🇷🇺 Микроша (Mikrosha): Part 3 (It's Working But ...) [TCE #0628]
28Thanks to #PCBWay for their ongoing support! https://www.pcbway.com/
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.
00:00 ... Intro
02:50 ... Panelisation by PCBWAY
06:25 ... The daughterboard
07:30 ... Preparing the daughterboard
11:30 ... Installing the new key switches
13:00 ... Testing fit with key caps (NOK)
16:50 ... Initial motherboard checks
21:30 ... Oscilloscope check on CPU, CRT controller & КР565РУ6 DRAM
27:00 ... Checking video signal
28:45 ... Checking video output (LCD)
34:20 ... Final motherboard look (key beeps!)
35:10 ... Channel thanks & outro
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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 ... ?
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From Part 1:
This just arrived from Belarus today - this Микроша (Mikrosha) was based on the home brew Радио-86РК (Radio-86RK) design and built in the Russian SFSR. This particular example was built in 1988 and cost (цена) 500 Soviet Roubles ... about US$120 (1988). This might seem cheap but consider that this equated to about two months salary.
As it's based on the 86RK it uses the Intel 8080 family of chips (well, the Soviet equivalents). This means that it also requires a tri-voltage power supply: ±5VDC and +12VDC. It does have one but the fuse has blown so I'll need to replace it before attempting to power on the power supply.
It apparently has some differences to the pure 86RK ... for example, the 8253 PIT used for sound. One other major difference to the other 86RKs I've worked on ... this appears to have baseband video output, rather than RF modulated ... yay!
Once I get a replacement fuse and check the pin out of everything from the documentation, I'll be able to try powering it up.
https://github.com/alemorf/retro_computers/tree/master/Microsha
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