Timex 1000 Wierd Issue

[Ogien]

I recently got a Timex 1000 (ZX81) and it actually works, well kind off. I had to shorten the keyboard ribbon because it was damaged from the heat of the RAM chip, but once I did that they keyboard works except for a few weird issues. Most of the keys work fine however when I press the 1 key I get ; but then if I press the 1 key again I get 1 so it looks like ;1. Its the same for 2 and 3 but 4 works OK. Also if I type in:


4 PRINT "ADAM"
RUN

I get 6DAM instead of the expected ADAM

Please help me debug this.

[1024MAK]

Welcome on board Ogien!



If you have a look in the ZX81 manual (download a copy if you don't have a paper copy) you can look up the hexadecimal (hex for short) character codes. Now compare the wanted character code with the character code of what the ZX81 printed. See that it is the least significant digit that is different. If you now convert the hex to a binary number, you can see that bit 2 is 0 when it should be a 1

Code: Select all

    dec  hex   7654 3210
A = 38 = 26h = 0010 0110 b
6 = 34 = 22h = 0010 0010 b

1 = 29 = 1Dh = 0001 1101 b
; = 25 = 19h = 0001 1001 b

Are the chips on your board socketed? If yes remove them, then replace them to see if this makes any difference. Do this, because this action helps clean the pins. When removing them, touch an earth / ground point to discharge any static electricity that you have gained (even better if you have a proper wrist strap). Then leaver them out gently working from both ends using a flat bladed screwdriver or a knife, so that when the chip finally comes free you do not bend any of the pins as it leaves the socket (easier said than done ).

If the pins are too wide when replacing them, hold one side of the chip with one row of pins flat on a table or other hard surface (use some thin card or layers of paper to protect the surface) and push down as you "rock" the chip slightly to bend the pins in just a little, repeat for the other pins.

If this does not help, or indeed if it does, post to let us know

Mark

[Ogien]

Thanks I will try that and let you know. I was wondering if this is a known issue with the Timex 1000s (like the burned keyboard ribbon is) or if something randomly broken on mine.

Adam

[1024MAK]

I was wondering if this is a known issue with the Timex 1000s (like the burned keyboard ribbon is) or if something randomly broken on mine.

I'm not aware of this symptom being a common "known issue" with Zeddies.

Poor contact between chip pins and socket contacts is a common fault on most old computer hardware.

On some other makes (e.g. Acorn BBC micros and Memotech MTX computers) RAM chip and or bus faults between RAM and the video chip can cause strange display issues (including the wrong displayed character symptom).

But the ZX81/Timex 1000 does not have a video chip. The ULA and Z80 CPU work together to generate the display.

Mark