Hi all, I am having some issues with my Unitron 80 column card (Videx clone). The machine was sat for 30+ years so I have no idea whether it had any pre-existing issues, but lets assume not!
When I enter PR#3 the blinking cursor on the 40 column display disappears (as expected), however when I then switch monitor inputs to the 80 column card, there is nothing displayed. Ctrl + bell results in silence, any pressing of keys followed by return results in silence, however I can press reset to be returned to the 40 column output. When I switch to 80 column I can also hear a change in the pitch of the electronic hum.
I have removed all of the chips, cleaned the sockets, re-inserted them, and cleaned the contact on the card and the slot, as well as checking continuity on the cable.
The cable is homemade and a bit janky, but it appears to be wired correctly.
The chips I have easily available to buy are:
The CD4013BE, 74LS02, 74LS139 (does it matter if this is an N or AN?), 74LS00, 74LS157, 74LS132, 74LS86 (does it matter if this is an N or AN?), 74LS166, UPD4016C-2, and the main controller chip, the HD46505 SP-1.
I can record the behaviour if you think it would be helpful!
I do also have intermittent issues with my Disk II card, so is there a chip on the motherboard which relates specifically to handling expansion cards? Perhaps that could be the issue if so?
Images of the card:
Any advice appreciated!