Kleincomputer KC 85/3: Part 3 (Repairs & First Boot) [TCE #0444]
45Let's continue working on the East German (DDR/GDR) Robotron KC 85 "small computer" that runs on the East German Z80 clone called the U880 (UB880 in this case).
A lot of excellent information ... and photos ... can be found here:
https://floooh.github.io/virtualkc/p010_kc85.html
After fully disassembling (with some effort) in Part 2, it was possible to make some small repairs. There were a few 100nF ceramic capacitors in poor state, and one that had been removed completely. I also further removed the RF video output cabling and fixed a bridge track on the expansion board.
I cabled up an RGBS cable for the GBS8200 according to the documentation, using the FBAS (composite video?) signal for the synchronisation input.
The B-row is the topside of the edge connector, and the A-row is the underside (which is pretty standard I guess). Pins are counted from the right to left.
Blue = B4
Green = B6
Red = B8
Synch = A10 (FBAS)
Ground = B3 (or B6, B7, B9)
And ... it seems to be working! It booted into HC-CAOS 3.2, where CAOS stands for "Cassette Assisted Operating System". The latest available version is 4.2 but I don't know if that's for the KC 85/4 only and I need to stick to the 3.x branch for the KC 85/3.
Now that I've figured out the RGBS pinout, I'll probably wire up a 5-pin socket (or 6+ pin to include audio?) and install that on the case. I want to also replace the Tesla MA7812 linear regulator in the power supply with a modern version from EZSBC.COM to reduce on waste heat and power draw.
If possible to upgrade the CAOS version to the latest, I'll give that a go ... will need to de-solder the ROM.
The most challenging part will likely be the re-assembly of the machine & case.