LMNC KAYPRO II Repair
The Parth Be Not Broken
This machine ended up having two problems, one was pretty obvious from the begining & was discovered the dayI started looking at it. The other was a little more hidden as it required a scope & inquisertive partner to find the issue.
The main problem with this machine was the graphics had random chars all over the screen, the kind of problem that is oftern related to RAM or ROM issues. So I at least had to right place to look for issues.
Likely these machines are quite well documented on the web & ROM dumps be plenty.
A shot of the KAYPRO II mobo. in all its ealy 80s glory. A resonalble amount of descreet logic on the board. The Z80 does have two PIO chips & a SIO chip. Two EPROMs, one for the main computer ROM & the other for the graphics char set. The DM74LS00N chip had a bent pin so I put that back & ony the final bottom foruth on the screen had the char coruption.
So the first issue was this machine that was found was the char EPROM had failed & was giving a diffrent output every time it was read. So as I had the EPROMs at home to burn a new one I did. The EPROM was a 2716. I have done a video on how to bern these in the past HERE. The image was found on the web after some searching.
The keys are all mecanical switches with a nice feedback. Also meny holes for other keys, not sure if these were ever added as options.
The rear of the keyboard PCB. You cna see the cervy tracecs indicating it was not designed on a computer, made sence for the early computers though.
The green screen CRT modual. Runs on 12V & does not take a composite input.
The PSU, 12V -12V & 5V. An example of an early switchmode.
The FDDs were cleaned & lubricated the same way as the IBM 5150 & 5160 drives I have serviced a few times before. Pretty much consists of taking the drive apart & lubricating all the mecanical parts, cleaning the heads & any other dirt from the drive. Link here to a video: IBM 5150 Drive B: Repair
So thye next part as to test the video RAM that consisted of four MM2114N-2L RAM chips. these are a little expensive & hard to get these days. However Sam order a RAM tester.
I took the mobo out of the computer & desoldered all the chips & then inserted them into sockets. After I put the machine back together I tested to make sure the problem was the same, it was which was good.
The RAM chips were then interted in the tester & to my suprise they all tested good. I did a few more tests of all the chips to make sure. I also tested all the logic chips that were socketed just ot make sure none of those were the cause of the issue. Nice thing about my EPROM programer is it can test logic chips. Those all tested good which left me a little confused. So it was onto the next step.
So it was time to play musical chips. If you hit the reset button enougth times you could make the coruption go away. However it would soon come back.
It did not like the disks I created for it.
An intresting sympton was when I removed the fourth RAM chips the coruption wwould disapear. However the screen space would fill with random chars. The coruption missing was a major clue.
Here is a shot with the forth chips removed. If any other RAM chip was removed the flikering chars would remain.
So my partner was doing some random probing on the RAM chips & spotted something diffrent between them. PIN one on the first three was conected together, however the forth chip pin one was not connected to the chain. This was very intresting, while not a data or address line it was a control line on the chips. So I soldered a bridge wire to the bottom of the PCB as that looked neeter. It conteded pin one to the rest of the RAM chips. I then reasemnbled the computer & fired it up.
It worked, that was the last problem, some how at some point in the machines life the trace on the PCB had somehow become broken & just needed a jumper wire to replace it.
Another shot of it working with a disk I created it does not like. The machgine has now been retured to the musium. Also fixed it wheil I had COVID.