monitor active sub- instantly blows output transistors

[Manuel Castellano]

Re: monitor active sub- instantly blows output transistors

Hello everybody.
I moved my Home Teather system with the VW8 included. (6 speakers). I installed everything in the new house and turn off everything. When I came backe the subwoofer never turned on again. No any red led on. I opened it an checked that the power is not getting out the transforemer. Could anybody helpe me with this?. Does anybody (onejay09) have the transformers spec to replace it?. Any advice?. Tks a lot in advanded. Regards.

[onejay09]

Re: monitor active sub- instantly blows output transistors

Yes it seems to be working perfectly :-) playing on high volume doesn't heat it up too much either, I'm quite surprised that it doesnt get hot to touch

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Happy to hear that.

The idle current is calculated from the voltage measured across the two 0.22 ohm resistors in series.
There is no need to risk taking this measurement if the heatsink feels only warm after the amp has idled for 15 minutes or so. And the audio sounds ok at very low volume, no cossover distortion.

[onejay09]

Re: monitor active sub- instantly blows output transistors

Hey guys, well thanks to all of your advice I have a working active subwoofer again, I replaced q507 and q508 through q511, checked for the strange fluctuating voltages I had on r519 r520 they were completely gone, so I built a light bulb in series with the ac cord and risked my new output transistors, soldered them in, closed my eyes fingers crossed and flipped the switch, fuse didn't blow, the bulbs flashed then went out, yess! Left it for 5 mins, I touched the heatsink it was warm! So i put it back together and tested it carefully, F YEA! works like new, thanks folks much appreciated!

I'm from UK, I'm sure I could use mouser or digikey but maybe not the fastest option, I'd try element14 think they've got a UK based sector, I agree that eBay is full of fakes and not ideal :-)

It must have been q508 2sd667 and q511 2sb647 the corroded pins were covered in that black rubbery glue, all over the wirewound resistors where the bad trans were, the heat must've been too much, I couldn't see the corrosion untill I got rid of the glue and removed the trans.

Yea I couldn't identify the transformer except the output voltage, even the schematic didn't say what the values were

Well this time I bought the output transistors as a matched pair, so it's working well

I also spent the night recapping the rest of the board, the small electrolytics all except 2

Anyway thanks guys happy with the result

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Ahh, I should've mentioned something about that black (rubberized?) stuff they've got on the board. Sometimes, solder joints can crack, yet the component won't wiggle readily, since it's being held by the glue.

It's probably there to keep vibrations from loosening the components. I think that may already have happened, though.

http://postimg.org/gallery/91qn1hge/05afb988/

Onejay, where are you located? How about Mouser or Digikey for parts? Especially semis, as Rod explains here:
http://sound.westhost.com/counterfeit.htm

Ok- it seems like that's another one of those "Federal" transformers. Whoever makes these amps has their TXs labeled as such. If it's anything like a similar amp I rebuilt a few years ago, I would definately consider resoldering everything. As well as paralleling output devices- there's room for them after all...

That is right. I had forgotten about the black glue. Its just as damaging as the yellow/brown stuff. I still see it in recently made products, mainly on outsourced power and BL inverter PCBs.

[rievax_60]

Re: monitor active sub- instantly blows output transistors

lol sorry, from now on ill reference to ground, my new parts came in today so gunna replace q508 q509 q510 q511, ill leave tip 35 36 out for now, so im thinking that the bias current or voltage is too high, how do i measure for that?

The voltages at R519 and R520 needs to be very stable and precise but I cant say exactly what.
R519 should be about +0.5v, R520 should be about -0.5v. Its actually the difference voltage, about 1v that is important. It is supposed to be just enough to cause the output transistors to be turned on by a very small amount so they pass a few milliamps of idle current.
Confirm that R529 and R520 test ok.
Voltage measurements taken from a good channel are useful when in doubt.

[kaboom]

Re: monitor active sub- instantly blows output transistors

Is there any sign of yellow/brown glue used on the PCBs? It causes corrosion.

Ahh, I should've mentioned something about that black (rubberized?) stuff they've got on the board. Sometimes, solder joints can crack, yet the component won't wiggle readily, since it's being held by the glue.

It's probably there to keep vibrations from loosening the components. I think that may already have happened, though.

http://postimg.org/gallery/91qn1hge/05afb988/

Onejay, where are you located? How about Mouser or Digikey for parts? Especially semis, as Rod explains here:
http://sound.westhost.com/counterfeit.htm

Ok- it seems like that's another one of those "Federal" transformers. Whoever makes these amps has their TXs labeled as such. If it's anything like a similar amp I rebuilt a few years ago, I would definately consider resoldering everything. As well as paralleling output devices- there's room for them after all...

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Is there any sign of yellow/brown glue used on the PCBs? It causes corrosion.

