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.
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.