punch 40 dsm

[bdgf2004]

i just repaired an old punch 40 dsm. the power supply fets were shorted (BUZ11, Q1, Q2) i replaced them with 2 IRFZ44N's, and 1 gate resistor (R3). 2 caps have vented in the power supply (C9-25v 100 uf, C10-25v 330uf) i replaced those with nichicon parts; and 2 diodes were shorted (CR6, CR7) i replaced those with 2 onsemi MUR440's.
in the audio section only one output fet was shorted (Q106) i replaced it with a 540N, all the driver componets were fine. i also repaired a trace on the board behind the RCA's, it looked like someone scratched it while adjusting the gain. i reset the bias and the amp worked beautifully. i measured 64.9 watts in the okay channel and 68.7 watts in the repaired channel @ 60hz, 2.3 ohm. then when i connected a speaker and played music it was perfect for a few minutes and it turrned off. the amp was not hot in fact it was barely over room temp. it had power and remote voltage (13.8v). i can unhook the remote and reconnect it and it powers on like normal and produces perfect audio for a few minutes and turns off again.

i saw in one thread that perry suggest replacing all the caps on the board behind the RCA's. i have not done that yet but i don't think that's the problem. what should i look at first? it seems like it would be something in the remote circuit but i could be wrong.

[Dr Zeus]

I hear 540N outputs might do that but I'm not really sure why. I'm sure someone can answer that Q for sure though;

Quote from Perry Babin

Very Important!Although the IRF540 and IRF540N have VERY similar specs, the IRF540N may not work properly in these amplifiers. I've seen problems such as oscillation on several amps and one would draw excessive current when driving a load. The current draw was enough to trip the protection circuit.

[bdgf2004]

thanks for the reply. i just rechecked and the part is IRF540.

[Perry Babin]

If you ever have strange problems in a punch amp using the IRF540N, you may want to try a different FET (like the IRF540A). The N version of the IRF540 doesn't always work properly.

The 10uF@16 caps in the DSM amps almost always need to be replaced. On the preamp boards, they cause a lot of damage. If they're not replaced, they'll make the boards virtually unrepairable.

Measure the DC voltage on all pins of the power supply driver board before and after it goes into protect mode. That may help determine what's shutting the amp down.

[bdgf2004]

while examining the power supply driver board i noticed 2 components burnt. the one in the yellow circle is marked A7 the one in the pink circle starts with A but i can't see the rest.

[Perry Babin]

They're both diodes (BAV99L). If the top terminals of each one go to pin 12 of the 494, they are the diodes that supply voltage to the 494.

[bdgf2004]

i got both diodes replaced with BAV99L and so far everything has been perfect for 25min and still running.

THANKS PERRY

[bdgf2004]

is it normal for the LM393 ic located on the power supply driver board to get hot. after about 40 minutes of playing the heat sinks are barely warm and the amp will cut off. if i touch around the top four pins of that ic it will immediatly come back on.

[Perry Babin]

I've never noticed if they get hot but, in general, comparators don't run hot. Are you sure that the heating isn't from the 4 driver transistors adjacent to the 393?

The next time it does it, touch it with a non-conductive probe so you don't touch/push on more than one component.

[bdgf2004]

it's definitely the LM393 getting hot. i used a toothpick to probe around with absolutely no result. as soon as i touch it with my finger it's back on. i even noticed that when i touch some of the through holes on the other (empty) side of the driver board it will come back on but i haven't narrowed it down to the exact ones yet.