MZB-3 CPU Card

A project log for Building an Interak

My attempt to build an Interak - a 1980s Z80 based computer

willWill 05/24/2023 at 09:150 Comments

I was fortunate to stumble across some old Kemitron Computer cards on eBay, amongst which was an MZB-3 CPU card. The Kemitron and the Interak share a common bus definition (ISBUS-A) and the same card form factor (International Size Cards). Some early Interaks even used Kemitron cards, so there should be no problem repurposing one. Hopefully all I'll need is a new Boot ROM.

[Looking at my pile of Kemitron cards got me wondering... what is a Kemitron? Could I build one? It sounds like I need to create a new project! Something like: Kemitron Computer |]

With a now (hopefully) working VDU card, it's time to try the MZB-3 CPU card. I fitted it into my system, powered it up, and... nothing! No change to the VDU output. That's disappointing.

I checked the supply voltages and they were fine, checked the clock and the dynamic RAM refresh clock and they were both fine too. Then I happened to look at the 5V rail on a 'scope - whoa, a huge ripple at the clock frequency - or so I thought!

Having swapped a few capacitors to no effect, I was thoroughly confused. Until I stumbled across an article online that said never use a x10 probe to measure anything near RF - the inductors in the probe pick up stray RF. The massive ripple I thought I had was in fact non-existent,

So what next? Maybe the EPROM has failed - allegedly they do after 20 years or so, although I've never seen it personally. Or a failed component?