There are safety interlock switches that are engaged by the front and back covers. If these covers are not properly seated, the Lisa will not turn-on.
The front panel interlock switch is at the lower left. The rear panel interlock switch is near the top of the power supply.
For the most part, the interlock switches are for the safety of the Lisa, as it is difficult to reach dangerous voltages with only the front and back removed. The interlock switches can by defeated using folded cardboard or some other non-metallic item to depress the switch while the cover is removed. Use due care and proceed at your own risk if you operate your Lisa with any covers removed.
A used Lisa may not turn on because it is missing some component. In order to turn on, a Lisa requires:
In order to show something on the screen, a Lisa also requires:
It is common for a Lisa to not power up or work properly due to poor contacts, below is a procedure for cleaning the contacts:
When handling Lisa components, take care to prevent damage due to ESD (electrostatic discharge).
Take out the Power Supply and card cage, and remove the CPU, I/O, and memory boards, as well as any expansion slot cards. Examine both sides of the card-edge connectors of each. Clean them with electronic contact cleaner or alcohol, and for extreme cases, a pencil eraser (be very gentle or the gold plating will be erased). Make sure that there's no oxidation left (or rubber from the eraser). Examine and if necessary, clean the card-edge receptacles.
Unplug any floppy/twiggy drives and any hard drives. Check their connectors are straight and clean.
Reseat the cards and make sure that they make contact properly. Some cards develop a warp and may not make contact all along the card edge.
Try to power the Lisa on with just the I/O board plugged in.
If that works, shut the Lisa off (you may have to pull out the power cord), and insert the CPU board without memory. This should cause the Lisa to beep and perhaps show an alternating video pattern (since there is no memory for the display.)
If this works, power the Lisa off again, insert a single memory board in the first slot. Power the Lisa on. If it doesn't work, swap it with another memory board. If it doesn't work, try the other slot. If it still doesn't work, you may have a bad memory board (or a bad CPU board.)[LisaFAQ] (Comment on this answer)