I've been struggling with a II+ that's not powering up properly.
The motherboard contains just one row of RAM chips - 16K of tested good RAM. ROM chips all test good on EPROM reader.
First symptom (and this is variable): Powers up to the (with beep) ] prompt but entering any keystrokes causes it to crash out.
Second symptom: it worsened to a blank inverse screen (still a beep) with what appears to be a black cursor in the top left. No input - reset does nothing.
Third symptom: suspecting character generator issues, I pulled the character ROM and the machine boots to a blank inverse screen, with beep and a black cursor.
It accepts typed commands but returns a beep (presumably a syntax error) on every command whether a good command (like "PRINT X") or a bad command (likke gibberish).
Installing a known-good character ROM returns to the second symptom.
From this it appears that the character ROM is somehow interfering withe the data bus, but I'm unsure how.
Any insights from the experts?
It shouldn't be connected, so if it is, perhaps a logic chip in the video circuitry is "leaking". Or worse, maybe a socket or board flaw causing a signal to leak across?
If one of the two 74LS257 at B6,B7 were bad, then when the microprocessor tried to read the keyboard, it could have data from the RAM instead. The chargen is also connected to the RAM outputs. It looks like a complex syndrome involving multiple failures.