Ленинград-1 (Leningrad): Part 4a (Troubleshooting starts)
13Rewired the keyboard - found a track that had lifted off the board that went to one of the keyboard pins - this could've been our dodgy connection.
It went via a pull-up resistor to pin 2 of a К555КП11 multiplexor (K555KP11, equivalent to a 74LS257). But it was also very close to a diode leg ... maybe touching if wires are moved ... so this was maybe out problem.
I fixed by running a bodge wire from the keyboard to the IC.
It seems more stable - booting to the standard Sinclair Spectrum screen ... but synch is all over the place ... similar to the Moldovan BYTE.
One comment made was about the lack of decoupling caps ... it has the "Soviet standard" or one per two DRAMs ... okay, okay ... I don't know if that is A Soviet standard ... what I mean is that I've seen it on more than one machine ... every 2nd DRAM only has a decoupling capacitor.
So maybe one thing I could try is adding more decoupling caps?
Also ... the ENTER button still doesn't seem to work ... so that needs fixing still.
Finally ... I have no idea what the left-hand DIN socket is for ... external keyboard? Something still to investigate.
#ZXSpectrum