🇸🇰 Tesla PMD 85: Part 6 (Video output!) [TCE

First, 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.

[@Aeduo]

el garbo is going to have a hard time with weird old computers. :p

[@jjermar1]

Glad to see you have video output. As for the keyboard beeping, a good way to test the disintegrated dome theory is to carefully flip the computer over while it's turned on. The beeping should cease then. But since you are now mentioning a different cause, maybe you are lucky and won't have to disassemble it. Well, eventually you will have to anyway as all these keyboards will have that issue sooner or later. If you ever want to disassemble the keyboard, not sure if somebody hasn't mentioned this here already, but basically you need to cut the plastic rivets, do whatever you need to do to clean/fix the key mechanism, turn the bottom of the key 90 degrees and screw it back with fitting screws.

[@Brfff]

Am going to try the keyboard today ... I know *some* keys are working ... like [SHIFT] [RST] ... but am dubious about others, so I'm expecting to have the issue. In Yura's (@yurasergienko6890) comment he pointed to video by a Hungarian fellow where he tells also about replacement rubber domes on Aliexpress: kzbin.info/www/bejne/iYuWiWWliLqcrLcsi=S5t4uMJRMcQf75sG&t=1089 Archived Aliexpress link in case someone reads this comment in 2044 and wonders what type they were: archive.is/0Sk09

[@MrWaalkman]

Hey! It lives! As for the conductive domes shorting out, didn't you have the keyboard unplugged the first time that you fired it up? As for a keyboard controller chip, I would think that it has one. And for improving my video output I bought an old Extron 112 (I think...) and it cleaned things up pretty well. Did you see the Aurora Borealis? No luck here.

[@Brfff]

Oh ... yes, you're right - it was printing out garbage before I connected the keyboard. Duh ... I'm a bit oblivious sometimes. Weather here has been pretty rubbish lately - just constant rain. Compared to 2019 or 2020 (just before Covid lockdowns started) when we were on strict water rationing ... we had to fill up pool up with a water tanker as we couldn't use tap water, and we couldn't water the freshly laid turf at all. Now there's too much water.

[@Brfff]

Fixed it! I had another look - it's just using one of those 8255s (interface adaptor) ... so tried swapping the original Tesla part for a Western one ... and it's working!

[@MrWaalkman]

@@Brfff Woo-Hoo! Alrighty then!

[@yurasergienko6890]

I also have a PMD, and I also encountered the fact that the keyboard is terrible. Some buttons get stuck and you have to press hard to press them. Other buttons give false positives and auto-returns.

[@Brfff]

I've been thinking about designing a replacement, modern keyboard ... if no-one has done it yet ...

[@yurasergienko6890]

@@Brfff There is a video on KZbin where a guy is trying to improve the keyboard. kzbin.info/www/bejne/iYuWiWWliLqcrLc

[@Brfff]

Got the keyboard to stop repeating - seems, in my case, it was a dodgy 8255 interface adaptor (Tesla MHB8225A) - replaced it and it seems okay now ... more testing tomorrow.

[@Brfff]

Excellent, thanks ... and he provides a link to Aliexpress where he got replacements!