General Category > Lisa Troubleshooting and Repair

Diagnosing no power

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stepleton:
+1 to blusnowkitty's suggestion to check +5V standby power. It's unlikely that this has failed (it's a really simple linear power supply) but the show won't start without it, so may as well give it a look. You can plug the PSU into the wall and measure between component-side pin 20 (+5V standby) and component-side pin 15 (one of the ground pins). I don't think the interlock needs to be engaged for it to work, but if you don't see a voltage, try defeating the interlock first.

Maybe you'll get lucky and all you'll need to do is replace a 7805...

paule:
Sorry, mistyped: computer_85, how did you "force the system to come on"?

sigma7:

--- Quote from: paule on March 04, 2023, 03:20:26 am ---how did you "force the system to come on"?

--- End quote ---

The COPS generates the signal to turn the power supply on or off.

In order to do that, the COPS runs on the +5STBY supply, which is always available when the Lisa is connected to line power. Hence the recommendation to check that +5STBY is available (it should be a bit more than 5V when the Lisa is plugged in).

If the COPS & front panel power switch aren't functioning, but +5STBY is available, you may be able to turn on the Lisa by forcing the "turn-on" signal manually.

The "ON" signal from the I/O board to the power supply is pin 120 of the I/O board card edge connector, which is on the back of the board so not so easily accessed.

More easily accessed is R42, which is the resistor for the ON signal that pulls it up to +5STBY when the COPS allows it. By adding a jumper across R42 (ie. short circuit across R42), the ON signal will be forced high, which would typically force the power supply to turn on if +5STBY is working and the front and rear safety interlock switches are engaged.

Note that this is in contention with the output of the 7417 that normally drives the ON signal to the off state. In most cases one can do this with TTL signals without damage, but it may cause overheating or damage to more sensitive parts, so minimize the amount of time the jumper is applied.

paule:
Thank you very much for that excellent explanation. Hopefully I be able to work on this some tomorrow.

paule:
So, good news. The Lisa Troubleshooting Guide was good as gold. It turns out that just removing and reinserting the IO board brought the system back to life (I actually swapped in a good one, powered on, then put the original back in).

However, the Widget did not turn on or did not spin up at least, and one of the RAM boards is problematic (71 - parity error).

Diaganosis continues...

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