Kaypro 2 - Both Floppies active, no display

[piddlepup]

Hi,
I have a Kaypro 2 that won't boot. There was a CPM disk in the A drive, but it looks like the surface may be scratched.

Symptoms:
- Nothing on display even when adjusting brightness.
- Both drives are lit up and the power light is lit.

Observations:
- Neck on the crt is glowing.
- The large Rifa cap is blown on the power supply, three caps are showing shrinkage, but no signs of leakage on the board. Someone wrote "Bad PSU" on one of the aluminum fins. No signs of after market soldering done on the board.

The power to the main board at the connector is showing 4.8V, 11.84V and -12.4V without the board connected.

My plan is to replace the PS with a modern replacement and check the monitor board for broken solder joints.

Any other suggestions?

Thanks

 

[durgadas311]

First question is whether this is a "Kaypro II" (2/83) or a model 2/84? When you say that both drives are lit up, do you mean that both "drive select" LEDs are on? Or simply that both motors are spinning? When you say the display is blank, do you mean that there is a "blank" taster scan when you turn the brightness up? Or is the display truly blank - not being scanned at all? Of course, for both models the video board (or CRT) could be bad in which case you get no raster.

On a 2/83, for the display to show no raster probably means that the circuitry to generate the video signals is not working (could also be the video board). Seeing a blank raster means that the CPU and ROM were functional enough to clear the display RAM - but it did not get far enough to print the greeting message on the screen. If the CPU/ROM were not functional, but video was working, you'd get a display full of random characters.

On a 2/84, a no-raster display might mean that the CPU and ROM did not function enough to initialize the CRT controller chip, but if there is a raster scan pattern when the brightness is turned up, that means the CRT controller did get initialized (and video RAM cleared) but the code failed to reach the point where the greeting is displayed.

If both drive LEDs are ON, that's different problem - but also has different meanings for different models.

 

[piddlepup]

Thanks for the reply Doug.

1. The sides and back say Kaypro 2. There is a PN 81-014 number on the back and the main board has a ROM that is labeled 81-292.
2. Both drive LEDs are ON as well as the power one. I can hear a hum, from the drives like a fan. This is with no disk in the drive.
3. There is no- raster signal on the display at all. I've tried adjusting the brightness and there is no change. Nothing is displayed on the screen.

So far, I've replaced caps on the PSU and the video board, reseated all the chips on the main board (and sprayed sockets with contact cleaner) but there is no change.

Thanks again for your assistance.
-Keith

 

[cj7hawk]

You can always run an old computer from a modern switch mode PSU, though if something is shorted, smoke will ensue. Usually a computer only requires 5v to operate to screen, so I use a lab supply and dial the current back to around 1A to test, and if I don't see 5V on the chips, I might lift this to 1.5A, but a Kaypro shouldn't consume more than 1.5A at the 5V rail, so if something is shorted, this is a useful test.

At 1.5A, any chips suppressing the 5V line will get warm, so a finger test can help identify possible candidates. Also tantalums are common for shorting and dropping rails.

A common reason for both floppy lights on at once is the drive cable is in backwards... Always check this on any situation when FDDs are both active, especially if they are spinning.

It won't affect the computer operation however.

Check the voltages with the board attached. If you have 5V established, and there's no video, it may be a board fault. To be fair, this is about where you want to have an oscilloscope to see if video data is going to the screen, which tells you whether it's the screen at fault, or the board. Kaypro's are all TTL IIRC, so video data should be obvious.

If there's no video and the main board has stable 5v, then you can start checking things like the CPU is running, and the ROM is being accessed. Video should occur even if the computer is not starting though, so checking for failed power rails is useful. Make sure the video ROM is also being accessed. Failed clocks can stop video output even on hardware boards.

What tools do you have for electronic fault finding?

 

[durgadas311]

Looks like you have a 2/84 model, so it has a CRT controller chip that doesn't produce any video signal until the CPU runs the ROM code to initialize it. If both drive LEDs are ON, that could indicate the "system port" has invalid data, as if the software crashed after it started to initialize things. Although, drive cable backwards could also explain that. Perhaps the next thing to check (after drive cabling and power) is that the ROM still has a valid image, but unless you have a EPROM programmer it may not be easy to check.