4 (input opamp) smoke. Maybe I am getting closer to the issue.