Last year, I repaired a Commodore PET 8032 for a friend of mine. The PET wasn’t working and needed its mains filter can removed, a keyboard service including a new shift-lock mechanism, and a general clean up – it’s matching 8050 dual 5.25″ FDD wasn’t working and needed its mains filter can removed with a new IEC socket fitted, and is still not reading and writing disks properly so requires further investigation.
The PET 8032 has a mains filter capacitor/inductor, and these have a habit of failing due to age, and pop open releasing a nasty smoke – probably tripping your circuit breaker too. These are before the power switch, so can go bang even if the computer is plugged in but not switched on. This unit still had its installed, so it needed to be removed. These are primarily for EMF compliance so aren’t really required for functionality’s sake.
The filter can is hidden away underneath the mains transformer, which needs to be removed for it to be accessible.
This one has three XY-class capacitors (1 x 100nF and 2 x 2500pF) and two inductors (2 x 100uH) across mains/earth, which leads to a pretty big bang when they fail.
I simply cut the two input wires to the filter close to the filter body to give me enough room to fit them back to the terminal block, making sure that live was both fused and switched before it goes into the transformer.
With this work done, the PET seemed to start up OK – however, the shift-lock mechanism didn’t stay latched and some of the keys didn’t register correctly.
I couldn’t find a working PET shift-lock mechanism for sale, and I imagine they all suffer from the same problem eventually – thankfully someone has created a CAD model for 3D-printing a replacement part using a modern push-lock switch, which doesn’t have quite the same travel but thankfully works extremely well.
I cleaned the keyboard PCB and carbon key plunger contacts thoroughly to get the keys working again, then fitted the replacement shift-lock mechanism.
With the system reassembled, everything worked as it should.
