Hi, I'm a retired Honda auto tech and I have helped many people in the past, hoping karma comes through for me. I have a pretty good understanding of what different components do, but the interactions are beyond my experience.
This is a KLH ASW10-120 subwoofer that I have had for many years. I hadn't noticed it working and the power led was out so I dove in. I found the 2A fuse was open, not violently blown. I replaced it and it powered up for a few seconds and then failed again. I took the amp apart and gave it a good look.
A long time ago, it had a hum from the speaker and I replaced the 2 large caps in the power supply that were bulged and all seemed fine. The replacements look good still and test very close to their spec (6800uF 50V) when off of the board. Through tireless searching online I found a schematic that looks correct (mostly) but it calls for 4700uF 50V. I think the electronics store didn't have the ones I needed so I substituted. Should the higher uF be OK for this? I am guessing yes.
I say the schematic mostly seems correct because it calls out the exact pcb number and transformer number I have but diodes D5 through D10 don't exist on my board as far as I can tell.
The transformer is always powered. With the leads unsoldered, it reads 72V open circuit and the fuse is not blowing.
Looking at the pair of 2 watt resistors R62 and R63, and I think the color bands have been baked to uselessness. The schematic calls for 680 ohms, one reads 797 and the other is 1.16k. I will replace both. It's odd that the bands don't match each other and they don't look like they could have ever been the colors for 680 (blue, gray, brown?)
The pair of 0.22 ohm 5 watt resistors R50 and R51 each test very close to spec, showing 0.3 ohms on my 40 ohm scale. R51 has the color bands slightly darker from heat.
I removed the 4 power transistors and they test ok using the diode test on my DVM but I know that's not a real-world test. My best clue at this point is that one of the small power transistors Q5 got burning hot in the short time before the fuse failed while its counterpart Q4 did not. Also, you can see in the photo that the little heat sink for Q5 has blackened the glue on the pcb where it contacts it.
This is where my abilities end. I need someone with knowledge to tell me the likely reason(s) for Q5 heating up. I'm guessing just replacing it is not the answer. If anyone feels any of the power transistors should be replaced, the part numbers are kind of outdated and I would welcome suggestions of suitable replacements. They are (Q4) D1563A, (Q5) B1086A, (Q6) A1516, and (Q7) C3907.
Thanks in advance for any help!
Rob
Here is the power supply PCB:
This is a KLH ASW10-120 subwoofer that I have had for many years. I hadn't noticed it working and the power led was out so I dove in. I found the 2A fuse was open, not violently blown. I replaced it and it powered up for a few seconds and then failed again. I took the amp apart and gave it a good look.
A long time ago, it had a hum from the speaker and I replaced the 2 large caps in the power supply that were bulged and all seemed fine. The replacements look good still and test very close to their spec (6800uF 50V) when off of the board. Through tireless searching online I found a schematic that looks correct (mostly) but it calls for 4700uF 50V. I think the electronics store didn't have the ones I needed so I substituted. Should the higher uF be OK for this? I am guessing yes.
I say the schematic mostly seems correct because it calls out the exact pcb number and transformer number I have but diodes D5 through D10 don't exist on my board as far as I can tell.
The transformer is always powered. With the leads unsoldered, it reads 72V open circuit and the fuse is not blowing.
Looking at the pair of 2 watt resistors R62 and R63, and I think the color bands have been baked to uselessness. The schematic calls for 680 ohms, one reads 797 and the other is 1.16k. I will replace both. It's odd that the bands don't match each other and they don't look like they could have ever been the colors for 680 (blue, gray, brown?)
The pair of 0.22 ohm 5 watt resistors R50 and R51 each test very close to spec, showing 0.3 ohms on my 40 ohm scale. R51 has the color bands slightly darker from heat.
I removed the 4 power transistors and they test ok using the diode test on my DVM but I know that's not a real-world test. My best clue at this point is that one of the small power transistors Q5 got burning hot in the short time before the fuse failed while its counterpart Q4 did not. Also, you can see in the photo that the little heat sink for Q5 has blackened the glue on the pcb where it contacts it.
This is where my abilities end. I need someone with knowledge to tell me the likely reason(s) for Q5 heating up. I'm guessing just replacing it is not the answer. If anyone feels any of the power transistors should be replaced, the part numbers are kind of outdated and I would welcome suggestions of suitable replacements. They are (Q4) D1563A, (Q5) B1086A, (Q6) A1516, and (Q7) C3907.
Thanks in advance for any help!
Rob
Here is the power supply PCB: