Back to The Serial Terminal from h**l!....
Having spent quite some time recapping most of the S100 cards in the IMS8000 computer I turned my attention back to the dreaded ADDS Regent 20 terminal.
When I started on this restoration I had in my mind all sorts of difficult scenarios resurrecting the 35 year old computer, digital oscillator board and keyboard console but to date the unpleasant surprise has been the serial terminal which has posed a string of issues that need resolving.
The biggest problem has centered around the Matsushita monitor chassis and picture tube. For reasons never likely to be known the glass pip on the end of the CRT was cracked off. Whether this was the result of an accident or whether it was deliberately let down in a workshop is subject to pure speculation.
As time as gone by I am starting to wonder if its in fact the latter as since making this discovery I have also noticed that the diode that rectifies one of the LOPT secondaries feeding around 70 volts to the top side of the video transistor has been removed and not replaced.
As a separate task I decided to source a monitor with similar scan rates and resolution as a potential donor replacement chassis for the original. It turned out that the old GT65 monochrome monitor for the CPC464 personal computer was very similar so I located one and purchased it.
On arrival I managed to feed the appropriate signals from the main processor board in the terminal to the monitor. After a bit of tweaking sure enough up came a stable picture but to my dismay the picture was far too wide and some 20% of the end of the lines were beyond the edge of the screen. Sadly this monitor has no width control and all manner of tweaking including the B+ rail had insufficient effect.
All was not lost...
What this proved was that the picture tube in the GT65 is good and has the same pinout, neck width and mounting positions as the failed CRT in the terminal. What it also proves is that the terminal electronics are working perfectly...
But if that wasn't enough...
To my frustration, having got a picture out of the terminal I quickly discovered at least 5 keys didn't work on its keyboard!!
I removed the keyboard and unscrewed the pcb assembly from the keytops only to be cruelly reminded of a past experience with a keyboard of the same design. The keyboard turns out to be a Cherry "Solid State" capacitive foam and foil type mechanism which are notorious for giving trouble if the foil pads have started to corrode.
Unfortunately it seems a report of beer being fed to the monster may not be so far from the truth!... sure enough all 5 failed keys were in a cluster with a curious sticky tidemark around them.
Fortunately this form of keyboard is modular so I quickly decided to remove the faulty keys and swap them for a number of lesser used keys having swapped over their key tops.
So at least I have all of the letters and numbers available again.
It seems this form of keyboard was not in manufacture for a very long time and I'm really struggling to find a specific model of keyboard from the era that could possibly be used as a donor for replacement key contacts.
So the next task is to remove the CRT from the GT65 (having remembered to discharge it first!!) and fit it in the metal chassis of the original monitor. With the help of another Regent 20 owner I then need to determine exactly what type of diode has been removed, fit it, and carefully attempt to power the terminal monitor up.
A slightly nicer task was recapping the Z80 processor S100 bus card which turned out very neatly:
And another successful task...
At 35 years old the contents of the IPL ROM (Initial Program Loader) on the I/O card was seriously living on borrowed time. The problem was that most modern EPROM programmers won't touch 2708 ROMS as they need several supply rails to read and write. I was really struggling to find a suitable programmer when I finally stumbled across the curiously retro Andromeda Research EPROM+ (AR-32a) that supports these chips as well as most of the fusable link Bi Polar PROMs on the digital oscillator board.
What I like about this programmer is the fact that the software is bootable and will run on just about any knackered old PC as long as it has a serial port. No drivers or anything are required - just stick the CD in and go... and they really mean it.. I grabbed a 15 year old laptop and within 5 minutes the system was up and running!
Only another 10 minutes later I had read the contents of the original IPL ROM (which is almost impossible to obtain!) and burn it onto another 2708 which booted and ran perfectly in the IMS8000... phew!!