This particular board has a rare mod installed that originates from Poland, most probably it was done by the company that sold the machine: APINA from Zielona Góra. There are several modifications done to the original board:
- 16K RAM extension (or actually a replacement of the original RAM), as an interesting add-on board soldered to the ZX81 board
- Composite video mod that adds a composite video signal to the RCA connector, while preserving the original RF signal on the same connector
- A larger radiator on the voltage regulator that was moved to a new position on the board
Here is a schematic of the composite video mod that was done to this particular board back in the eighties:
Close-ups of the RAM add-on board:
ZX81 is powered with 9V DC though a mono mini-jack type of connector with plus on the tip of the plug. Original power supply is 0.7A
The original keyboard membrane broke into pieces at the connector.