Альфа БК (Alpha BK): Part 2 (Troubleshooting Begins) [TCE

We first looked at this machine about a year ago, way back in TCE #0297. I didn't have any 8080-related spare parts (such as the 8257 DMA controller or 8275 CRT controller) so assumed "can't fix it right now" ... basically, powered it on and got no video output ... "too hard" basket.
But I have those parts now so finally decided to have another crack at it.
I have done some troubleshooting - essentially, I'm still not getting any video output. However, it does seem that there is some output when I check on an oscilloscope ... but is it inverted compared to "normal" Western baseband analogue video signal ... ?
Connecting it up to the Eyoyoyoyoyoyo LCD display shows nothing ... it senses that there is some sort of signal ... the "no signal" message disappears ... but there's nothing there it recognises. Same result with the El Gato USB capture dongle.
I think I might check Sergey's re-implementation of the Radio 86RK for a hint, and perhaps also check the original documentation (see below).
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):
en.wikipedia.org/wiki/Radio-86RK
Lots of info here - including a modern version of the original Radio 86RK design:
github.com/skiselev/radio-86rk
Information about the Alpha BK:
github.com/alemorf/retro_comp...
ОТКЛ (отключить): OFF
ВКЛ: ON
ВНЕШ УСТРОЙСТВА: EXTERNAL DEVICES
МФ: magnetophone, i.e. cassette recorder
ТВ: TV
Soviet parts used with Latin transliterations & equivalent Western part:
КР580ВВ55А (KR580VV55A): Intel 8255 programmable peripheral interface (PPI)
КР580ВГ75 (KR580VG75): Intel 8275 CRT controller
КР580ВТ57 (KR580VT57): Intel 8257 programmable DMA controller
КР580ВМ80А (KR580VM80A): Intel 8080A CPU
КР580ГФ24 (KR580GF24): Intel 8224 clock generator & driver
КС573РФ2 (KS573RF2): 2716 2KByte EPROM
КР565РУ5Д (KR565RU5D): 64Kbit DRAM (I think)
К589ИР12 (K589IR12): multimode buffer register
К155ИР1 (K155IR1): 7495 4-bit shift register
К155ИР13 (K155IR13): 74198 8-bit shift register
140УД608 (140UD608): op. amp.

[@Brfff]

ОТКЛ (отключить): OFF
ВКЛ: ON
ВНЕШ УСТРОЙСТВА: EXTERNAL DEVICES
МФ: magnetophone, i.e. cassette recorder
ТВ: TV

[@enix9983]

Thanks, very interesting!!!

[@MrWaalkman]

Shouldn't both clock signals have a 50% duty cycle? And I think that you need to look at both signals at the same time to verify that the phase angles of the clock signals are shifted (90 degrees? 180? who knows?). Might be time for a new 8224 clock signal generator.
And I wondered about the 500mv signal levels myself. LOL!
As for your video signal, the 8275 gets it character code from pins CC0 - CC6. Which should be a 7-bit ASCII code for the character that gets displayed on the CRT. Hook a LA up to those pins and bang on the keyboard to see if anything sensible is going to the 8275.

[@Brfff]

I'm zeroing in on the character ROM. You are quite correct, the 8275 sends an address to the character ROM which is then latched & bit-shifted (74198 analogue). I can see address activity on the ROM's address pins ... but no data coming out of the EPROM ... it's a 2716 which I have difficulty burning (hence why my Radio 86RK build video hasn't seen the light of TH-cam yet). But worst case, I can install a blank 2716 in there so there's at least some sort of data coming out. Oh, that reminds me ... I didn't check the !CS pin yet ...

[@Brfff]

"Shouldn't both clock signals have a 50% duty cycle? And I think that you need to look at both signals at the same time to verify that the phase angles of the clock signals are shifted (90 degrees? 180? who knows?)."
That would require me to actually read the data sheets and understand how it's supposed to work, rather than randomly poking at things until I stumble upon the problem ... which I can then backtrack from and then pretend I knew what problem I was looking for all along

[@MrWaalkman]

@@Brfff Far be it from me to interfere with a well laid out plan. :) And yeah, I'm from the "If I have to read the instructions, it probably wasn't worth knowing in the first place", so I can relate.

[@MrWaalkman]

@@Brfff Curse the 25 volt club! Try this one: NMC27C16 Sweet 13 volt programming.
techblog.bagu.uk/update/retrocomputing/2020/08/27/eproms.html

[@Brfff]

I've got an "old skool" AR-32A programmer that should do it ... one reason I bought the "Book 8088"
www.arlabs.com/eprom_plus.html