My ZX81 is not respondiung to Keypad

[Ian13]

Hi, I am new to this forum, so excuse me if this question has already been answered before.

I have an issue 3 ZX81, i have bypassed the TV modulator so I can use it with a PC monitor connected through a Composite video to VGA converter. I get the 'K' cursor on the screen, so signs of life. I have just purchased a new keypad and plugged it in and there is no response from it. The original keypad had the classic cracked ribbon cable.

I am guessing there is some sort of hardware fault. I am guessing the troublesome ULA chip might be to blame. Mine is running very hot.

I have got a circuit diagram and using a logic probe can see there is activity on the keyboard connector when the appropriate key is pressed. this appears on the appropriate pin on the ULA chip. Should the ULA be polling these pins or do they generate an interrupt when a key is pressed?

Is there anything obvious to check? Has my ULA gone Faulty? and is it easily repairable (I guess not)?

let me know if anymore information is required.

thanks in advanced.

[Ian13]

Should my 'K' be flashing, it has been a long time since I last used it?

mine is not

That may explain why The keypad is not responding!!!!!

Any pointers.

Thanks.

[1024MAK]

No, on the ZX81 the inverse video K does not flash.
The Z80 CPU polls the ULA which includes an input port. Part of this port receives the input from the keyboard (five input lines). The keyboard itself is just a simple switch matrix built using a two membrane layers.
In order to work out which key is pressed, the upper eight CPU address lines (A8 to A15) connect via diodes (D1 to D8) to the keyboard eight way connector. The 16 bit I/O address to read the ULA input port is then used to hold one of the upper address lines low, thus one of the eight half rows is active. Should a key be pressed, then the corresponding input to the ULA will go low.

If all five ULA keyboard inputs respond correctly, then it is likely that the ULA is indeed faulty. But also check for dry joints. Especially the A0, /RD and /IORQ pins.

Oh, and when testing a ZX81, I'm too lazy to keep reconnecting a keyboard, so I use a thin solid core insulated wire (with stripped ends) to briefly link a contact in the five pin keyboard connector to a contact in the eight pin keyboard connector

Mark

[Ian13]

Thanks.

I will see if there is any activity on all the ULA keypad pins, and check for dry joints. Dry joints can certainly cause problems on old PCB's

I will resolder all of the ULA and Processor pins, The ROM and the RAM must be working for it get the 'K' on the screen.

[Zxkim81]

Hi there
I had the same problem and it turned out that the ram was at fault
The kursor K is static until you type something.
Hope this helps.
Kim