Kaypro 4 Keyboard problems

[eight088]

Hey guys

I recently got a Kaypro 4 and I'm having problems with the keyboard. When I first powered it up, it came up with the usual insert boot disk screen and I also noticed that the keyboard also seemed to work (caps lock light would go on and off with each press and the speaker would also click when I pressed a key). So I got pretty excited that I have myself a working Kaypro, so went to my PC and made a boot disk for it.

After making the disk I then attempted to boot from it and success! It worked! Thinking it was working, I went to type in a command and they keyboard no longer seemed to be responsive. The caps lock light is always on whenever the machine is turned on, and no keys work.

After looking at some schematics, I tested the cable first and seemed to be okay, but there seems to be no signal coming from the keyboard to the SIO chip. (Or the hex inverter). So leads me to think it's an actual problem with the keyboard. I don't have much in the way of diagnostic equipment, but I do have a dmm and a diy logic probe (really need to get a better one though). Looking at basic things first on the keyboard, I've noticed that a couple of diodes have a voltage drop which is the same in both directions. One reads 0.5v and the other reads 0.02v (dmm in diode mode). Reading them in ohms, one is open circuit and the other is 46ohms, in both directions.

Is it likely a faulty diode is at fault? I didn't think diodes go faulty all that often, but they might be worth replacing? Only problem is I'm not sure what type they are, and I can't find a schematic for the actual keyboard.

Thanks

 

[durgadas311]

Just one note about those keyboards, they are not like "modern PC" keyboards where "Caps Lock" generates a code and the computer must actually turn on the Caps Lock LED. The Kaypro keyboard is self-contained, and the caps lock LED is controlled by electronics inside the keyboard. The Kaypro keyboard also has a very simple RS232 interface, and runs at 300 baud.

One caution before replacing diodes, if they are signal diodes (not used to rectify AC or prevent reverse-voltage on DC) then your measurements with a DMM may be misleading. In addition, measuring resistance of a diode that is in-circuit can give false readings, as there may be other paths for electrons that throw off the reading.

Can you tell if you have the original Kaypro cable for the keyboard? They use a different pinout than other, similar-looking, computers and I've heard of damage caused by using the wrong cable. Also, I forget whether the standard (U.S.) telephone HANDSET cable is the right pinout, but those are rarely satisfactory due to the use of too-thin wire (if you can even call that wire). Also, damage to even the official Kaypro cable is common, due to how it would be stretched so much when in-use. Length, and quality/gauge, of the wire/cable is critical.

 

[eight088]

Update...

Turns out there's a definite problem with the keyboard. I was able to get my hands on another kaypro, and tried that keyboard and it works fine! Stoked to have a working Kaypro! I dug a little further into the faulty keyboard, and I found that the reset line is being held active on the 8049 controller. The reset line is connected to one of the diodes I was suss about. I checked on the working keyboard and the diode measured 0.8v one way, and 0.5v the other way, and the original keyboard measured 0.5v in both directions.

Cheers