General Category > Lisa Troubleshooting and Repair
Help please for my Lisa 2/5
Lisa2:
Will the system stay on if the I/O board is removed? This may tell you if the issue is on the I/O board or elsewhere in the system.
pintoguy:
If I insert the board from a good 2/10, the light stays on. This is why I almost certain it's the IO Board
sigma7:
--- Quote from: pintoguy on September 01, 2023, 11:22:16 am ---... almost certain it's the IO Board
--- End quote ---
Check the socketed chips for one in backwards (happens to all of us).
On the questionable I/O Board (out of the Lisa), check the resistance from +5V to ground, and from +12V to ground. If either of those are nearly shorted, the PSU will click.
Depending on the sensitivity of your ohmmeter, you may be able to locate a short within a few chips... keep moving the probes around until you find the area where the resistance (eg. from +5 to ground) is the least.
IIRC, this is a battery corrosion repaired board... visual check for further corrosion?
pintoguy:
Thanks very much James
--- Quote ---Check the socketed chips for one in backwards (happens to all of us).
--- End quote ---
Yep, all OK. I even removed them all - same result
--- Quote ---On the questionable I/O Board (out of the Lisa), check the resistance from +5V to ground, and from +12V to ground. If either of those are nearly shorted, the PSU will click.
--- End quote ---
800 Ohm or so for +5V-gnd and 4Mohm for 12V-gnd, so OK I guess ? I didn't check the -5V though. My PS actually does not click. The power just shuts off after one second or so. So perhaps not a short ?
--- Quote ---IIRC, this is a battery corrosion repaired board... visual check for further corrosion?
--- End quote ---
Yeah, I'm almost sick of looking at the board and checking all the traces after 2 month of that :). But I'll persevere
sigma7:
--- Quote from: pintoguy on September 02, 2023, 01:11:02 pm ---800 Ohm or so for +5V-gnd and 4Mohm for 12V-gnd, so OK I guess ?
--- End quote ---
4 MegOhm isn't right, way too much resistance -- I'd guess you didn't get the right test point or a good connection. On a sample here, +12V to ground is about 59 KOhm. +5 to ground is about 500, close enough to yours that I'd not suspect it.
--- Quote ---My PS actually does not click. The power just shuts off after one second or so. So perhaps not a short ?
--- End quote ---
Ah yes: no. If there is a short, the PSU will click as the over-current protection kicks in, then resets, repeatedly. No clicking very likely indicates the problem is not a short.
Presumably then the COPS power control circuit is telling the PSU to shut off.
Refer to sheet 2 of the 5 page schematic...
Check the ON signal at U7F-12.
If it is low, then that will turn off the power.
I see that "ON" is pulled up by R42 to "+5B2" which came up earlier, so re-check that +5B2 is roughly 5V.
Presuming that "ON" is low and +5B2 is around 5V; pushing the power switch should make "ON" go high... does it go high briefly or stay low?
Depending on what you find, work back through the circuit to the COPS and see if it is generating the ON state or not, and/or see if some other part of the circuit is over-riding that.
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