APEX JR. subwoofer amp blowing fuses, bad caps found

[timsch]

Hello all,

I've got an APEX JR. subwoofer amp that was blowing fuses after several years of not being used. I opened up the amp and found black goo on the board around the 4700 uF capacitors. The goo looked like melted plastic skin from the caps. Both caps look to be damaged in the same way, with melting on the pin end. No bulging that I can see.

Other than replacing the caps, what else might be bad in this situation? I have good soldering skills, but not much experience troubleshooting stuff like this.

 

[kellys_eye]

Could be..... but large electrolytic capacitors are often 'glued' to the circuit board to give more mechanical rigidity and that black goo could just be the 'glue' they used.

Need a wider shot of the whole arrangement and/or the schematic to make a more specific decision. Those caps are also shown as rated at 85degC which, if the power supply is SWITCHED-MODE, could well mean they are dud as switched-mode supplies most properly used capacitors rated at 105degC. I can't tell from the close-up shot whether you have a switched-mode or linear PSU system.

 

[timsch]

Could be..... but large electrolytic capacitors are often 'glued' to the circuit board to give more mechanical rigidity and that black goo could just be the 'glue' they used.

Need a wider shot of the whole arrangement and/or the schematic to make a more specific decision. Those caps are also shown as rated at 85degC which, if the power supply is SWITCHED-MODE, could well mean they are dud as switched-mode supplies most properly used capacitors rated at 105degC. I can't tell from the close-up shot whether you have a switched-mode or linear PSU system.

Thanks for the reply.

If it were glue, I'd expect to see a more uniform oozing around the cap, and not so far away from the cap. At the bottom of the caps above the goo (although it's not gooey, it's dried), there are bare spots of what look to be aluminum, which makes me think this area had the skin melted off.

Here's a link to the product with a schematic link

Here's a wider view.

 

[kellys_eye]

I missed the missing plastic! Remove the capacitors and power it up again. It'll hum like hell but if the fuse doesn't blow then you've found the fault.

 

[timsch]

It'll work without the caps? What's their purpose?

 

[Martaine2005]

It'll work without the caps? What's their purpose?

They are filter caps. The purpose of removing them is to see if one is shorted. If fuse doesn’t blow after removal, then they are the problem. Replace with same values but 105 degC not 85 degC.
Don’t continue using it without replacing them. For test purposes it’s fine.

Martin

 

[Bluejets]

Black stuff is glue.
Cannot see any "aluminium bare spots"...........

With power off and disconnected, best discharge the caps with a suitable resistor, then remove and test caps with a cap tester/esr meter.
Running the unit without these caps in place could well do more damage than you already have.
One instance where a high wattage series lamp comes into play for short testing.

 

[kellys_eye]

Black stuff is glue.

That's what I said.

Cannot see any "aluminium bare spots"...........

Some plastic missing at the bottom in one pic - could be as a result of 'scratching' rather than melting though. Dunno at this stage.

I'd point out to the OP that the usual cause is the final stage output transistor(s) - one probably gone short. Quick check with test meter (collector to emitter) will show any issues.

 

[kellys_eye]

Replace with same values but 105 degC not 85 degC.

Shouldn't be necessary in a non-SMPS situation although the small additional cost isn't worth arguing over.

 

[Bluejets]

Some plastic missing at the bottom in one pic - could be as a result of 'scratching' rather than melting though. Dunno at this stage.

Ah, yes, see it now....looks like cockroaches have been at it.

 

[timsch]

Got the caps off with just a soldering iron. No glue holding them down that I could see. They measured ~4500 uF with my fluke meter.
The fuse blew immediately again when I powered the amp up.

The black stuff on the board does flake off when I scrape it with a screwdriver. It is also conductive, measuring ~2-3 MOhm whenever I put my probes on a continuous section of the black stuff. Assuming it is melted cap "skin", it looks like it could have flowed under the speaker output pins (2-pin connector near the top cap socket in the pic below).

...


I'd point out to the OP that the usual cause is the final stage output transistor(s) - one probably gone short. Quick check with test meter (collector to emitter) will show any issues.

Can you give me a pointer on how to test the transistor and where it's located?

 

[kellys_eye]

I can't read the schematic too clearly but I think we're looking at Q105 and Q106 as potential problems. Use your meter diode function to check between all three pins of each transistor and report the results.

 

[Bluejets]

It's glue.
Chances are the semi's will need to be removed to test successfully also.
You will need to read the marking to determine just what type of component it is and find the specs on Google search.
What does the bridge test..???

 

[timsch]

Unfortunately, I've not been able to find Q105 or Q106 on the board itself. There are a number of transistors on the board that are the same type/shape as Q102 in the middle of the image below. I saw this morning a component, also 3-pin, below & to the left of Q102, that look like it may also be bad. This is on the board that does not hold Q105/6, so not the output stage, if I guess right.



A problem I'm having is that a number of these transistors board markings are covered up with the white adhesive goop they put everywhere. So I'm a little stumped with even this 1st step for troubleshooting.

It's glue.
Chances are the semi's will need to be removed to test successfully also.
You will need to read the marking to determine just what type of component it is and find the specs on Google search.
What does the bridge test..???

By semi's I guess you mean all the IC's? I don't know how to identify the bridge or how to test it.

 

[kellys_eye]

Follow the blue-brown-blue wires from the transformer to the circuit board connector. Right beside that connector is a black device with four pins. This is the bridge rectifier. Do a diode test between each of the 4 wires on that device and post the results.

The main output transistors are bolted to the larger aluminium heatsink just in front of the transformer. These transistors 'may' have insulating washers under them so remember to replace them if you remove them (probably not as they are plastic bodied devices) but these are the transistors that need testing too.

 

[timsch]

Do these require removal to test properly? I just got measurements from the soldering-side of the board for these components, but then reread Bluejets post about removal.

 

[Bluejets]

As in #13...test the bridge rectifier.
Plenty of info on "how to do" on the web without having to replicate it here.

 

[timsch]

Follow the blue-brown-blue wires from the transformer to the circuit board connector. Right beside that connector is a black device with four pins. This is the bridge rectifier. Do a diode test between each of the 4 wires on that device and post the results.

The main output transistors are bolted to the larger aluminium heatsink just in front of the transformer. These transistors 'may' have insulating washers under them so remember to replace them if you remove them (probably not as they are plastic bodied devices) but these are the transistors that need testing too.

The info I found online regarding in-circuit testing of ICs indicated "may" have to remove, like in post #13, so these tests are done in-circuit using Fluke 177.

Transistor Toshiba C5198/A1941
V_BC = 0.580 / 0.525 V
V_CE = (0.585/0.838) / (0.533/0.874) V (values differed when switching multimeter probes)
V_BE = 0.996 / 1.001 V

Bridge RS402L
Pin 1-2 = 0.536 V
Pin 1-3 = 0.544 V
Pin 1-4 = 0.982 V
Pin 2-3 = OL
Pin 2-4 = 0.544 V
Pin 3-4 = 0.529 V

The transistors both measure within spec according to the datasheet. The bridge I'll need y'all's expertice on.

 

[Bluejets]

Please post what data sheets you have there so we don't have to go searching.
Show what meter range you measured the semiconductors on.

 

[kellys_eye]

Bridge is ok. Transistor seems ok too (the schematic link you give shows different designations than on your actual board i.e. schematic shows Q105/Q106 but the board is marked Q409 and .... dunno, can't see!)