• Please review our updated Terms and Rules here

Kaypro 1 text issue

xp0nentz

xp0nentz

Experienced Member
Joined
Sep 1, 2025
Messages
265
Hello, so i got a Kaypro 1 2 months ago, and it works fine but the text is displaying blocks and what looks like _s, .s,and pipe(|)s instead of the normal ASCII characters
i'll send an image if i can, but can someone please tell me what's wrong?
 
Last edited:
Maybe screen memory, or character ROM?
And as usual, re-seat all socketed chips before replacing them.
Over this much time, corrosion may play a role, even if you don't see it.
 
This is how it looks, sorry for the low quality. Yes it does display everything normally, just it has this garbled text. This is the screenshot from before I bought it, since I don't wanna take any pictures myself.
 

Attachments

  • Screenshot_20250902-154126.png
    Screenshot_20250902-154126.png
    300.7 KB · Views: 30
I'd need a clearer picture in order to have any idea what the bit errors might be. What we don't know for sure is whether data is getting corrupted when it is written to the CRT RAM, or when it is read (refreshed). But the general data path is from CPU bus to U14 latch, to video RAM U23, then during refresh back out to U8 latch to char gen ROM U9. Note that it's not impossible for the address input lines of the ROM to get shorted/open, so reading that in a PROM programmer might confirm the right contents (there should files available to match the ROM part number). Since it got this far, I'd assume the CPU data bus is fine. You could try swapping the two video RAM chips at U23/U15. If the RAM is bad, you'd expect at least a change by swapping them (unless they are both bad in the exact same way).

For taking pictures of the screen, it helps to reduce the brightness, shine a light on the screen, and make certain you're not too close to focus.
 
durgadas311 said:
I'd need a clearer picture in order to have any idea what the bit errors might be. What we don't know for sure is whether data is getting corrupted when it is written to the CRT RAM, or when it is read (refreshed). But the general data path is from CPU bus to U14 latch, to video RAM U23, then during refresh back out to U8 latch to char gen ROM U9. Note that it's not impossible for the address input lines of the ROM to get shorted/open, so reading that in a PROM programmer might confirm the right contents (there should files available to match the ROM part number). Since it got this far, I'd assume the CPU data bus is fine. You could try swapping the two video RAM chips at U23/U15. If the RAM is bad, you'd expect at least a change by swapping them (unless they are both bad in the exact same way).

For taking pictures of the screen, it helps to reduce the brightness, shine a light on the screen, and make certain you're not too close to focus.
Click to expand...
Alrighty here
 

Attachments

  • PXL_20250620_044800700.MP.jpg
    PXL_20250620_044800700.MP.jpg
    629.6 KB · Views: 21
Looks like the character rom is cactus....

Your first image made it look like some characters were coming up, but were getting interposed.

If you can find a different character rom just to swap it in and check, that will save time doing any deeper electronic troubleshooting until you confirm whether it's good or not.

Otherwise you'll need to trace the circuitry that drives the character rom and see whether something isn't right there.

From the picture the fault appears to be on the outside half of the character rom circuit? So it could also be the counters that progress through the ROM addresses, or the output from the video RAM to the address lines of the character rom, but I'm guessing it's most likely the ROM itself.

I assume you've booted the computer and are typing things in, except that characters aren't coming up - is that correct?
 
It looks like something more than just character generator ROM. The patterns look more like something involving interlace malfunction. Perhaps the CRT controller chip, hopefully not the custom-logic chip. Probably going to take some deeper diagnostics to tell what exactly.
 
