MTX Thunder Elite 404

[Dr Zeus]

This amp simply power cycles approximately every 6 seconds. The LED near the RCA labeled 'Shorted' flashes for ~3 seconds every ~6 seconds. When the LED is off, the amp in essence 'charges' itself and produces ~24vDC at the rails, but after ~3 seconds the shorted LED powers up and the rail voltage drops.

None of the power (1F27SP) or output (IRF540) transistors are shorted. I went ahead an pulled all eight IRF540s off the PCB and the power cycling still occurs.

I was able to measure the TL494. These readings were taken in-between the power-cycling while the shorted LED is mainly not lit, unless I used a '~'.
Pin 1: 2.300
Pin 2: 2.2 ~ 4.7
Pin 3: 3.450
Pin 4: 0.865 ~ 4.7
Pin 5: 1.513
Pin 6: 3.178
Pin 7: 0.016
Pin 8: 13.62
Pin 9: ~0 ~ 0.075
Pin 10: ~0 ~ 0.075
Pin 11: 13.62
Pin 12: 13.68
Pin 13: 4.97
Pin 14: 4.97
Pin 15: 4.97
Pin 16: ~0 ~ 0.075

[Perry Babin]

Can you post a photo of the inside of the amp?

[Dr Zeus]

Sure. I just checked the opAmps on the 'top' board and they all get +-19vDC during the 'cycle'. Also the LM339s are getting 13vDC on pin 3



TL494 is most-left:


Half the outputs


I'm calling this the obnoxious LED board. 5 opAmps.

[Perry Babin]

Did anything change when you removed the outputs?

[Dr Zeus]

Did anything change when you removed the outputs?

Nothing changed at all. So far all the transistors I've poked at are measuring up well.

I let it power cycle for ~15 mnutes hoping it would make something get warm enough to start following the trail. The power supply fets get just slightly warm but nothing could get this adventure started. I'm thinking if I ground the TL494 pin 16 then it should stay on? This amp was almost tossed before I got so it could be an experiment.

[Perry Babin]

Can you take some photos with your good camera (Kodak?) of the area around the LM339s? Email them to me at full resolution. Set the camera to the highest resolution possible.

babin_perry@yahoo.com

[Dr Zeus]

I sent you an email with the image at 100% quality. Attached to this thread/post is a version at ~40% of the original's quality.

[Perry Babin]

It's going to take me a while to study this. I likely won't have time until tomorrow.

[Dr Zeus]

Hey I'm back to working on this little MTX amp. I'm kind of lost with it to tell you the truth and had it aside for a little while.

[Perry Babin]

It would help quite a bit if I could read the numbers on the resistors. Try setting the camera to macro mode. You probably won't be able to capture the area all in one photo so take 3-4 photos to cover this area and email them to me.

babin_perry@yahoo.com