TMS9918A Video Upgrade: Part 1

The TMS9918A video project consists of two PC boards, a small modification/addition to the main computer board, and a +5, +12, -5 VDC power supply (an expansion board of some type is necessary). Video board “A” contains the TMS9918 VDP chip, 16K video ram and one support chip for I/O interfacing. Board “B” contains the TMS9918A enhanced BASIC 2764 EPROM, a switchable 2764 EPROM (8-16K) socket and the logic necessary to decode the system and bank switch the character generator space (for initialization and printing to the 2040 printer).

The computer board modification consists of a diode OR-ing the NMI input to the Z80, allowing the VDP to interrupt the CPU instead of the ULA chip. This is a transparent modification and does not hinder the normal video display when the boards are removed.

The power supply is necessary for the video ram. Although power requirements are modest for -5 and +12 VDC and heat sinking is not required, the +5 VDC regulator should have a hefty heat sink especially when using a TS 16 K ram pack. When using any ram (TS1016, Memotech 64K) or other boards that require 9 VDC on edge trace 2B (see edge connector drawing), the following ‘mod’ will allow this supply to power the whole computer:

On the computer end of your expansion board, cut the trace going to the computers’ edge trace 2B. Connect four (4) 1N4148 diodes in series (anode to cathode), then connect the anode end to the lower left trace (looking at it from the computer end) on the expansion board (12 VDC), and connect the cathode end to trace 2B on the expansion side of the cut.

NOW, LET THE FUN BEGIN!

First, install the diode/resistor on the computers main board, a 1N4148 diode and a 4.7K ohm resistor (yellow/violet/red). Check the main computer drawing for your type of computer and board. Make the necessary cuts and connections.

Reassemble your computer and test it to see if it still works. If it does not, then go back over it and check for solder bridges, etc., in the area surrounding your artwork. Do NOT proceed until your computer works with this ‘mod’ installed (ZX80 with a ROM CS not mod installed).

Next, have your power supply ready for hook up. -5 VDC is connected to the top left trace of the expansion board (22AWG wire or larger). This is just left of the TS trace 1A (remember, Johns’ expansion board is a true 50 pin board). Connect the 12 VDC line to the bottom left ‘extra’ trace and the +5 VDC line to the 5 VDC Sinclair trace (IB). Connect the power supply ground to the expansion ground (traces 4B and 5B). Hook up the 4 diode connection as previously stated, and you are ready for a cool running, externally powered computer. You won’t have to use that 9 volt plug on the side of your computer anymore. Check all connections and power up your computer. If it does NOT work, then power down immediately and check your connections again. Look for the proper edge connections and shorts to adjacent tracks. Check for proper voltage on each connected line.

You have now made the necessary computer board mod and power supply hookup. Before we begin the video boards, take a moment to relax, get an old issue of BYTE or RADIO ELECTRONICS and look up a source for thE TMS9918A chip and its’ companion the 10.738635 MHz crystal.

The following are sources that we know of:

• JDR Microdevices 1-800-538-500 in Calif. 1-800-622-6279
• ACTIVE Electronics (Mass.) 1-800-343-0874
• MILGRAY Electronics (with a company PO) is in Wash. DC. and has the VDP chip for $25.00, purchase order only.

JDR and Active has it for $39.95 and $32.95 respectively, but Active is VERY, VERY SLOW!! The crystal is about $4.00.

VIDEO BOARD “A” ASSEMBLY INSTRUCTIONS

Assuming that you make your own boards, the full size layout is available for board “A” and “B”. It is beyond the scope of our text in this article to go through the steps required to make PC boards. We can supply those of you who are interested with information on PC assembly, if you supply us with a SASE.

Assuming at this point, that you now have possession of board A, These instructions will get you through its assembly.

1. Install all feedthroughs on the board using 30 AWG solid wire, in every location that has a round “doughnut” pad, on the COMPONENT side of the board. DO NOT install feedthroughs in the rectangular pads on U1s’ socket, pins 21-24.

To install a feedthrough, insert the wire from the boards’ component side, to extend about 1/8 inch out from the non-component side of the board. Bend the wire over on both sides, lay the board down flat (non component side up) and solder the wire on this side. Turn the board over, cut the wire off about 1/8 inch above the board, bend the wire over and solder this side also. Repeat this until all the feedthroughs are in place. Clean the flux from the board with some acetone or flux remover and inspect your work for bad solder joints or shorts. Touch up as necessary.

2. Install resistors R1, R2 and R3. Next solder all IC sockets onto the board.

USE CARE in soldering U3-U10 (16K RAM) sockets. The memory array is VERY prone to shorts. Clean the flux off and again check your work.

3. Install transistor Q1 by bending its center pin forward (toward flat front) and placing it in the EBC pads with the flat facing the boards’ left. Solder in place.
4. Install C1, C2 just left of R1, in their capacitor marked holes. Install C3 just right of Q1, denoted by the standard polarized capacitor symbol. Be sure the positive pin of C3 goes in the hole marked with the “+” symbol. If C3 is crowded by R2, then it can be laid down with its’ top toward the edge connector. Bend the leads as necessary and solder in place.
5. Install the 10.7386 MHz. crystal by first cutting the leads to 1/2 inch length. Insert the leads only far enough through the board, at its location marked with a crystal, to solder them in place. Twist the crystal 90 degrees and tape it down (gently) to the board, shaping its leads as necessary. Clean and check your work.
6. If it all looks good, then flip the board over and install the 7 tantalum bypass caps (Cbp) on the non component side (carefully).