Assumed so, no idea what to do since kaypro 1 chr roms are probably very hard to find, best guess is chr rom since everything related to the crt from the outside is perfectly ok, these are all types characters and I don't know what's wrong at all, it's pretty confusing to me
 
xp0nentz said:
Assumed so, no idea what to do since kaypro 1 chr roms are probably very hard to find, best guess is chr rom since everything related to the crt from the outside is perfectly ok, these are all types characters and I don't know what's wrong at all, it's pretty confusing to me
Click to expand...
The character generator ROM is a "standard" pin-out and could likely be replaced by an EPROM or modern EEPROM. Anything 2732 compatible should work. But, the ROM you have is probably mask-programmed at the factory so is less likely to fail in the field. Still possible, and a probably the first thing to try. You can just try and read the one you have and compare to the image available online. The socket should accept a 2764, which can easily be replaced by a modern 28C64 EEPROM. Those are usually easier to program than 2732/2764.
 
Programming 2764's can be an issue, but mainly due to the historically higher programming voltage ( eg, 25v or 21v ) with most modern programmers not going above around 17v. Hence why the EEPROM version might be better.

Which version Kaypro do you have? Do you have a photograph of the main board? Some have discreet counters generating the video signal and others use a CRTC IC to generate them.

A programmer suitable for 28c64 should cost somewhere around USD$40 or so - not expensive.
 
Last edited:
Another option to make your own 28c64...


xp0nentz said:
If it were to be with the crt what would it look like
Click to expand...

It's not with the CRT - that looks fine in the images.

The suggestion was the CRTC - The CRT Controller chip - it generates the timing and addresses to drive the CRT and the address signals that go to the video character ROM.

Generally, it *could* look like what you're seeing - if it was generating the same address due to a short or missing address lines, then you might see something where one of four possible bytes for the upper character is repeated four times, then the same for the lower four bytes. This could be figured out by looking to see if the address lines to the character rom are all oscillating and changing regularly, but generally you'd probably need a logic analyzer to diagnose the issue.

You can get a cheap logic analyzer also that may be of assistance in diagnosing the problem.

Something like this:
Logic Analyzer.jpg

Would cost around US$ 5 shipped.

David.
 
The Kaypro 1 was the last model made of the 8-bit ones. Only the first-generation Kaypro 10 used discrete logic (vs. custom ASICs) for the DRAM and CRT support circuitry. But all of these later models used the 6545 (or 6845E) CRT controller chips. If you don't have a programmer, picking one up might be a good idea. Then you can read the existing character generator and see if it is good or bad, and if need be program a 28C64 to replace it. Or someone near by and program a 28C64 and send it to you.
 
The video is generated by two main chips, the 6545 CRTC and the 81-189 custom logic chip. The patterns look like the top half of the character cell is using a different character code than the bottom, which is difficult to explain by simple shorted/dead address lines. Since the Kaypros use interlaced video, it seemed possible there was something amiss with the CRTC or 81-189 affecting timing. Just a guess, but it should be simpler to check the character generator ROM first.
 
durgadas311 said:
The video is generated by two main chips, the 6545 CRTC and the 81-189 custom logic chip. The patterns look like the top half of the character cell is using a different character code than the bottom, which is difficult to explain by simple shorted/dead address lines. Since the Kaypros use interlaced video, it seemed possible there was something amiss with the CRTC or 81-189 affecting timing. Just a guess, but it should be simpler to check the character generator ROM first.
Click to expand...
To me it all looks the same
 
xp0nentz said:
To me it all looks the same
Click to expand...

Go here: https://www.manualslib.com/download/1022840/Kaypro-Kaypro-Series.html and download the technical manual.

Look on Page 52 - I assume that's your video schematic.

You can find an image of the board here: https://inanis.net/wp-content/uploads/2014/03/kaypro1_mobo_hidef.jpg

If that's the one, then this is your character generator ROM.

Kaypro PCB.JPG


Although a 28c64 as recommended is larger than the 2732 installed, it's a large enough socket and suitable circuit for it to fit.

You may want to copy the file twice into memory though using a 28c64 to ensure you don't accidentally use the blank side of the chip.
 
Were you able to remove the cover and get a picture of the mainboard? It would need to be hi-resolution and clearly focused. It would be good to see if the character generator is the right part and installed properly.
 
You must log in or register to reply here.
Back
Top