🇷🇺 Микроша (Mikrosha): Part 4 (Video Output Fixed!) [TCE #0629]
37I got the video output working! It was staring at me ... and I was commenting on it ... the whole time!
Like any 86RK-based machine, it does take a while to warm up and actually start working - pretty much all of them require the [RESET] button to be pressed several times before it becomes stable.
In addition, this machine actually seems to need to warm up ... first thing in the morning it will refuse to work, but after being powered for a few minutes (and, indeed, being powered off/on a few times too), it'll eventually boot and then work more reliably.
The video output issue was due to the missing 10µF electrolytic and shorted 22nF ceramic capacitors which meant that the DC component of the signal wasn't being blocked. Once I added these back into the circuit, the video output started working (once the computer was working, of course).
So, the next issue is sorting out what I'm going to do with the keyboard. I don't know how much longer the original Soviet-era key switch mechanisms will work.
00:15 ... Checking the schematic
03:30 ... Checking the motherboard
05:30 ... Replacing the capacitors
07:00 ... Testing, testing
09:30 ... SUCCESS!
13:20 ... Checking signal on oscilloscope with 51Ω resistor
17:00 ... Checking with 75Ω resistor
19:55 ... Checking video signal components
26:55 ... Channel thanks & outro
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From Part 3:
This is a two part video: first we look at the daughterboard I designed to possibly replace the original key switches, and then we continue fiddling to see if the machine is working.
The key switch daughterboard design was covered in TCE #0625:
• Simple KiCad For Simple Vintage Computer H...
The design can be found on my Github - obviously some further work is required:
https://github.com/0ddjob/Conversion_Boards/tree/main/Soviet-Keyswitch-Replacement
A common issue with the Radio 86RK design is that they never seem to boot "first time" ... it usually takes a few presses of the [RESET] button to jolt them into life. This one is no different.
After a LOT of coaxing, the machine does in fact seem to be working ... I'm getting key press sounds! ... but the my LCD (and composite-to-HDMI converter) can't lock onto the video signal.
So that's the next hurdle to sort out.
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From Part 2:
Sorry, a bit of a long-winded one with no payoff at the end - the machine isn't working at the moment.
I removed all the key caps and gave them a wash. The underlying key switches look a bit dodgy ... might look at creating some sort of converter boards so I can use Cherry MX key switches.
The power supply seems to be working okay - I had to replace the fuse holder as the original Soviet 500mA slow blow fuse is smaller (15x4mm) than the M205 (20x5mm) fuses I can source. But that was pretty simple and the power supply seemed to be working quite fine.
The Микроша itself seems to be quite dead - there was a clock signal being generated by the 8224 (КР580ГФ24) but I didn't see any activity on the 8080 (КР580ВМ80А) or the DRAMs (КР565РУ6). No chips looked particularly hot so no obvious shorted-IC that could be causing the problem.
There's a lot of bodge work ... cut tracks, bodge wires, re-soldering work ... so no idea what has been done nor why - it's one of the bodgiest 86RK clones I've come across. Could the issue be as simple as a shorted address or data line ... ?
https://github.com/alemorf/retro_computers/tree/master/Microsha
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