I like this adventure naming scheme we've got going
I've had this thing for at least a year now, and never quite made it around to working on it. It arrived with a little rust, and a cut off power cord.
Even when I did start working on it I couldn't find any information at all beyond an original advertisement and people's fond memories.
I'll post up more photos over the weekend, but mentioning it now because I had a bit of a win tonight.
- one of the large 2200uF caps had blown off the board taking the trace with it, cap was fine, so I bridged it with solder to recreate the original trace, and used hot glue to keep the cap in place
- a 74LS series IC was burning hot, replaced this with a fresh component from another board
- resistance on +5V line kept getting lower and lower after each power on, so tantalum capactiors were removed (resistance went back up and remained stable) before I got shot in the eye
- used an oscilliscope to verify HSync and VSync signals - but no video feed, and nothing on the CRT
- visual inspection of CRT board, nothing stood out
- used oscilliscope to check the RAM chips were getting and sending signals (easy with 4116's because there is seperate Data In and Data Out pins)
- removed chips to see if conditions changed - they did - 3 symptom modes - flashing Caps Lock, flashing ON, no lights - so it was doing something - symptoms often varied for no reason on some occasions, rarely I'd also get beeps on cold starts
- replaced NEC 780 (Z80) CPU and CPU socket - symptoms now consistent - with everything fitted, ON light was flashing on keyboard, no more weird beeps.
- found a Texas Instruments CPU/Graphics controller chip and it's 8116T DRAM - ordered replacements as the TI chip fell apart when I removed it from the socket
- contacted Herb who sells/scans S100 manuals, and had a Visual 200 user manual, turned out a successful POST results in the CAPS LOCK light turning off - which it was. He also mentioned that the system should display a white cursor and nothing else on startup.
- suspected video circuit, so installed new TI chip
- quickly reassembled while my wife was watching, gave her fair warning it might go bang but ended up saying "oh wow, I've got a steady ON light, what's on the screen?" "a white box" she said!
I still need to test the EIA (RS232 in cool kid talk) interface works, and need to put all new foam pads in the keyboard, but I'm extatic with the fact that this 1981 workhorse is alive again.
I'm also amazed the 34 year old video board, which relies heavily on electrolytic capacitors and large current flows, works without any adjustments.
Quick video I shot on my cellphone before I had to come back inside: https://twitter.com/BaudBand/status/664723410719604736