Crazy MCR Power Supply Issue = Head + Brick Wall + Brick Wall + Brick Wall...
The 5+ on my MCR 90412D stopped working again (only outputting +3.45), so I bought new parts for anything that hadn't been replaced even if it "tested good".
I replaced the parts one-at-a-time, then tested the board, starting with the U2 controller -> Q105 -> Q104 -> Q103... Nothing changed. Parts 2,3,4,5,6,8,9,10,11 (pic1) have been replaced at this point.
I read Jacklick's post (https://forums.arcade-museum.com/showpost.php?p=3565917&postcount=3) mentioning replacing tantalum cap 107 with an electrolytic, so I put a real nice 100uF 50V in.
The board put out +5.25 for about a second, then dropped sharply to +3.45 kind of like I'd blown something in the circuit.
I replaced C107 with a standard 100uF 16V electrolytic, but the output was still +3.45.
As a last attempt with C107, I grabbed a tantalum C107 from a donor board, but it still output +3.45.
Back to square one. I replaced U2 and the +5 acted like it was trying to level out, but never reached +5v, then fell back to +3.45.
Since then, I've replaced Q105 -> Q104 -> Q103 with new ones (just in case one of them were bad), but still only +3.45v.
On the LAST attempt, I just happened to leave the meter connected after powering down the 90412D. While measuring at C106, the power initially went to +4.38v then fell back to +3.47.
After turning off the power, C106 slowly fell from +3.47 to +2.37, THEN ROSE AGAIN TO +4.25, slowly fell to +3.33, Then Rose Again To +4.17 before falling off to +0v.
So, power up went like this:
Power down went like this:
What could be wrong with my board?
The 5+ on my MCR 90412D stopped working again (only outputting +3.45), so I bought new parts for anything that hadn't been replaced even if it "tested good".
I replaced the parts one-at-a-time, then tested the board, starting with the U2 controller -> Q105 -> Q104 -> Q103... Nothing changed. Parts 2,3,4,5,6,8,9,10,11 (pic1) have been replaced at this point.
I read Jacklick's post (https://forums.arcade-museum.com/showpost.php?p=3565917&postcount=3) mentioning replacing tantalum cap 107 with an electrolytic, so I put a real nice 100uF 50V in.
The board put out +5.25 for about a second, then dropped sharply to +3.45 kind of like I'd blown something in the circuit.
I replaced C107 with a standard 100uF 16V electrolytic, but the output was still +3.45.
As a last attempt with C107, I grabbed a tantalum C107 from a donor board, but it still output +3.45.
Back to square one. I replaced U2 and the +5 acted like it was trying to level out, but never reached +5v, then fell back to +3.45.
Since then, I've replaced Q105 -> Q104 -> Q103 with new ones (just in case one of them were bad), but still only +3.45v.
On the LAST attempt, I just happened to leave the meter connected after powering down the 90412D. While measuring at C106, the power initially went to +4.38v then fell back to +3.47.
After turning off the power, C106 slowly fell from +3.47 to +2.37, THEN ROSE AGAIN TO +4.25, slowly fell to +3.33, Then Rose Again To +4.17 before falling off to +0v.
So, power up went like this:
Power down went like this:
What could be wrong with my board?
