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Imsai VDP-40

G

guibrush

Member
Joined
May 3, 2023
Messages
40
Hi, a friend of mine got his hands on a nice Imsai VDP-40.
IMG_4837.jpeg

We are trying to bring it back to life, but have some problems getting a working prompt. We have certainly 2 different problems as we have different results when the machine is powered on:
- sometimes we have only a black screen
-sometimes we have some garbage on the screen:
IMG_4846.jpeg
sometimes we have the repeating pattern, letting me thing that the CPU is initialised, but no ROM is found:

IMG_4840.jpegsometimes we have a prompt:
IMG_4847.jpeg
but unfortunally the keyboard isn’t responding.
Sometimes the keyboard bip without ending too.

We don’t really understand how the boot process should be, I was assuming that the monitor rom is loaded at boot and then a command have to be typed to access the rom on the floppy controller to load the disk, but I’m not sure. Did anyone know how it’s supposed to work?

We dumped the content of the floppy controller ROM


Thanks for your help!
 
Very cool! Loving that wood-grain :) These patterns you're seeing on the screen are the result of an un-initialized display controller. In my (relatively short) experience, the cause is bad RAM, preventing the CPU from loading the ROM/boot code. Would you mind sharing which cards you have installed?
 
the keyboard issues could just be deteriorated sponge in the contacts if that's the type it uses. There are replacement kits available.
The metal discs on the sponge can fall and make phantom keypresses. Maybe the reset line is floating so not always getting a reset.
A proper reset will give a cursor like one of your photos.

Larry G
 
I had an IKB1 back in the late 70s early 80s and I'm pretty sure they weren't foam/foil. More likely corrosion on the connectors or a loose connector is the problem. Check to see that the ribbon cable is properly on the parallel port. You will need to lift the lid and look about. Surely that is an IKB1 without some LEDs? But I have never seen a VDP-40 except in adds so I could be wrong.
 
By the way I do have an original "VDP~40 Reference Manual". I expect You have a MPU-B 8085 Microprocessor card installed and a VIO-D video card. I would be very interested to see which other cards are installed and how they are configured, jumped and switched as well as how the cables are connected.
 
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