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Commodore Plus 4 Repair

28 Pins Of Failure

So henI first got this machine it did not work properly. I som etimes could get it to boot, but it would crash as it warmed up. I did some Googleing around to try & find the issue.

First was to fix the keyboard, a pretty standard procedure at this point. Just takes time due to the amount of keys that needed conductive paint, usualy just do the whole keybaord.

The next thing I tryed was the RAM, desolderd & tested it. Was all good. Quite a few computers have bad RAM, pretty much garenteed to be dead if its MOS RAM. So I soldered in some sockets & put the chips back.

Next thing I tryed was the CPU. I got one of those Comodore 64 CPU replacment kits off of eBay & pluged it in swapped the required ROM with the provided EPROM. No diffrence was made, reliability was the same. So it was time to takle the PLA.

Just for fun the keybaord, it needed the contacts redoing. I did the carbon contacts with conductive paint & thta fixed the dead keys, common on these old keybaords that use carbon contacts.

The real culpret to why the bastard had died. Pretty standard for Commodore machines as something about the chip fabircation proccess made MOS chips prone to chip rot & thus they very oftern fail. This is one that did just that.

After some research it turned out that the Plus 4 PLA wa the same pinout as the C64 PLA, probaly the same chip with a diffrent "code". You can see where I socketed the RAM. Also added heat sinks to the CPU as they are prone to over heating & dieing. Anyway, back to the PLA. So then I read you could program the PLA on an EPROM, this means they are esentaily mask ROMs. The pinouts also almost matched that of a standard 512Kb EPROM.

At first In found C64 PLA dumps so that ment there was a chance there as a Plus 4 dump. So after a lot of searching I was successful in finding a website that hosted the PLA images. I downloded it & have hosted my own version here: pla.251641-02have also fddfd

Also have a link down at the bottom of where I got the original PLA code. Anyway I burned it to a ST M27C512-15FI EPROM I then wired the EPROM to a socket & did the pin rewires so it would match the pinout with that of the commodore. Once I pluged it in the Commodore Plus 4, it fired to life & stayed reliable when I did a soke test. So now I have a fully functional Plus 4. I even got a tape drive later so I have been able to verify useing an EPROM does not cause the same issues that it does on a Commodore 64.

Here is a YouTube video I did on the proccess of replacing the PLA, sadly I am the only one who seems to have done this repair. I hope by putting this information out there others will be able to restor the Commodore Plus 4's & C64's etc.

The website should provide nice suplimentry cover to my YouTube videos on restoring al this old tech.

PS: I also built it a custom PSU.

The finnished PSU.

Commodore Plus 4 running soome tests with the Commodore Plus 4 tape drive I got off of Look Mum No Computer. Worked a treat provng the PLA EPROM replacment was a success.

I got the tape drive a year or two after repairing the machine its self. So I can at least burn software onto tapes for the machine to test it out properly.

Links To PLA Firmware & Other Related Info:
http://www.devili.iki.fi/Computers/Commodore/Plus4/Service_Manual/page_08.html
https://www.commodorefree.com/magazine/vol4/issue38.html
http://www.zimmers.net/anonftp/pub/cbm/firmware/ALLFILES.html
http://www.zimmers.net/anonftp/pub/cbm/firmware/computers/plus4/index.html

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