🇩🇪 Robotron KC 87: Part 2 (Powering on) [TCE #0548]
38There is still an issue with the power supply - there is some sort of fault (short?) on the DC side which causes the 500mA fuse to blow almost immediately. The PSU is supposed to generate ±5VDC and ±12VDC, which is unusual (in my limited experience) for a Z80-based computer - maybe we don't need all of them ...
00:00 ... Intro
00:04 ... Trigger warning!
00:30 ... Power supply
04:00 ... Pin strapping
05:10 ... Baseband video output?
06:45 ... Checking video signal on oscilloscope
09:00 ... Testing the video output
10:20 ... Testing keyboard
11:34 ... Channel thanks & outro
I have a Mean Well GP25A13A-R1B with a 5-pin DIN plug that outputs ±5VDC and +12VDC ... I can't really see any use the the -12VDC on the board (maybe serial, but I'm not interested in that for now) so let's give that a try.
With just those three voltages (and GND) connected, I could see a reasonable-looking B&W/luma video signal on the oscilloscope ... it could be working!
And indeed it does seem to be! My guess for where to tap the baseband video signal also seems correct.
So ... we seem to have a working machine. Very exciting!
I have two options for the power supply - replace the original, faulty one with an internal Mean Well (or equivalent) replacement (if it will fit) or just install a 5-pin DIN socket and forgo the internal PSU completely.
I will also need to re-wire the video output so it outputs the baseband signal. And finally, wire up a cassette/magnetophone plug so I can try to load some software!
Regarding "East Germany" ... this seems to trigger some people. I can't help that the GDR/DDR was commonly called "East Germany" by the English-speaking Western Bloc. It isn't a sign of disrespect ... the FRG was always called "West Germany" ... we never called it the "FRG"! I caught up with some work colleagues (who grew up in the Soviet Union and Hungary) last week and I asked them what they usually called the DDR in Russian and Hungarian ... "East Germany"! I still have, and use, my dad's Praktica camera - I love it. So I certainly mean no disrespect about the DDR ... I wouldn't be spending time restoring their computers if I did.
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From Part 1:
The Robotron Kleincomputer (Small Computer) KC 87 is an East German (or DDR if "East Germany offends you) microcomputer released in 1987 and designed around their U880 (or UB880) processor, their clone of the Z80. The other computers in this range were the Z9001 and the KC85/1 ... not to be confused with the KC85/2, KC85/3 and KC85/4 that are different computers produced by a different factory.
An ASMR description of the computer ... 🧐
• Soft Nails ~ ♥ [ASMR] Kleincomputer Ro...
https://nadjabuttendorf24.com/kc87.php
Maybe the PSU schematic:
https://www.sax.de/~zander/kc/kcsch_6.pdf
Some other interesting links I've found:
https://oldcomputer.info/8bit/kc85_1/index.htm#pin
https://hc-ddr.hucki.net/wiki/doku.php/z9001
https://oldcrap.org/2018/05/02/robotron-kc-87/
Discussion about replacing the 166-5551 encapsulated mains filter (in German):
https://www.robotrontechnik.de/html/forum/thwb/showtopic.php?threadid=16520
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