[kaboom]

Re: monitor active sub- instantly blows output transistors

Use the center point of the two large electros in the power supply for your common reference. If the board is removed from the chassis, any bonding link is broken. Therefore, hooking the negative meter lead to the chassis will give meaningless results.

With the two finals out, what's the voltage from the output bus, before the spkr relay, to common? Test point is the junction of R516 and R517.

Normally, bias current is measured as a voltage across the emitter resistors of the finals, then calculated by Ohm's law.

Ex- 0.02V / .22ohms = ~91mA.

Don't worry about this now- there are other major problems for now...

Ok so weird results measuring across r519 and r520

R519 ranging from 0.6to 1.7 volts fluctuating up and down
R520 ranging from 0volts to 0.5

Unstable bias, and possibly offset. Those output devices are being "told" to conduct when they're not supposed to.

if I measure from soldered side I dont get a reading!

Flux or dirt on the board?

Make sure all resistors and solder joints for the dual-differential (Q501-Q504) are good. Heck, redo them anyway.

You've got funny readings on both sides (R519 and R520) so the dual-diff is my first suspect.

Also, check the two 47 ohm (fusible?) resistors, R512 and R515, for cracked solder.

If the dominant-pole caps, C505 and 507, intermittently break down, the whole amp will become upset.

[onejay09]

Re: monitor active sub- instantly blows output transistors

lol sorry, from now on ill reference to ground, my new parts came in today so gunna replace q508 q509 q510 q511, ill leave tip 35 36 out for now, so im thinking that the bias current or voltage is too high, how do i measure for that?

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Ok so weird results measuring across r519 and r520

R519 ranging from 0.6to 1.7 volts fluctuating up and down
R520 ranging from 0volts to 0.5, if I measure from soldered side I dont get a reading!

Oh forgot to say that I put q510 q511 back in to test before and after I replace them, now last time I replaced the output transistors I couldn't find tip36cw or tip35c and used tip35c tip36c without the w. What's the difference?
R519 r520 was measured to negative at the rectifier

Its all getting a bit hard for me to follow now, too many variables.
Even if Q10 and Q11 diode test ok, I wouldnt trust them unless any possible surface tracking is cleaned from between the legs. I use a small hacksaw blade.
Have you tested R519 and R520?
voltage measurements are usually taken with respect the 0v/ground/chassis unless there is a reason to use another reference.
I cant see a difference yet between the TIP36C and TIP36CW.

[onejay09]

Re: monitor active sub- instantly blows output transistors

Ok so weird results measuring across r519 and r520

R519 ranging from 0.6to 1.7 volts fluctuating up and down
R520 ranging from 0volts to 0.5, if I measure from soldered side I dont get a reading!

[onejay09]

Re: monitor active sub- instantly blows output transistors

Oh forgot to say that I put q510 q511 back in to test before and after I replace them, now last time I replaced the output transistors I couldn't find tip36cw or tip35c and used tip35c tip36c without the w. What's the difference?

R519 r520 was measured to negative at the rectifier

[rievax_60]

Re: monitor active sub- instantly blows output transistors

Those transistor readings look ok.
With Q10 and Q11 removed, I should have asked you to take voltage measurements at R516 and R517.
There should have been no voltage readings at R519 and R520 with the output transistor removed and Q10 and Q11 removed.

[onejay09]

Re: monitor active sub- instantly blows output transistors

voltage at r519 is 44.2v voltage at r520 is 45.2v

diode mode
q507 vbe mpsa13
1+/2- ol
1+/3- ol
2+/3- 771
3+/2- ol
3+/1- ol
2+/1- 1.737

q508 c3328
1+/2- .650
1+/3- .653
2+/3- .ol
3+/2- .615
3+/1- .ol
2+/1- .ol

q509 a1275
1+/2- .ol
1+/3- .ol
2+/3- .703
3+/2- .ol
3+/1- .635
2+/1- .641

anything strange?

[onejay09]

Re: monitor active sub- instantly blows output transistors

voltage at r519 is 44.2v voltage at r520 is 45.2v

diode mode
q507 vbe mpsa13
1+/2- ol
1+/3- ol
2+/3- 771
3+/2- ol
3+/1- ol
2+/1- 1.737

q508 c3328
1+/2- .650
1+/3- .653
2+/3- .ol
3+/2- .615
3+/1- .ol
2+/1- .ol

q509 a1275
1+/2- .ol
1+/3- .ol
2+/3- .703
3+/2- .ol
3+/1- .635
2+/1- .641

anything strange?

[onejay09]

Re: monitor active sub- instantly blows output transistors

Ok is it safe to test withlit the board connected to the plate with the transformer ground wise or should I run a wire to ground the board?

[rievax_60]

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?

A 75w bulb in series with the mains is not certain to save the output transistors if a problem is missed.
I think its best to do some tests before the output transistors are fitted like I mentioned before.
You can still do the tests if Q508 and Q509 are ok.

[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?

[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?