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Trying to fix home subwoofer amplifier. Any help is appreciated.

jener8tionx

Apr 6, 2013
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Thanks for the updated images. The reworked area is starting to look a bit better now.

Yes, C135 looks like it needs to be replaced. The joint nearby looks OK now, I think, but I think you should carefully inspect the whole board for dry joints. Run this Google images search to get an idea of what a dry joint looks like: http://www.google.com/search?site=imghp&tbm=isch&q=dry+joints

I have gone over it quite a bit and I think that was the only dry joint.

Yes, that electrolytic is definitely bulging. Is it C113 or C136? It's a non-polarised electrolytic capacitor, rated at 100V. Neither Digikey nor Mouser have any suitable replacement. Actually I don't think the designer should have used that type of capacitor. A polyester or polypropylene capacitor would be a lot more appropriate in that position. See http://www.digikey.com/product-detail/en/QXK2E685KTP/493-3526-ND/2117905. I would also replace the other one.

C113 is the bulging cap according to the label on the PCB, but I see what you're thinking. C 135 and C136 go together. When I fix this spot I will just replace C112, C113, C135, and C136. Again the components are rather inexpensive, but the cost of multiple orders adds up.

That is not the only dodgy component choice in that design. I've noticed a few poor design decisions and I think the designer may have been inexperienced.

This is from a subwoofer that retailed for almost $2000 back in the day!

The failure of that non-polarised electrolytic may be related to the failure of C135.

Thanks for the detailed answers to my questions.

Point 3. "The PCB shorted" is not a meaningful thing to say. I would describe it as a section of track vaporising.

So do we assume that there was some problem internal to the board that caused the track to blow up and damaged D3 and Q8? Is it possible that you made a mistake or something accidentally touched something else, to cause the problem?

I wasn't touching the board when the PCB track vaporized. The problem was a component. I have the transformer leads marked so they were on correctly. Also, it happened a second time (vaporization) as detailed in my first post.

Point 6. Can you measure the resistance of R76 and R78 on the undamaged board. They should measure properly in-circuit, i.e. you don't have to desolder them. Try the multimeter one way then the other and post the two values you get.
On the working board R78 is 788ohms and R76 is 825ohms. I think you are right about heating causing the colors to change on the resistors.

Do you have the original R76 and R78 from the faulty board? Do the colours look similar to the ones on the good board?
The colors look the same on the resistors from the old board as the as the good board.

The colours will have changed due to long-term heating of the resistors. I think the ones in the picture _could_ have been 820 ohms... I think you're right.The closest one, which is the most discoloured, reading from the right end, could be grey, red, brown, gold (the brown stripe could have turned orange) and the back one, reading from the left end, could also be grey, red, brown, gold. Do you agree?

Pont 7. Yes, higher wattage and longer leads will both help. If you use the rectangular type, you could attach them both to a single small heatsink.

OK, well, it sounds as though the project is on hold for a while due to weather.[/QUOTE]

We're supposed to get some warm weather next week, so we'll see.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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OK, thanks for the helpful answers.
If you're ordering and replacing a bunch of parts, you may want to replace the other TL072s that may have been damaged along with U4. U1, U2, U3 and U5 are all connected to the same rails as U4 and could equally have been damaged. U8 and U9 are slightly isolated from those rails by some 10 ohm resistors, but they could have been damaged too. You have to be pretty careful replacing SMT ICs. You might want to buy some desoldering braid to help you remove the old ones.
 

jener8tionx

Apr 6, 2013
14
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Apr 6, 2013
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OK, it's been a while, but I finally got back to this project. Here's my latest update:

I removed C113 and C136 and took pictures:

I ordered NPE replacements, but now that I reread Kris's reply, I will order a couple polyester or polypropylene capacitors instead.

C113 is the one with damage. I did not damage it like that when i removed it.



 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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It's interesting that the one from the other side isn't showing any signs of damage. It could be that the bad one went faulty internally by itself. It could also mean that there's a separate fault in that side. Proper replacement caps will be bigger than the originals but I'm sure you can find room for them.

But I hope you're not just going to replace some more parts then power it up and hope for the best... Various components have failed, and so far the reason(s) for these failures are unknown. You need to diagnose it step-by-step. To start with, you need to leave the output MOSFETs disconnected.

Then, the first step is to make sure that the +/-15V rails are right. You need to check this before you connect them back to the op-amps, otherwise they could be damaged.

Edit: I estimate that the +15V and -15V transistors are intended to supply about 40 mA each maximum. So to test that part of the power supply, you need to break the feeds to the rest of the circuit, measure the voltages with no load, and measure the voltages with a 1W, 360 ohm resistor from each rail to 0V.

When it comes to testing the two sides of the bridged amplifier, it's a bit tricky because they won't work properly without feedback, so the output stages need to be enabled somehow. I think the best approach would be to insert series resistors in the +V and -V rails to the output stages, so that if something goes wrong, nothing gets blown up (again). I hope you're prepared to spend significant time on this amp. I doubt it will just be a matter of finding all the damaged components and replacing them.
 
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jener8tionx

Apr 6, 2013
14
Joined
Apr 6, 2013
Messages
14
It's interesting that the one from the other side isn't showing any signs of damage. It could be that the bad one went faulty internally by itself. It could also mean that there's a separate fault in that side. Proper replacement caps will be bigger than the originals but I'm sure you can find room for them.

But I hope you're not just going to replace some more parts then power it up and hope for the best... Various components have failed, and so far the reason(s) for these failures are unknown. You need to diagnose it step-by-step. To start with, you need to leave the output MOSFETs disconnected.

Then, the first step is to make sure that the +/-15V rails are right. You need to check this before you connect them back to the op-amps, otherwise they could be damaged.

Edit: I estimate that the +15V and -15V transistors are intended to supply about 40 mA each maximum. So to test that part of the power supply, you need to break the feeds to the rest of the circuit, measure the voltages with no load, and measure the voltages with a 1W, 360 ohm resistor from each rail to 0V.

When it comes to testing the two sides of the bridged amplifier, it's a bit tricky because they won't work properly without feedback, so the output stages need to be enabled somehow. I think the best approach would be to insert series resistors in the +V and -V rails to the output stages, so that if something goes wrong, nothing gets blown up (again). I hope you're prepared to spend significant time on this amp. I doubt it will just be a matter of finding all the damaged components and replacing them.

I'm sure that you are right. Nevertheless, my stubbornness will force me to try swapping components first. My plan it to replace the capacitors, then power up without the output transistors and check the 15 volt rails. If they are still off, I will try to replace other components. I still feel like there must be a component that I can replace to get this thing back on track.

Also, I'm not sure if I have the skill to properly diagnose this the way you are suggesting. I have no idea how the digital components work in this thing and I don't have the right tools to properly diagnose the digital components.

Thank you a ton for all the help, and I'm sorry that I don't think I can do the things you are suggesting. I will keep trying and I will update my progress - or lack thereof.
 
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