🇧🇾 Квант (Quantum): Part 3 (Z80 replacement & clock investigation) [TCE #0458]
17Troubleshooting continues. I replace the original East German U880 CPU (the DDR version of the Zilog Z80) with a socketed Zilog version (made in 2021). Unsurprisingly it doesn't magically fix the not-working state of the machine.
Thanks to @skjerk's earlier comment about some signal levels being around 4V, it got me thinking about the clock. It was at about 4V when arriving at the Z80's clock pin.
The design is pretty much the same as other homebrew Sinclair ZX Spectrum implementations - crystal output passing through several NAND or NOT gates then into some flip flops to halve the frequency.
http://www.zxdesign.info/schematics.shtml
In our case we pass the crystal at 13.824MHz through three inverters (К555ЛН1 = 7404) and then a pair of flip flops (К555ТМ2 or К155ТМ2 = 7474 dual D-type flip flops).
The final output is marked as the /RAS signal and supplied as the clock input to the Z80.
When monitoring the output it seems that the final output of the К555ТМ2 sometimes collapses ... so I wonder if that chip needs replacing?
From Part 1:
Let's have a look at this Belarusian Sinclair ZX Spectrum clone called the КВАНТ (KVANT) or Quant(um) in English.
It's labelled "КВАНТ МИКРО ЗВМ" where ЗВМ is an abbreviation for "Электронная Вычислительная Машина" ... Electronic Calculating Machine ... so it's a "Quantum Micro Computer".
Lots of excellent information (in Russian), including documentation & circuit fixes:
https://zxbyte.ru/inter.htm