Альфа БК (Alpha BK): Part 3 (Got Video Working!) [TCE #0453]
30Sorry, a bit of a long one ... but I do finally get a video signal out (quite noisy), however the machine is working intermittently ... it was working with a greeting & flashing cursor, but then decided to stop working (still displaying video though).
So, what did I do? I will try to summarise ... TL;DR it seems the CPU (КР580ВМ80А) was at fault.
There wasn't a valid video signal - nothing the screen could use, no luma information.
First I noticed that there was no data coming out from the 2KByte character ROM (D9) - the address lines were active, but data lines were not. I then noticed that the EPROM's !CS (or !OE?) never transitioned from high to low ... so that's why there was no data coming out.
The EPROM's CS or OE (can't remember which) was tied to the 8275 CRT controller's VSP (video suppression) pin 35 - this pin disables CRT output. Ahh, this makes sense - hence why there is no video signal.
So I de-soldered the Soviet КР580ВГ75 and installed a 40-pin socket. This is not a very good fit due to the difference between Soviet 2.5mm pitch and Western 2.54mm/0.1" pitch ... only 0.04mm difference between two pins, but it adds up over 20 pins.
I installed a genuine Intel 8275 ... but no difference.
Then I noticed the 8275 has a !CS pin of its own. This CRT chip select comes out from a 3-to-8 address decoder (D13) ... this is a Soviet К555ИД7 (or 74LS138). This never deviated either, so the 8275 was never being activated. The decoder input is A12, A13 and A14 from the 8080 (or Soviet КР580ВМ80А) itself ... so I de-soldered the 8080 and socketed a replacement.
And ... I got a video signal. Just a flashing cursor and a noisy screen ... but more than I had before.
I tried reseating the firmware EPROM (D17) ... the only socketed chip on the board ... and I got the boot screen ... although the characters were a bit messed up. They intermittently changed ... so maybe an issue with the character EPROM (not socketed)?
I tried putting the board back into the very heavy case (that is really starting to annoy me now - very awkward to handle whilst trying to also troubleshoot), but it stopped working - still got video output though.
So maybe the dodgy Soviet EPROM socket needs replacing too.
From the Part 1 video description:
The Alpha BK is a mass produced version of the famous Радио 86РК (Radio 86RK), an Intel 8080 (clone) based home brew computer detailed in "Radio" magazine (in 1986):
https://en.wikipedia.org/wiki/Radio-86RK
Lots of info here - including a modern version of the original Radio 86RK design:
https://github.com/skiselev/radio-86rk
Information about the Alpha BK:
https://github.com/alemorf/retro_computers/tree/master/Alfa-BK