Kaypro 4 PSU Troubleshooting

I picked up a Kaypro 4 last night from someone I know. It's in good condition overall, and it does work. I'm really impressed with the condition of the CRT. It's still very bright, and it doesn't have any blemishes.

However, the power supply is a different story. Most of the time I can get it to power up fine, but neither of the floppy drives will work - they aren't getting 5V on the molex power connector. Both drives seem to work when powered with a separate PSU.

Also, there are times when the computer itself just doesn't turn on. When the power switch is turned on the fan runs fine, but there's no sign of life from the CRT, the power indicator doesn't light up, and the keyboard beeper doesn't work.

Any ideas where I should start with troubleshooting this system? I'm beyond excited to own it (it's been a long time since I've messed with a system this cool), but I want to go about repairing it properly. This is literally the first Kaypro machine I've been able to find on the used market locally, and I would feel terrible accidentally blowing it up or otherwise damaging it. I still haven't forgiven myself for killing the motherboard in my IBM PS/1 a few years ago.


  • Small update: I got the entire system running together. Both of the floppy drives are being powered externally by a Dell ATX power supply, and the computer itself is running from the internal power supply. It's not the most stable thing in the world, but it does work. The internal power supply is definitely weak though, as the CRT and the power indicator both occasionally dim a fair amount. This seems to result in the system hanging. It's recovered (by doing a warm boot) a couple times, but other times I've had to turn it off and back on.
  • Well, replacing electrolytic capacitors are a place to start. I hate removing parts that I don't know for sure are bad, but unlike most other components that either work or not, when electrolytic capacitors start to go bad they can create odd intermittent problems that can be impossible to pinpoint.

    As I understand it, power supplies are often the first part to go due to the high frequencies in a switched power supply.
  • @SomeGuy Yep, they're definitely on my list. None of them look visibly damaged, but it's been close to 40 years since they were new.
  • Solder joints failing around the hi amp load stuff.
  • Very good point.

    Probably best to go over the entire thing with a magnifying glass, look for bad solder joints, cracks, or any other less obvious damage. It could be something simple.

    Something like that very well might not completely prevent it from operating.
  • @02k-guy Thanks for the reminder about solder joints. Normally this is the type of project I'd just do by myself, but being me I know I'd forget to check for something, and I hadn't thought about that. I'll definitely check for those when I get the time.

    @SomeGuy You're right - it could be something simple. I hope it is anyway. If not I'll just do my best to rebuild it entirely, but I like to know why something is working the way it is (or not working, such as the case may be).
  • Trust me I own at least 8 or more Kaypros and have been through it ALL REALLY I'm 65 years old and I know these machines! I do what I call a RECAP on every Kaypro I have.
    I buy all of the capacitors that are on the boards and replace them all with new ones. Capacitors are the biggest problem with any old system if the degrade over time then they glitch and you may be in the middle of writing some code that you haven't saved yet and it is all lost! And I have rebuild with new parts old power supplies and got systems up and running also. Kaypros are very misunderstood systems but I have bought many and had to replace all of the ROM'S as people changed them out not knowing anything about the series of kaypros and the versions of the rom's to them and how they relate! I understand that the younger generation wants to know but you must do all of your research before you jump into projects like this old stuff or you destroy all of it just by stupidity! Have a nice day all!
  • Just go through the Power Supply component by component and buy new ones and replace them and it will work! I do it all the time.
Sign In or Register to comment.