Capacitor and fuse blown in audio amplifier (integrated in active speaker)

[Flisken]

This active speaker was completely silent (but power light lit up) so i opened it up and a fuse (to the left in pic) was blown. When I replaced it, sparks appeared around the fuse and small pops came from the speaker (should have been a warning to me maybe). Anyways... when i turned the power back on, a capacitor (pic center) literally exploded.

I guess replacing the cap isn't going to help.. Can anyone point me in some direction as to where to begin. As you can probably tell I'm not an engineer and don't know what I'm doing

ThNks in advance

 

[Harald Kapp]

Welcome to the forum.

There are lots of possible causes. It is not unlikely that the capacitor is the culprit. Sometimes electrolsytic capacitors develop a short circuit.
To exclude other possibilities: remove the defect capacitor. Measure the resistance between the two holes where the capacitor was placed. It the resistance is near 0 Ohm, chances are that another componet has developed a short circuit.
If the resistance is noticeably larger tahn 0 Ohm, remove the Ohmmeter. Insert an ammeter between the contacts of the fuse. Turn on the power and wath the current. Is it on the range expected from the ratings of the amplifier (at least much lower than the fuse rating)? If so, replace the defect cap by a new on, insert a fuse of the correct rating and chances are you will be fine.
If the current is way too large, we will have to continue searching for the defect.

 

[Flisken]

Thank you for helping! I was hoping to be be able to fix this myself because the speaker isn't really worth paying a tech to fix.

Resistance across the place where the cap was is in Megaohms so that seems like a lot more than zero... Current across the fuse on the hand is, as you put it, "way to large". It never actually got above 2.5A (the fuse rating) because another fuse blew, but there where blue sparks and huge signal out from the amp. I guess this means short circuit somewhere... Damnit, was hoping i could just replace the cap.

Thanks again! Your post was very informative and I really ahould learn more about this stuff

 

[Harald Kapp]

You can try to trace from the fuse versus ground with an Ohmmeter to locate the short.
Also inspect the PCB visually for any signs of burn or obvious shorts.
Possible culprits are the transistors in the output stage or any other (large) capacitors.
Remove the speakers before you perform furher tests.

Are you sure the polarity of your supply voltage is correct? A wrong polarity can lead to vey high currents, too.

 

[Flisken]

Yeah, I agree I should have removed the speaker but the terminals are soldered and I was hoping I wouldn't have to redo that. Being lazy doesn't seem to be saving me any work . :/

I have to unscrew the PCB from the panel to inspect it properly so I'll do that tomorrow (almost night over here). I haven't noticed anything obviously wrong so far, though.

The polarity "should" be right. I'll double check it tomorrow, though.

Thanks again! most helpfull

 

[KrisBlueNZ]

It would be helpful to know the brand and model number, so we can try to find a service manual, or at least a schematic diagram.

 

[Flisken]

Yes, it's a fraser wyatt p1000 and the amp/input module is called p1000-ehd. I haven't been able to find any schematics but maybe you know better where to look

 

[KrisBlueNZ]

Hmm, you're right. I can't find much at all on it.

How old is it?

Can you upload some more photos? Start with a view of the complete unit, from above and below, then closeups of both sides of the board.

 

[Flisken]

I honestly don't know their age but they look pretty beat up so would guess at least 5 years.

Here are some more pics.

Complete amp unit.

 

[Flisken]

Main board top side. The blown fuse and capacitor was in the bottom left. The broken cap was exactly where the shadow of the large purple cap is.

 

[Flisken]

Under side of mainboard. This was hard because of attached wires.. Sorry

 

[Flisken]

Input module. I think this is fine.

 

[Flisken]

Little boards attached to the chassiunder the main board. No idea what this is, only resistors on them and they're screwed to the panel.. Unimplemented panel features maybe?

 

[KrisBlueNZ]

Those four little boards screwed to the metal are the output transistors. Can you try to get pictures of them viewed from above? Or at least read the markings on them using a mirror?

Also can you post either a picture, or the markings, on the 8-pin IC near the front of the main board, about 1/3 of the way across.

 

[Flisken]

The 8 pin IC says TLO72OP, M in a circle, and T8707 on separate rows... I have to do a bit of unscrewing to read the transistors... Didn't even see them before... Brb

 

[KrisBlueNZ]

OK, TL072 is a common dual op-amp. 8707 is the date code - 1987, 7th week. So the amplifier is probably around 25 years old.

Do you have a multimeter? While you have the transistors uncovered, you can see whether they're short-circuit. Most likely at least one of them will be. Measure resistance between the collector (the middle pin) and the emitter, which is the right hand pin if you're looking at the part number marking, with the tab up and the leads pointing down. A shorted transistor will measure a few ohms, or less.

 

[Flisken]

There are four pairs of output transistors. They are all labeled PH, BDT 62C or 63C, and m8723 or m8727.

 

[Flisken]

Woah thanks.. Is it expected that both in a pair of transistors are shorted? The top right pair both seem to short between collector and emitter. Is this likely to be the cause of my troubles?

 

[Flisken]

One more.. In the bottom left pair is shorted

 

[Flisken]

Seems I can buy these transistors for cheap. Any specific brands/countries of origin i should look for or avoid?