Hi all. New member here, attempting to repair a BP7002 amp for a set of towers i purchased locally for next to nothing knowing one amp didn't work but the other worked perfectly.
i read through hundreds of threads on the subject and being familiar with fixing a couple vintage receivers, i thought i'd give it a try. i bought another non-working 7002 amp off ebay to use instead of mine (in case i mess up and for spare parts) where the led comes on, but no sound through the sub- although after gettin git and hooking it up, it has random hissing/static, kind of like the sound you get when scanning a tuner in FM between stations. Glad i bought one though because when i removed the bad one from the tower, it had been tinkered with and had some burn marks on it and the led didn't even come on - so i put it aside and focused on the ebay one. i compared the ebay one to my good one, measuring about every voltage possible and everything checks out. there are a few that are "slightly" off but in looking at the boards and the schematics i found, there was a rev B to rev C change that added/changed a few resistor values and Q types, so the small variances seem to make sense. Regardless, nothing is grossly off, all very, very close.
The ebay amp also suffered from the led light going out sometimes and voltages skewing when the light went out, and a weird noise when plugged in....i assumed that was the power supply and was planning to replace all the electrolytic caps anyway as they are known failure points (except the huge ones), which i did, and now both those issues have been fixed (4 of the caps actually tested bad) HOWEVER the scratchy/static sound from the sub is still there with no input, just plugged in, and no sound. I tried on both towers as well and different outlets. It seems this just cannot simply be voltages, or caps.
Moving to the amp board, I checked the three big output transistors on the heat sinks, and the several small ones, as well as all the diodes, several resistors, and the two caps (in circuit) and it all checks out nearly identical to the good one i have.
At this point, i'm thinking it's one of the IC's....or I'm missing something, otherwise I'm at a loss. Any thoughts on anything else i can/should try?
Any help is appreciated.
i read through hundreds of threads on the subject and being familiar with fixing a couple vintage receivers, i thought i'd give it a try. i bought another non-working 7002 amp off ebay to use instead of mine (in case i mess up and for spare parts) where the led comes on, but no sound through the sub- although after gettin git and hooking it up, it has random hissing/static, kind of like the sound you get when scanning a tuner in FM between stations. Glad i bought one though because when i removed the bad one from the tower, it had been tinkered with and had some burn marks on it and the led didn't even come on - so i put it aside and focused on the ebay one. i compared the ebay one to my good one, measuring about every voltage possible and everything checks out. there are a few that are "slightly" off but in looking at the boards and the schematics i found, there was a rev B to rev C change that added/changed a few resistor values and Q types, so the small variances seem to make sense. Regardless, nothing is grossly off, all very, very close.
The ebay amp also suffered from the led light going out sometimes and voltages skewing when the light went out, and a weird noise when plugged in....i assumed that was the power supply and was planning to replace all the electrolytic caps anyway as they are known failure points (except the huge ones), which i did, and now both those issues have been fixed (4 of the caps actually tested bad) HOWEVER the scratchy/static sound from the sub is still there with no input, just plugged in, and no sound. I tried on both towers as well and different outlets. It seems this just cannot simply be voltages, or caps.
Moving to the amp board, I checked the three big output transistors on the heat sinks, and the several small ones, as well as all the diodes, several resistors, and the two caps (in circuit) and it all checks out nearly identical to the good one i have.
At this point, i'm thinking it's one of the IC's....or I'm missing something, otherwise I'm at a loss. Any thoughts on anything else i can/should try?
Any help is appreciated.