I've got an Onkyo SKW-791 subwoofer in right now that isn't powering on. Or at least the indicator light isn't coming on and the system it goes to can't detect/connect to it. It's drawing 0.7W from the wall when plugged in according to my kill a watt. It has a hissing noise constantly whenever it's plugged in, but the hiss isn't coming from the speakers this is when the board is pulled out of the housing and sitting on my bench. The hissing nose is still present even when the speaker is plugged in, but nothing comes out of the speaker.
The first thing I did was replace the audio amp (IRFI4212H)(light blue on the topside picture) because I was not getting diode readings on all the legs when it was out of circuit. I compared the old one to the new one and I was missing readings on two of the pins. I put it in and nothing changed, but the pins are all still reading the same as they were before I put the chip in so I don't think it's blown.
I got it to "turn on" once by just plugging and unplugging it a bunch of times and it was drawing 7W from the wall and the hiss went away, but I haven't been able to make it do that again. The one time it did draw more power I hit it with my thermal camera and was getting ~150F-160F on the resistors next to the two large caps on the amplifier side of the board (marked in orange on bottom red on top), but they are reading fine in and out of circuit. The other area that was getting warm (130F) was just below the audio transistor (around the two diodes below the black circle on underside of the board.)
I only found one short on the board and it was on the AC pins of the bridge rectifier on the amplifier side of the board(light orange). When I removed the rectifier the short was still there, but I was only able to trace it back to the primary transformer. when I checked there was no AC present on this bridge. On the DC + leg of the bridge I'm getting 45V and on the - 55V.
I found 12V on one of the diodes near the secondary side line, but I don't remember which one it was. I have not been able to find 5V or 3.3V anywhere on the board. Most places I check give me 44v-45V or nothing. The 78m12a (dark blue underside) next to the audio IC read 44V on all three pins when I checked using the gnd from the transistor on the bottom circled in red, but 0V when I checked using the gnd of the heatsink (black underside). I got the same readings on everything else when I checked using both gnd points. On the 78m12a when I checked in diode mode none of the pins seemed connected to any gnd point I checked from anywhere on the board, but none of the legs were shorted together.
I did check all the large caps on both sides and they are charging up to their rated values. When I pulled them out and tested them in my component tester the capacitance and ESR where within spec. I haven't checked every individual capacitor, but I did random sample a couple from around the heat spots and did not have any that were out of spec.
I'm not really sure where to go from here other than pull all the caps and test them. I'd say throw in the towel is an option, but I'm to stubborn for that.
The first thing I did was replace the audio amp (IRFI4212H)(light blue on the topside picture) because I was not getting diode readings on all the legs when it was out of circuit. I compared the old one to the new one and I was missing readings on two of the pins. I put it in and nothing changed, but the pins are all still reading the same as they were before I put the chip in so I don't think it's blown.
I got it to "turn on" once by just plugging and unplugging it a bunch of times and it was drawing 7W from the wall and the hiss went away, but I haven't been able to make it do that again. The one time it did draw more power I hit it with my thermal camera and was getting ~150F-160F on the resistors next to the two large caps on the amplifier side of the board (marked in orange on bottom red on top), but they are reading fine in and out of circuit. The other area that was getting warm (130F) was just below the audio transistor (around the two diodes below the black circle on underside of the board.)
I only found one short on the board and it was on the AC pins of the bridge rectifier on the amplifier side of the board(light orange). When I removed the rectifier the short was still there, but I was only able to trace it back to the primary transformer. when I checked there was no AC present on this bridge. On the DC + leg of the bridge I'm getting 45V and on the - 55V.
I found 12V on one of the diodes near the secondary side line, but I don't remember which one it was. I have not been able to find 5V or 3.3V anywhere on the board. Most places I check give me 44v-45V or nothing. The 78m12a (dark blue underside) next to the audio IC read 44V on all three pins when I checked using the gnd from the transistor on the bottom circled in red, but 0V when I checked using the gnd of the heatsink (black underside). I got the same readings on everything else when I checked using both gnd points. On the 78m12a when I checked in diode mode none of the pins seemed connected to any gnd point I checked from anywhere on the board, but none of the legs were shorted together.
I did check all the large caps on both sides and they are charging up to their rated values. When I pulled them out and tested them in my component tester the capacitance and ESR where within spec. I haven't checked every individual capacitor, but I did random sample a couple from around the heat spots and did not have any that were out of spec.
I'm not really sure where to go from here other than pull all the caps and test them. I'd say throw in the towel is an option, but I'm to stubborn for that.
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