Smooth flicker free graphics has been the “impossible dream” for Sinclair ZX80 or MicroAce owners, at least until recently. It has been particularly frustrating to have the new 8K ROM and realize that this capability exists on the chip but is denied the ZX80 user! The MicroAce Video Upgrade was designed to eliminate this problem, and it works very well.
The Upgrade comes as a kit with a high quality PC board, a sack full of parts, and some instructions. These are a bit sketchy but the board layout is clear, and, if you have some exerience selecting and assembling electronic components, you should have no trouble although it is certainly not a “Heathkit.”
Installing the kit is another problem. I assembled the board in less than an hour, but spent several days figuring out where to put and how to attach it. It will not fit inside the ZX80’s case and some provision for mounting must be made. I placed mine in an external chassis box which also houses my keyboard beeper and connected that assembly to the computer with 14 conductor ribbon cable. The electrical problems I had with the Video Upgrade stem from the fact that I have a VHF modulator in my system. Installation procedures for these differ from those equipped with UHF modulators and the differenes are not adequately explained. After some false starts and some unnecessary board surgery, the following was determined:
a) If you have a VHF modulator do not make the indicated cuts between:
1) IC21 pin 1 and IC19 pin 5
2) IC19 pin 5 and R32+35
b) Do make the indicated cut in the SYNC track (IC19 pin 6 and the base resistor at TR1).
c) The indicated connection between IC21 pin 2 and R32+35 is unnecessary.
You should also be prepared to build a simple buffer circuit for the output of the Video Upgrade. The circuit for this is shown adequately in the instructions. However, you have to get the parts from Radio Shack and no mounting provisions is made on the PC board. I installed mine in the 14 pin dip socket interface.
After assembling and installing my Video Upgrade kit, I applied the power and it worked the first time although an adjustment was required to center the K vertically inside the cursor. My characters were actually split horizontally and rolled vertically so that the middle of the cursor was a space with the bottom of the K at the top and the top of the K at the bottom. The adjustment is to be expected and is adequately explained in the instructions.
In operation the Video Upgrade is great. The SLOW and FAST commands work exactly as described in the 8K ROM manual.
With the modification, the ZX80 produces pleasant, flicker free displays and smooth animation in the SLOW mode. It is, however, awfully slow. The only solution to this appears to be machine code graphics. The only lingering peculiarity is the fact that the top row of print is inclined slightly to the right (this occurs in the SLOW mode only; PAUSE or FAST displays are entirely normal). MicroAce says that fixing this would require a complete redesign of the ZX80 board. The distortion is, in my opinion, a very minor problem.
Since I installed the Video Upgrade, my software has become increasingly display oriented. Even if you are not interested in “games,” the MicroAce Video Upgrade is worthwhile modification to your ZX80. It will turn it into a ZX81 for only $29. Quite a bargain!