View Full Version : Commodore PET 2001-8 Repair...

December 3rd, 2006, 11:53 AM
A while ago I bought a PET 2001-8 (with the original "Chiclet Keyboard") at an estate sale, and recently I have finally started to try to get it running. With a little work, I have gotten the screen to come on and display garbage. The cassette motor also runs continuously. I have verified, by means of a logic probe, that the power-on reset circuit brings the 6502 reset line high about a second after power is applied. My oscilloscope shows clean power on the +5V bus (as measured at the internal cassette reader connector). I have carefully removed and re-seated every socketed component on the board, with little resulting change in behavior. An hour of swapping SRAMs around (I have three "new", presumably good, spares) also had no useful effect. The ROM chips installed are a "Version 2" set from Skyles Electric Works, which is supposed to make the machine act like the later "N" series models. Unfortunately I do not have another set of ROM chips to try.
Any suggestions regarding how to proceed would be much appreciated. The equipment I have available includes oscilloscopes and a basic logic analyzer.

December 26th, 2006, 09:31 AM
I suggest that you start on the 4116 Ram Chips. A common problem with all vintage computer with a garbled screen is a dodgy ram chip. Power on you pet and feel if any of them are getting warmer than the others. If one of them is then it is likely that this may be the cause. If not then change one at a time.
I am working on my second Commodore Pet trying to get it going but I am getting nothing on the screen, similarly I have tried changing the socketed chips with my known working Pet but no difference. I am aboutto change the ram chips. Good Luck !

December 26th, 2006, 03:48 PM
Here are a couple of places that may help: Here (http://www.6502.org/users/andre/petindex/repairs.html) and here (http://www.geocities.com/davegostelow/PETrestore/PETrestore.html).



December 26th, 2006, 06:43 PM
With all of the vintage computer stuff I have found for myself, the only problems I've found are the CPU and the ROM's (YMMV).

CPU's are easy enough to replace but the ROM's can be tricky, especially the MOS 32xx ROM's as they're slightly different to the Intel ones.

With my SYM-1 for example, to replace the ROM's I had to put some jumpers in. I used an Atmel 28C64 8K EEPROM and wired it like this, after programming it.

Solder pins 26, 27 and 28 together on the EEPROM (the two Vcc's and /WE).
Bend out pins 23 (A11) and 20 (/CE).
Solder a bit of single core wire to pin 23 and push it into pin 18 of the socket (same as pin 20 on the EEPROM).
Solder pins 22 (/OE) and 20 together with a bit of wire.
If the original MOS ROM is an 8K (2364) then solder a piece of wire to pin 2 of the EEPROM and push it into pin 21 of the socket (same as pin 23 on the EEPROM).

Once this is done stick the EEPROM into the socket with pins 1, 2, 27 and 28 sticking over the edge of the socket.

Hope this gives you something useful to work with!

February 21st, 2007, 04:10 AM
I hate to bring too old of a topic back but I had to post about this. I fixed my PET 2001-8 with the chicklet keyboard and here's how.

First, I would try reseating the CPU, make sure the pins are clean and working good. Next, try the RAM IC's. The chicklet model used MOS 6550 (discontinued years ago) static RAM chips that fail. I had just enough good RAM for the 1 meg video and 1 meg onboard. It just wasn't enough RAM so I purchased and built the RAM/ROM board for the chicklet PET. You can find it here:


I highly suggest this board as it gives you all the versions of ROMS and 32K RAM.


February 21st, 2007, 06:21 AM
Here's a little trick that could save you lots of unnecessary soldering and unsoldering when troubleshooting soldered in RAM.

First, turn it on and let it sit a while and then touch each of the RAM chips. If you don't get the chip logo burned into your fingertip, you'll know none are shorted.

Second, to check for opens, take a RAM chip of the same type and, with the power OFF, slip it over the first on-board RAM making sure that a) it's contacting all the legs of the target chip, b) it's going in the same direction and, c) you aren't shorting any of the pins from either chip together.

Turn the unit back on. If it works, then the target chip is open somewhere, if not, turn OFF, move the test chip down the row and repeat until the RAM fires up right and that's the bad soldered RAM. Replace it.

If you've tried them all and it doesn't work, but when it was on one of the chips, the "garbage pattern" changed, then you are in the unlikely situation of having two shorted chips, so, let the one on the RAM that showed the change and repeat with another test RAM to find the second bad chip.

It's down and dirty, but, it works.

February 21st, 2007, 05:56 PM
I had just enough good RAM for the 1 meg video and 1 meg onboard.

God, I hope you meant 1K and 1K. If you've got 2MB in a Pet I'm going to have to insist on pictures. . . :jumping7:

February 22nd, 2007, 02:39 AM
LOL, yeah, 1K and 1K, sorry.


February 27th, 2007, 09:22 PM
i once accidently dropped by PET4016 from 3 inches on concrete, it survived with no damage whatsoever.

i guess your machines just arent as good ^-^;;

i do need to maintenance the keys tho.

the 8 sticks easy.

the only thing that needs repair is that one 8 key and blowing dust and particulates out from under the keys.

hooray for the fifty pound invincible behemoth that is the Commodore PET 4016!

yes i do intend to keep it as a historically accurate, as close to factory condition, Commodore PET 4016 for as long as i can ^-^;;

it still functions factory minus the sticky 8 key.

March 6th, 2007, 01:58 PM

i have 2 Pet boards; 1x 320137 assy with 2114 RAM's and 1 320132 Assy with ?!?! 4045 ?!?! RAM's...I think these are also 2114..

my question is: can I mix 6550 RAM's with 2114? are they equal?
I have also and old 320008 board with 6550 RAM's.. and I fear to test them on the 320132..

the problem with the 320132 is the scrambled text... mostly a RAM problem.. I could not resolve it with changing all the chips.. also changing the video rams with the other rams did not help.. the only thing what is changing is the type of scrambled things on the screen...


March 7th, 2007, 07:13 PM
2114's and 6550's are NOT compatible, I would check ebay. I had a 4016 and they are great tanks!