🇸🇰 Tesla PMD 85: Part 6 (Video output!) [TCE #0446]
22First, repaired the PMD 10 power supply by replacing the 0.1µF class X safety capacitor that failed - I'm assuming it failed short circuit as the F2.5A fuse also blew. Replaced this capacitor with a similar Miflex branded class X capacitor and put a new fuse in (F3.15A as it's all I had handy).
Thanks to@jjermar1's advice in TCE #0418 comments, I've now got B&W video output. Simply, put a 100µF electrolytic in series with the video output to block any DC (I guess) on the video signal that modern displays might not like ... positive leg to the video output wire and negative leg to the DIN connector pin.
So it boots but it's like a key is being held down - in either the system monitor (no ROM) or with BASIC module installed. Jakub also suggested, in TCE #0419 comments, that it could be due to the conductive rubber domes in the keyboard having disintegrated and causing a short on the keyboard traces ... so pulling apart the keyboard will be next.
Update: Fixed the keyboard issue, maybe. Replaced the original Tesla MHB8255A part (interface adaptor) with a western version ... and now it boots (without BASIC module) to "Os ready" ... no more continuous key repeating!
For the algorithm (from earlier videos):
The PMD 85 was an MHB8080A-based machine produced for a few years from 1985 in the Slovak part of Czechoslovakia.
The MHB8080A is/was the Czechoslovak clone of the Intel 8080.