Didn't know if I should create a new thread or write on an old thread. Hopefully new is ok. 
I've gotten the repair broken things bug and so have been attempting to troubleshoot and fix what I can find on the cheap.
Ok, on to the questions. Powered up this solid state amp on a dbt and the light shines bright. Unplugged and discharged i immediately went for the transistors. All test ok through my dmm at or near 0.500 on the proper legs. Going to need to walk my way back to find the issue. We'll get more into that soon...
After pulling the main transistors i noticed some had mica and grease and a few had just rubber. Is that normal? I also had two that had double mica? Accident or purposeful?
I've gotten the repair broken things bug and so have been attempting to troubleshoot and fix what I can find on the cheap. Ok, on to the questions. Powered up this solid state amp on a dbt and the light shines bright. Unplugged and discharged i immediately went for the transistors. All test ok through my dmm at or near 0.500 on the proper legs. Going to need to walk my way back to find the issue. We'll get more into that soon...
After pulling the main transistors i noticed some had mica and grease and a few had just rubber. Is that normal? I also had two that had double mica? Accident or purposeful?
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If the transistors are ok but you get an overcurrent condition, either the bias circuit is messed up or one of those insulators is shorted. If the screws are over tightened, touching the heat sink, or a burr cuts thru the insulator it can happen. Especially if someone else worked on it first.
Which appears to be the case - those rubber insulators are the more modern “greaseless” type and are not original. The others are not mica, but kapton. Mica are clear, kapton are yellow. Kapton are also thinner and more easily subject to cut-through. Doubled-up insulators might be an indication there was previously a problem with cut-through. That might have been how somebody “fixed” it once before - which is dead wrong. You may want to replace them *all* wen you put it back together, to insure that this doesn’t happen. If you do reuse the old ones clean all the old grease off with isopropyl alcohol, and use fresh heat sink grease on the kapton ones. If they are cracked, split, or deformed replace them for sure.
Which appears to be the case - those rubber insulators are the more modern “greaseless” type and are not original. The others are not mica, but kapton. Mica are clear, kapton are yellow. Kapton are also thinner and more easily subject to cut-through. Doubled-up insulators might be an indication there was previously a problem with cut-through. That might have been how somebody “fixed” it once before - which is dead wrong. You may want to replace them *all* wen you put it back together, to insure that this doesn’t happen. If you do reuse the old ones clean all the old grease off with isopropyl alcohol, and use fresh heat sink grease on the kapton ones. If they are cracked, split, or deformed replace them for sure.
Other oddities on this amp. Thoughts and opinions welcome!
1. found a MPS-U10 that the legs were broken? and replaced with wires? Really not sure, I need to cut off the heatshrink.
2. One channel has a ceramic capacitor on the backside of the output transistors and one does not.
For the replacement of the MPS-U10 would you say this is an acceptable replacement or does anyone have a better candidate.
Original - MPS-U10 pdf, MPS-U10 description, MPS-U10 datasheets, MPS-U10 view ::: ALLDATASHEET :::
Replacement Option - ZTX657 Diodes Incorporated | Mouser
1. found a MPS-U10 that the legs were broken? and replaced with wires? Really not sure, I need to cut off the heatshrink.
2. One channel has a ceramic capacitor on the backside of the output transistors and one does not.
For the replacement of the MPS-U10 would you say this is an acceptable replacement or does anyone have a better candidate.
Original - MPS-U10 pdf, MPS-U10 description, MPS-U10 datasheets, MPS-U10 view ::: ALLDATASHEET :::
Replacement Option - ZTX657 Diodes Incorporated | Mouser
Interesting, thank you! Yeah, I've been finding strange things here and there. Thank you for the correction on the type. I assumed mica... Do you have a recommended thermal pad replacement you like to use?
That's a great point on the short to the heatsink. I noticed some of the mountpoints have a plastic spacer inside the screw holes and some are missing those pieces. I'll have to see if I can test this with the dmm.
That's a great point on the short to the heatsink. I noticed some of the mountpoints have a plastic spacer inside the screw holes and some are missing those pieces. I'll have to see if I can test this with the dmm.
If you are missing the spacers, you can heat shrink around the screw threads. It will compress to fit and keep it insulated as you tighten the screw. I use Sil-pad K10, 6 mil thick for pretty much all flat-pack transistor sizes, as I can just buy 11x12 sheets and cut it. They are available in most transistor outlines. I just happen to have a few hundred TO-3 micas that I got from Skycraft eons ago.
I had a home brew amp short to heat sink during a gig once, as the heat sink heated up. Made a god-awful KA-POP and blew the fuse. Paralleled that speaker pair off its sister amp, and completed the show. Didn’t blow anything other than the fuse - after re-polishing the heat sink and installing a new transistor and insulator all was well. Trashed the original transistor mounting surface, but still tested good. All it did was discharge the main filter cap, because B+ was collector.
I had a home brew amp short to heat sink during a gig once, as the heat sink heated up. Made a god-awful KA-POP and blew the fuse. Paralleled that speaker pair off its sister amp, and completed the show. Didn’t blow anything other than the fuse - after re-polishing the heat sink and installing a new transistor and insulator all was well. Trashed the original transistor mounting surface, but still tested good. All it did was discharge the main filter cap, because B+ was collector.
OK stupid question maybe. How best do I test if its shorting to the heatsink? I tried the continuity mode but both probes on the heatsink stay OL. Am I missing something obvious? I assumed check heatsink to collector but I decided to test just heatsink thinking both probes on the heatsink should beep too. Am I wrong?
