Mosfet Short, capacitor problem?

[jakelm1975]

Hi all,

Repairing a BK AV5000,

Found on amp board (5th channel) all 4 Mosfets shorted.

Found 2 other issued.

100uf (100v) cap had very high resistance. (compared to all other caps as reference)

Emitter (farthest resister from fuse).47ohm 5w resistor had no current pass threw at all.

That particular channel was continuing to blow both 4amp slow-blow fuses.

Ordered all new caps for board.

Any thoughts? Something else I should look into?

 

[JonSnell Electronic]

Schematic would help but capacitors should have no resistance.

 

[Alan0354]

One possibility if the cap is for bypassing the voltage rail for those very transistors. When the cap opened and you loss the AV ground to the drain, the MOSFET might start to oscillate at high frequency. It is very common for MOSFET to blow with everything shorted when oscillate. Actually I have a lot of experience when I was working on switching supplies and power control circuits. They die quietly, never get hot, just turn on and just die......stone cold. Normally when a MOSFET blown, you have burn and smoke. If the board looks clean, no burn marks or signs of overheat, you might want to consider that.

 

[djk]

Check the output zobel as well, if the either the cap or the series resistor go open the amp will oscillate and burn up.

 

[jakelm1975]

Take a look at the specs and schematics. Is replacing the caps a good start?

I did take some time to check about 99% of the resistors. Pulling one side of the resistor out of the the board, most were very close to spec, and others were almost dead on. The only thing I am unable to text is the diodes and caps. So I just decided to replace them.

But all 4 Mosfets will need to be replaced. All have shorted out.

 

[jakelm1975]

Also, this might sound like a silly question. But how can all boards have the same parts and same specs and fed by same size main caps, yet be rated at different wattage?

Channels 1-2-3, are rated at 100w @ 8ohm
While 4-5, are rated at 60w @ 8ohm
Yet all parts are the same

 

[jakelm1975]

schematic for av5000 series 2, not the series 1 as i have but specs are pretty close if not exact

 

[jakelm1975]

OK update, replaced the 4 mosfets. And the 100uF caps, checked resistors for value.

Now the channel is not blowing fuses, BUT

63dcV speaker output!! With no input and just power on. With extreme heat from that channel. And bias pots and dc off set pots not working

 

[jakelm1975]

Green = changed
Red = Checked
And all diodes ok.

 

[sgrossklass]

With some a severe fault I would expect C10 to have been damaged, and maybe Q9 as well. (Replace Q8/9 in pairs.)