This information was generously submitted by Bruce Philip aka eightbit and he agreed to let me add it to my website. Kudos to Bruce! Note: Bruce first noted that his PET would not work at all, then would start to come up after a few minutes. He found the connection between its power supply and system was intermittent. Plugs and sockets, including IC sockets, can present frustrating problems such as this, which is why I always reseat connectors and IC chips in their sockets as an initial part of diagnostics. Ray Carlsen Ray, In case you were interested, I have fixed the PET. What a whirlwind of a ride. I had to take a few crash courses in troubleshooting and reading schematics :) Here are my notes and pictures in case you were interested on what this was: RAM and video RAM I found that much of the original 4116 DRAM was bad, so I socketed the RAM and replaced it with NOS 4116s. The original 2114 video RAM was also bad. I socketed those positions and replaced the 2114s with known-good parts harvested from a VIC-20. Since I was already doing the RAM work, I expanded the machine from 16K to 32K. I installed sockets and RAM in the previously unused second DRAM bank and changed the motherboard jumper shunts from the 16K configuration to the 32K dynamic-RAM configuration. After that, the screen improved considerably. Instead of the repetitive O's and arrows, I now had a screen full of fairly normal random PET characters. This was an important clue. It looked like the random-character screen you briefly see when a PET is first powered on, before the CPU reset completes and BASIC clears the screen. Mine was simply staying there. Stuck RESET and C68 I checked 6502 pin 40 (/RESET) and found it stuck LOW at approximately 0.19 V. I traced the reset signal backward through the board to the 555 power-on reset circuit. Interestingly, while probing around the 555, the PET actually booted normally into BASIC twice. That was the clue that really made me concentrate on the reset circuit. Some of my measurements were: 555 pin 2 (TRIGGER): about 0.02 V and staying LOW 555 pin 3: HIGH 555 pin 6: about 4.48 V 6502 pin 40 (/RESET): about 0.19 V The fault turned out to be C68, the capacitor associated with the 555's power-on trigger. The original capacitor was marked 104 (0.1 µF / 100 nF). Out of circuit, my original C68 measured approximately 175 nF. I replaced it with a new 104 / 0.1 µF ceramic capacitor and that immediately fixed the reset problem. The PET now resets correctly on every power-up and proceeds normally into BASIC. While I was in that area I also replaced the nearby 103 (0.01 µF) ceramic capacitors because I already had new ones, but C68 was the part that fixed the stuck-reset condition. One more problem – marginal 6502 After fixing reset, the PET would boot normally and give me the READY prompt and flashing cursor, but after several minutes it could freeze and return to a garbled screen. This time 6502 pin 40 remained HIGH, so the reset problem wasn't returning. I swapped the 6502, and the machine became stable. Interestingly, the original 6502 still passes my chip tester. Apparently it's marginal under actual operating conditions even though the tester considers it good. That's something I'll keep in mind in the future: a PASS from a chip tester doesn't necessarily mean the IC is reliable in the actual machine. Keyboard repair The keyboard was another problem. Originally, only H, O and P worked. I've seen a lot of PET keyboard repairs where people repaint the graphite contacts on the rubber plungers with conductive paint. I found that wasn't necessary. I used a Q-tip dampened with 91% isopropyl alcohol and rubbed the graphite pad on each plunger about 8–10 times. The Q-tip turns black and it looks like you're removing black paint. Some surface material is indeed coming off, but there is plenty of graphite on these plungers; I didn't scrub aggressively or try to remove it all. This removed the old surface layer and exposed a good conductive surface again. After doing this to all of the plungers, every key works like new. No conductive paint was needed. Final result The PET now: Boots and resets reliably every time Has 32K of working RAM Has working 2114 video RAM Passes a BASIC RAM test using $55, $AA, $00 and $FF patterns through address $7FFF (32767) Remains stable with the replacement 6502 Has a fully working keyboard So in the end I had bad 4116 DRAM, bad 2114 video RAM, a bad/out-of-spec C68 in the reset circuit, and a marginal 6502. Along the way I also upgraded the machine from 16K to 32K and restored the keyboard. The biggest troubleshooting lesson for me was the change in the garbled screen after repairing the RAM. The first screen really was caused in part by bad memory. The second "garbled" screen was actually the normal PET power-on random-character state—it just wasn't progressing past it because the CPU was being held in RESET. If I encounter another PET stuck on that type of screen, 6502 pin 40 is going to be one of the first things I check. The other big clue was the machine unexpectedly booting twice while I was probing the 555. In hindsight, the PET was basically pointing me toward the reset circuit. -Bruce