MTX 6500D

[m4ick]

This had a shorted output and all of the R220 resistors were blown in that bank.

R121 and R123 were burned.

I removed the output and the two burned resistors and it powers up and appears to be oscillating.

When I put R121 and R123 back in and powered it up R123 went up in smoke.

Any ideas whats causing that?

[m4ick]

Also, this thing has 42N20Ds in it, I though you had problems with those parts in this amp. It must have been fixed before.

[m4ick]

Please let me know if more information is need for someone to help.

[Perry Babin]

If all of the 0.22 ohm resistors are within tolerance, check Q101 and Q102. Check for open junctions and leakage as well as shorts.

[m4ick]

Being that it has 42n20s in it anyway, should I just remove all of the output fets and then continue troubleshooting, or will it not oscillate with those out of circuit?

[Perry Babin]

Do the solder connections on the outputs appear to be original?

I tried a lot of transistors in these amps. I had the best luck with IRF3415s.

It will not oscillate without the outputs. This is a self oscillating class D amp. Only the 'clocked' class D amps will produce the high frequency oscillation without the outputs in the circuit.

Did you check Q101 and Q102?

[m4ick]

Two solder joints on the outputs look, well, bulbous.

I have fet 103 out of circuit. ALL .22 resistors in that bank are out of circuit.

q101 and q102 are not shorted, do I need to remove them for more definitive results?

[Perry Babin]

With only one FET out of the circuit and everything else in in place with no other defective components, it should oscillate.

If the 0.22 ohm resistors are out of the circuit, it won't oscillate and that could explain why the resistor is burning.

Yes, you should remove them to test them.

[m4ick]

I mean, should I test q101 and q102 out of circuit to see if they are open?

As far as being leaky, should I remove R123 so it doesn't smoke and check voltages on q101 and q102?

[m4ick]

Sorry, I posted the last reply before you posted yours.

Ok, I will replace the 0.22 resistors and power it on. I imagine that if i'm pulling q101 and q102 that I'm checking to see if the are open, or as I call it, "not dioding up".