Do you have any of the following problems with your ZX81 or TS1000?
1: “Built-in NVM” of SWN 4:1 does not properly run WRX16 hi-res.
2: Certain keyboard lines go dead after installing a large keyboard, joystick plug, etc.
3: Erratic operation when adding certain peripherals, even though you KNOW you have an adequate power supply.
4: The TS2040 printer gives your system a fit.
If the answer to any of the above is “yes,” take heart. We’ve found yet another design oddity in the ZX81. Correcting it is almost ridiculously easy; no parts need to be added, and no circuit board traces need to be cut. You only have to clip out a few resistors!
The ZX81/TS1000 has “pullup” resistors on the CPU data lines. Presumably, these were included to help give more drive. In the most basic systems (16K RAM only), these help to insure that logic HIGH is indeed high. However, most add-ons have a tendency to pull the data lines high anyway; in this case, it can happen that the data line voltage is pulled too high to give a reliable logic LOW. This especially shows up when running WRX16 hi-res based programs; some systems will give nothing but a black screen, others will have pixels that randomly flash on and off.
I realize that some folks may object, on the principle that “if the resistors weren’t needed, Timex wouldn’t have included them.” All I can say is that on the five different machines we’ve done this with, ALL showed superior performance after the surgery, ALL run WRX16 without a hitch, and NONE have showed any adverse “post-op reactions.”
These eight resistors are located between the heatsink and the large keyboard tail, at the front of the board. They are arranged in two banks of four 10K resistors each. All you need to do is to clip one end of each of these resistors, and bend them up so that there is no possibility of contact. Note that these are 10K (brown-black-orange-gold). DO NOT clip the 470 ohm resistors that are nearby (yellow-purple-brown-gold).
Interestingly, the design used in the Larken / Silicon Mountain SCRAM board seems more immune to problems than the “built-in NVM.” Still, it is conceivable that even this device will have trouble with some systems unless the data line pullups are disconnected.