On Sun, Jun 4, 2017 at 5:43 PM, Ray Arachelian via cctalk <cctalk@classiccmp.org> wrote: > I have a pair of 8032 Pets. One of them has a short on the motherboard > that causes the fuse to blow whenever it's turned on - looks like its > previous owner tried to repair it as the power connector is a bit burned
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... > and a new, non-pet power connector has been added. That was one solution. One of my PETs has that (and it's a PITA because I can't easily swap out the board with a working one). > Likely one of the power transistors in the heatsinkis blown Those aren't power transistors. Those are regulators in TO-3 packages. > I'm trying to find out if the keys conductive or capacitive. Conductive. > If they're conductive, it may be that the conductive rubber is worn out > and despite my cleaning both the contacts on the board, and the rubber > on the keys with a contact cleaner they won't work right. 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. 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. > ... dead pet in the other one and most of the keys work, except for > much of the top numeric row, including, sadly the 2 key which has the > quote on it, which prevents the loading of programs off my petdisk. 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. -ethan
