desertrout
Well-known member
I have Plus that is reading steady low voltage - 9.2V and 3.9V - and showing an odd pattern on the screen (see attached). General sequence of events, things to know:
- AB was working fine, but caps were older so decided to recap
- recapping resulted in flupping, swapped old caps back in but symptom didn't change (I didn't think to adjust the voltage pot, brain fart). New caps checked good, so put them back in
- poking around the switching side of the board, I clumsily shorted something and blew Q10, awesome
- after some additional testing, I replaced some open/bad resistors, replaced CR21 just in case, swapped in a known good Q9 and Q10 (from a working Plus)
- powers on, no bong, shows attached screen, voltages read steady 9.2V and 3.9V instead of 12 And 5. Adjusting R56 makes no difference --- it works, I can measure the pot changes, just doesn't translate to a change in voltage going to the logic board or elsewhere on the sweep side of the board.
- replaced and CR18 and CR19 zener diodes just in case, no change
- the logic board is known good, so I'm assuming the pattern is due to low voltage
I'm no EE, so I'm trying to narrow down the issue by comparing measurements between this and a working Plus, and looking at https://vintageapple.org/gamba2/images/plus_analog.PDF to hopefully build some understanding. Everything on the supply side of the board seems good. Everything on the sweep side of the board seems off / low. The only big thing I'm noticing is the voltage on pin 2 of U3 (the optoisolator) on the non-working Plus is really high (6V+) but low on the working Plus (~1.2V), but I don't know if that's indicative of anything tbh. I'm thinking of swapping in U3, but I'm also getting tired of what feels like chasing my tail.
Any ideas of where I should be looking, what component(s) may be at fault?
- AB was working fine, but caps were older so decided to recap
- recapping resulted in flupping, swapped old caps back in but symptom didn't change (I didn't think to adjust the voltage pot, brain fart). New caps checked good, so put them back in
- poking around the switching side of the board, I clumsily shorted something and blew Q10, awesome
- after some additional testing, I replaced some open/bad resistors, replaced CR21 just in case, swapped in a known good Q9 and Q10 (from a working Plus)
- powers on, no bong, shows attached screen, voltages read steady 9.2V and 3.9V instead of 12 And 5. Adjusting R56 makes no difference --- it works, I can measure the pot changes, just doesn't translate to a change in voltage going to the logic board or elsewhere on the sweep side of the board.
- replaced and CR18 and CR19 zener diodes just in case, no change
- the logic board is known good, so I'm assuming the pattern is due to low voltage
I'm no EE, so I'm trying to narrow down the issue by comparing measurements between this and a working Plus, and looking at https://vintageapple.org/gamba2/images/plus_analog.PDF to hopefully build some understanding. Everything on the supply side of the board seems good. Everything on the sweep side of the board seems off / low. The only big thing I'm noticing is the voltage on pin 2 of U3 (the optoisolator) on the non-working Plus is really high (6V+) but low on the working Plus (~1.2V), but I don't know if that's indicative of anything tbh. I'm thinking of swapping in U3, but I'm also getting tired of what feels like chasing my tail.
Any ideas of where I should be looking, what component(s) may be at fault?
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