Resources

Wiki
zimmers.net – schematics, programs, ROM images and many more
Classic computers – history of some repairs
6502.org
RAM/ROM replacement board

Internals

Motherboard

There are seven sockets with ROM chips named U9-U2 (U1 is the one soldered at the bottom of the picture). Their content depends on the revision of the system and the basic version installed. My system has Commodore Basic 3.0. 6502.org page has a decent description of ROMs and their differences. My board has the following ROM chips (thanks to dave_m of Vintage Computer Federation for the details):

• D9 – 901465-03 : Kernel : Boot process and key Operating Routines (0xFxxx, 4K)
• D8 – 901447-24 : EDIT Routines including screen and keyboard handling (0xExxx, 2K)
• D7 – 901465-02 : BASIC Routines (0xDxxx, 4K)
• D6 – 901465-01 : BASIC Routines (0xCxxx, 4K)

Original 4K MOS 901465 ROM chips are the equivalent of the TMS-2532 EPROMs. They are not the same as commonly available 2732 chips, but can be made compatible with a help of a simple adapter (please read an interesting story about the TI ROM chips compatibility). However, original 2K MOS 901447 ROM chips are the equivalent of 2716 EPROMs.

My motherboard also contained an extra chip soldered to it – 4xNAND 74LS20. The reason for this chip remains unknown, but until it was removed, PET did not work properly.

Original PET does not have a reset button. It is pretty inconvenient, because every power cycle increases risk of CRT failure. A reset pushbutton can be added to the pin 5 (Control Voltage) of LM555 chip. It should close this pin to the ground through some resistor (<1K).

Tynemouth Software manufactures RAM/ROM replacement boards for PET. The board is an interposer between the motherboard and CPU and contains 32K RAM and various ROM images including Commodore BASIC V1.0, 2.0 and 4.0 as well as DOS Wedge and PET tester ROM. It is a very useful device for detecting RAM/ROM chips defects, because it allows for a replacement of the individual chips.

Keyboard

Keyboard in my PET was completely broken – only space key worked if pushed really strong and the other keys did not make any effect. It was due to the graphite coating of the rubber pads being totally worn out. I used Electon 40AC conductive silver-filled lacquer to cover all key pads and the keyboard started working again as new.

Power Supply

Analog Circuitry and CRT

Reassembly

Some Dirt

Documents