On 06/05/2017 11:40 AM, Ethan Dicks wrote: > The power connectors on many PETs are burned - the current draw is a > bit much for the connector so oxidation happens which increases > resistance which adds heat and the cycle repeats... > Gotcha, that makes sense, and I take the original connectors are unobtainium (or will just cause the same issue) hence the alternative connectors.
> Those aren't power transistors. Those are regulators in TO-3 packages. I'll see if I can test it and possibly get a replacement. It does look like some of the solder points are a bit burned, possibly by end user attempted repairs, or maybe power issues. This pet had a printer interface installed in it, the end user soldered bunches of pins off the IEEE interface to a ribbon cable on both the top and bottom and then to an IEEE M/F card that presumably went on the IEEE488 port previously. There's also a DB25 on the side where the secondary tape interface is (which is now covered.) Removing these two did not get rid of the short, so most likely it's in the power section of the board. > Check the resistance across the matrix to see how "bad" it is. If > it's several K-ohms, you still have a problem. > > As kids we wore out a keyboard playing Space Invaders - just the A, 4 > and 6 keys. No gold, bare copper on those pads. I had to buy a new > keyboard PCB from the dealer. Very true, I was one of those kids back then. :-D Though to my eye, I don't see the PCB pads worn at all, so more than likely it's the rubber. > Outside of actual damage to the PCB, it should be cleanable with > alcohol. The pads are carbon-infused rubber. Look at the pads for > indentations/marks. What I found worked with a recent TRS-80 repair > (individual key switches so it was easy to get each resistance) was if > I just cleaned them, I still measured several K-ohms each. I also > rotated the pads 90 degrees to get some "fresh" rubber onto the > contacts. Those measured a few hundred ohms each and work reliably. Sounds like a plan. It seems to be keys 1-7. 8 works if I hold it down with a lot of force for a couple of seconds. I'll check the schematic to see if that counts as a whole row in the kb matrix . > An entire row could be a bad wire or bad trace or a bad component on > the PET. You can take a jumper wire and short the pins in the matrix > to generate key events. If an entire row doesn't show up from that, > it's the PET, not the keyboard. There should be a 74145 on one side > of the matrix and a 6520 on the other side, from memory. The 6520 is > socketed and easy to swap out. Additionally, I've had success > replacing a 6520 with a 6821 which is much easier to find (it's a > common part in vintage pinball machines, and I happen to have some NOS > from a product I worked on in the 1980s). > > Testing each stage (individual keys, end-to-end resistance tests, > matrix tests on the mainboard...) should help identify specific > problems and where they are. > Where can I find parts these days if I need'em? Digikey?
