Fried board??? photos enclosed

[Garydean]

This is a Bose powered sub woofer from 1992. I took some readings at the output section, a few transistors measured 38v some had no readings at all. I couldn't get through the whole lot of 18 output transistors when I heard it click into overload. At this time I've got 14v +- on some of the output transistors. The substance around some of the components look like dried globs of pudding. Could this thing have been zapped by a surge like lightening?


Thanks,
Gary

BTW no sound is coming from any of the satellite speakers that are connected to this unit as well.
When I took the back off the sub, someone had undone all he molex connections. A previous tech may have pronounced it DOA.

[defaced]

That brown stuff around the caps is glue. Sorry I can't be of more help regarding the amp - I don't know much about SS stuff.

[Enzo]

WHat he said - glue.

SO what is the thing doing? No sound just to the satellites? Or no sound period?

Those xstrs on the heat sinks are not all output transistors. The TO126s are either drivers or bias components.

Did you contact Bose to ask if there are schematics available? DOn't just look on their web site to see if they are online. DO that, but when they are not there, directly ask them. Worst thing can happen is they say no, so nothing to lose.

[Garydean]

Thanks for the reply. I'm getting no sound at all thru anything. I turn the amp on and hear the relay click in, but no sound thru any speaker, not even a hiss. I get sound thru the headphones so I'm guessing the preamp is good. After 5 minutes of having the amp on I hear another click and the voltage readings of the outputs drop from 38 to 14v. The preamp, CD changer, is connected via cable to the sub bass.

thanks,
Gary

[jrfrond]

Once that glue, which has an organic base turns brown, it carbonizes and becomes conductive, essentially forming a crude resistor, and can upset dc and ac balance in circuits where it is surrounding both leads of a component.

Before you do ANYTHING, use a small screwdriver, awl or some other sharp instrument to scrape this stuff off and then retest. It can and does totally disable gear when present.

[Enzo]

John, a lot of that stuff is brown coming out of the gate. Glue that TURNS brown from heat or whatever has indeed become conductive, but brown glue is probably OK. Stick two ohmeter leads into it and check if in doubt. I see brown glue all the time.

[jrfrond]

John, a lot of that stuff is brown coming out of the gate. Glue that TURNS brown from heat or whatever has indeed become conductive, but brown glue is probably OK. Stick two ohmeter leads into it and check if in doubt. I see brown glue all the time.

If you look carefully around the filter caps, you can see that there are tan (light brown?) areas of glue that have not yet carbonized. The rest I see all is a darker solid brown and sports a chalky surface appearance, indicating that it may have indeed carbonized. In any case, even if only SOME of it has carbonized, it represents an area that needs to be tackled before component troubleshooting. Remember the old SPX-90 power supplies with this issue? Most times, removing the glue repaired the unit.

[Garydean]

Certainly interesting. I stripped off all the brown glue and still I'm getting nothing. Should I be getting something even if some of the outputs are bad. BTW it a Bose acoustimass powered speaker system from 1992. After the amp (sub base) is on for about 3 minutes I hear a click and the voltages drop in half (thermal?), but still nothing out of the speakers before or after the second click. Could I inject a signal from a generator at the point where the audio input plug connects?.... I'm not sure what pin that would be tho, to see if perhaps there may be an issue with the connecting cable or it's sockets.

thank you
Gary

[Enzo]

When you called Bose to ask for the schematic, how did that turn out?


John, I have one of those SPX-90 supplies across the room from me. I wrote off glue problems as the material being hygroscopic - like the old Fender conductive eyelet board. I don't know where the high temps would come from to carbonize the glue in these cases here.

[jrfrond]

The interesting thing about this muck that they use over in some Asian factories is that it seems to carbonize on it's own with age and doesn't require heat as a catalyst. I first saw this problem in Mitsubishi VCR's back in the early 80's. They used the stuff to tack down wires on the solder side of the PCB's, and all kinds of crap happened! Like I said: why? I don't know. It very well could be a hygroscopic issue. I just know that it does happen and it does exist, and unless I am totally off-base, that is what I am seeing in these Bose pics. To put it another way: if that were on my bench right now, that would be the first thing I would do. Could be a very easy fix.

[metalangst]

You could inject signal at various places, put a coupling cap on the sig gen lead in case you touch voltage. If one output is bad it may be dragging the system down. disconnect all speakers, try one speaker at a time. Also, use an ohmmeter to check all the ss components.