monitor active sub- instantly blows output transistors

[onejay09]

hi im working on a project i bought from ebay,
its a monitor audio vector vw8 active subwoofer, its 100watts i think its a push pull ab amp
i replaced the 50v 4700uf caps two 25v 220uf caps, the bridge rectifier, and the two 223j poly caps on the rectifier circuit, when i replaced the tip 35c and tip 36c output transistors and powered it up it instantly dead shorted the tip transistors and blows the fuse, i gave it to a audio repair shop and they replaced the driver transistors and again the new output transistors shorted again, im at my witts end with this amp and i really need some advice.
ill upload some pictures and a schematic which i just recently got hold of directly from monitor audio tech support

the board runs without the output transistors in...
the transformer is center tapped earth at 66-33-66v ac, ac to dc would put dc at 90v so how come the caps are 50v
also the volume potentiometer tested weird at 0 resistance up or down would this blow the outputs?

please help!

pictures here

[budm]

Re: monitor active sub- instantly blows output transistors

You are dealing with power amp section that uses direct couple circuits, ALL the transistors and diodes need to be carefully checked to short, open, and leakage resistance. The out transistor drive has high gain Darlington (Q509/511, 508/510)set up which will drive the output transistors very hard if there is any resistance leakage or shorts. The transistors and Diodes should be checked off the board. Check carefully on the active BIAS transistor Q507, if it is open or not active, that can cause the BIAS current to go way up.
You should also use 75W lamp in series with the Line side of the power cord so you will not keep blowing up the new parts, the lamp will light up brightly and stay on bright if there is short circuits.

[onejay09]

Re: monitor active sub- instantly blows output transistors

seeing as i dont have a variac the bulb sounds like a great idea, hope it saves me from buying even more parts, ive read a few threads on here and you seem to be very helpfull everytime someone needs help thanks budm, im ordering more parts today, ill get back to you asap!

[onejay09]

Re: monitor active sub- instantly blows output transistors

could it be that the transformer wind is shorted? and if any of the op amps were shorted would they cause the same issue?

[tom66]

Re: monitor active sub- instantly blows output transistors

If the transformer were shorted, then removing the transistors wouldn't fix the issue.

[onejay09]

Re: monitor active sub- instantly blows output transistors

right ok, also just wondering should there be any voltage reading from speaker out?

[rievax_60]

Re: monitor active sub- instantly blows output transistors

What voltages do you measure at R519 and R520 now?

[budm]

Re: monitor active sub- instantly blows output transistors

It should be 0V, but the speaker relay will not come on if the the speaker protection relay circuit detects for than 1.5~2v at the amplifier output. See circuit on page 2 bottom right of the page.

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Assuming no faults, I would expect to see a bit over +0.5v and -0.5v.
Because the feedback loop is still closed via R516 and R517, I would expect the amplifier to be balanced at zero volts ok.
Im am looking for a possible problem with the Vbe multiplier transistor, Q507.

[onejay09]

Re: monitor active sub- instantly blows output transistors

well i pick up the subwoofer tomorrow from the repair shop as they failed to repair it, but as i remember reading positive and negative speaker output it was either 3 or 30v dc without the output transistors in

[onejay09]

Re: monitor active sub- instantly blows output transistors

ill test again when i get back, just it to be sure

[onejay09]

Re: monitor active sub- instantly blows output transistors

just to be clear, am i testing from ground to r519 etc or across the resistor, still learning...

[rievax_60]

Re: monitor active sub- instantly blows output transistors

just to be clear, am i testing from ground to r519 etc or across the resistor, still learning...

measure from ground to either end of the resistors. First check that the resistors are ok.

[onejay09]

Re: monitor active sub- instantly blows output transistors

im going to be doing lots of testing tonight at work, and ive got no internet connection there, so i will post the results tomorrow morning, im going to save this page offline tonight before i leave for work

so ill check
q507 = mpsa13
q508 = 2sc3228
q509 = 2sa1275
q510 = 2sd667
q511 = 2sb647

r519 r520 are these wirewound cement resistors? how would these fail, short open of closed?


what would cause the tips to instantly short?, too much bias? what else should i check if any.
would the volume a50k dual pot cause this failure if it is shorted?


i appreciate your guidance and help so much, id be lost without you all!

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Resistors usually fail open.
If a fault causes both output transistors to turn on hard at the same time, they will fail, usually short.
This can be caused by a shorted or leaky driver transistor. Or the Vbe Multiplier transistor not doing its job properly. It is supposed to control the voltage difference between the bases of the driver transistors at some precise voltage, about 2v, so that the output transistors pass a few milliamps of idle current.
The worst possible reason is oscillation of the amp. It can be very difficult to deal with. We will forget about this possibility.
I cant see how the volume pot could be involved at this stage.

Dont fit the output transistors for now.

[onejay09]

Re: monitor active sub- instantly blows output transistors



Q510 q511 both look like this, I'll post my readings when I get up. All main resistors test ok, going to go over them all again tonight with a list of the expected values for a more precise outcome, ive bought a d667 and a b647 and a c3228 and also an a1275 wait for delivery...., and another pair of output transistors, gunna power the board today and take some voltage readings too. do you think these 2 transistors have failed?

[rievax_60]

Re: monitor active sub- instantly blows output transistors

The picture will not load for me.

[onejay09]

Re: monitor active sub- instantly blows output transistors

strange, can you see this time?

[rievax_60]

Re: monitor active sub- instantly blows output transistors

I think I see a piece blown out of the epoxy, so its likely to be blown open circuit.
The green between the legs looks like corrosion caused by a liquid spill.
Are there other signs of liquid spill?

[onejay09]

Re: monitor active sub- instantly blows output transistors

no its just Q510 and Q511, i cleaned this one up and the corrosion stripped it back to copper, maybe their toast, anyway ordered some new ones off ebay, how do i go about using a 75watt bulb in series with the power lead?