MTX Thunder 81000D troubles

[mbates14]

Got an 81000D that worked, but supposidly got extremely hot and distorted audio.


I found R327 blown open, and the 42N20D FET associated with it was shorted.

Also the Zener diode D326 was bad. H2 marking. guess a 12v zener?

Replaced all 3 parts and powered the amp up. With a load on the speaker, i can hear clean sound but R327 starts smoking again.

No load, the FET and resistor get really hot really fast.

Quick voltage check i have roughly 65v on the drain, and -70v on the source.

Something seems wrong. any ideas? im lost.

[mbates14]

anyone?

[Perry Babin]

Assuming that R327 is the large 100 ohm resistor and the FET is soldered to the board...

This is to drain the filter capacitors when the amp is switched off.

If you can't determine what's driving it on (or what's not driving it off) while the amp is on, let me know and I'll try to trace it down on a junker I have here.

[mbates14]

Would a rail imbalance cause this?

Because he said if he turns the amp up its really distorted. and the amp starts to heat up really fast.

Its funny though i found that 100ohm resistor melted and blown apart. along with the FET being shorted.

As soon as i remove the remote wire, i see smoke commin from the 100ohm resistor for a quick second.

and when the amp is on, the FET gets blistering hot. theres about 2.3V sitting on the gate when the amp is on, measured from the speaker terminal ground.


If i connect a speaker load, and turn the amp up a little bit. The 100 ohm resistor smoke starts barreling out of it.

it wont smoke if the amp is unloaded.

I also measured the output of the amplifier via the scope, and the audio is passing through unclipped, but it seems to have a "zig-zaggity" noise within it. almost like a triangle wave modulated inside the 100hz sine wave

[mbates14]

P.S. of course the heating up and distortion he mentioned was before i got it. and the fet was shorted, so it was possible it was pulling the rails imbalanced.

I might just remove the fet i put back in it and see how it does.

But if you can, back-trace that gate circuit please. because maybe when the FET shorted, it screwed something up there. There is a zener diode and a resistor right there in the gate circuit, and the resistor was fine. the zener diode i wasnt getting any continuity in either direction on the meter. so i swapped it out with a new one. 12v zener.

[mbates14]

I got to thinking also, if he ran it with the FET shorted for awhile, when obviously it blew out the 100ohm resistor, it also means roughly 70v was sitting on the gate circuit for awhile. so besides the zener, whatever in that gate circuit probably died.

[mbates14]

When amp is on, Gate voltage climbs to about -70v steady.

Source climbs to -77v steady.

When remote wire is removed, gate climbs rapidly to about 2.3 to 3v positive.

[Perry Babin]

I haven't had a chance to dig the board out of the stack of amps. It may be tonight before I get a chance.

I don't know if you can rely on voltages referenced to ground for this circuit. I think (could be wrong) that it simply floats between the rails. If you want to see the gate voltage, you'll have to place the black probe on the source leg and the red probe on the gate leg.

[mbates14]

strange... I put my probes between G and S and soon as i see the red LED kick up in the diag circuit, it drops to 0v across.

soon as i unplug remote/power it spikes to about 8v. and gets hot. but thats normal when off...

Strange, it was smoking while on. but soon as i touched the fet leads with probes. now it all seems to be fine. VERY strange... maybe loose connection. might have to go through and double check and resolder everything.

[Perry Babin]

There's probably an open resistor that's supposed to be holding the FET off. Look at the resistors in the area and near the LM339 with a lighted magnifying glass to see if you can find one that's burned.