Imsai VDP-40

[guibrush]

Hi, a friend of mine got his hands on a nice Imsai VDP-40.


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:

sometimes we have the repeating pattern, letting me thing that the CPU is initialised, but no ROM is found:

sometimes we have a prompt:

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

https://www.swisstransfer.com/d/27e68f93-7851-4499-9bd5-a5554eb9f7a0

Thanks for your help!

 

[nullvalue]

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?

 

[retrogear]

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