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• MZB-3 CPU Card

  Will • 05/24/2023 at 09:15 • 0 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 | Hackaday.io]

  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?

• VDU-K (Graphics Card)

  Will • 05/24/2023 at 08:56 • 0 comments

  Starting my Interak build with the 'Graphics Card' (to use more modern terminology) seems wise. It will allow me to see what the other system cards are doing when I get to them, and it has the bonus of being standalone. A VDU-K card should display a screenful of random characters even with no other cards present in the system.

  Sadly, my VDU-K had been stripped of all ICs, and they're not as readily available as they once were. 

  Trawling a few component suppliers (and eBay) turned them all up though. The timing generator and character set EPROMs were going to be the most troublesome - I was expecting to have to program those - but another request into the Interak community revealed someone with spare EPROMs and the ability to program them. They're a helpful crowd!
  

  My first check was just to power up the bare board, check the supplies were ok and that no electrolytics went bang. I'd always recommend replacing vintage electrolytic capacitors, but in this case I've left the originals in place, for now anyway.

  Ok, time to populate the board.

  A final check over (the EPROM VPP/CS links threw me for a while) and now for a test...

  Yay! Those look like random characters to me. I need more time to test the composite video output and to see why the tv video is slowly rolling, but I'm impressed that there's any output from a 40 year old card!

  ---

  A quick browse of eBay, and I'm now the proud owner of a small compact monitor with a composite video input. Even without a proper cable it's a lot clearer than a TV picture. Still just random characters though.
  

  Time for a proper card front with switches and connectors... and on to my next problem! It seems my cardframe is also deeper than it should be - and with a faceplate fitted the cards don't reach the backplane! I briefly considered trying to relocate the backplane further forward, before deciding a small extension to the faceplate mountings would be easier. Two small strips of aluminium later and my card now fits fine, complete with faceplate.

  I still only have a screen of random characters, but at least all of the switches and outputs are working, and the card seats nicely in the cardframe. Time to take a look at the MZB-3 CPU card.

• 19" Sub Rack and PSU

  Will • 05/23/2023 at 15:23 • 0 comments

  First I need a card frame to slot my Interak cards into, a backplane and a suitable PSU. I thought I'd found something usable in an eBay job-lot of surplus comms kit, but it wasn't quite as straightforward as I hoped...

  My sub-rack came with nice dual redundant PSUs, which looked great but sadly only output +/- 14.5V. An Interak needs (mostly) +5V, and occasionally +/-12V too. I thought briefly about keeping the existing power supplies and regulating the voltage, but as only two rails were wired for redundancy (and I wanted three) there seemed little point. I junked the existing SMPS and fitted another of the same size but with correct voltages to one of the plug in PSU cards. I took the opportunity to fuse the individual supplies at the same time, as I'm planning to add crowbar protection later. Only fitting one PSU card has the bonus of leaving space for floppy drives at the other end of the card frame too.

  I was fortunate to find an original Interak backplane on eBay. It looks to be in good condition, other than it needed a haircut on the track-side. Whoever mounted the edge connectors didn't put the most care into trimming the leads - no doubt fine when it was safely mounted in a rack, but 40 years of storage has bent the legs, some almost shorting together.
  

  But how does the backplane mount? The 'new' backplanes on sale today are larger and drilled with fixing holes but mine has nothing like that. 
  

  My first question to the Interak community soon yielded the answer - I need edge connector fixing brackets. The only problem there is they seem to be obsolete and unobtainable! By sheer luck I found three old edge connectors on eBay that still had them fitted, which has at least allowed me to trial fit the backplane. Beyond that I'll probably need to manufacture them.
  

  Things are looking good. But do the cards fit? Erm, No! It seems this card frame wasn't made for the 4.5" x 8" International Size Cards that the Interak uses. 

  The fix was to remove the top and bottom rails and then remove about a 1 mm of thickness from each. Success, the cards now fit the opening.

  Things are looking good again, I just need to fit the guide rails and the case is all done. 

  Wait a minute, is the card bowing out the case? Yes it is! With the guide rails fitted there's insufficient clearance again. That isn't going to work. Cards might fit (snugly) in the middle of the rack but the ends of the case are unusable. The easiest option would seem to be to re-profile the guide rails so they sit lower in the card frame.

  That works. I'm just not looking forward to filing-down another 25! Maybe re-profiling a pair at a time as I progress to the next card will be ok though.

  Now the card frame really is finished. Time to move on to the actual cards.

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