In this article, I will discuss some of the problems that can occur with the 16K RAM pack for the ZX81 , TS 1000, and TS 1500.
As I said in my last article, RAM pack ‘wobble’ is the most common problem and can be cured by cleaning the connector, raising the RAM pack off the table, and using Velcro/double-sided tape to secure it to the rear of the computer.
This month we will go inside the RAM pack and see what we can do to solve some of its problems.
If you have crashes every time you power up your computer with the RAM pack attached, and it works fine without, and you have tried the above cures, then possibly you have a faulty RAM pack. Generally, the screen will try to initialize but can’t and might scroll wild characters up the screen.
A different sequence is the computer initializing properly and seems to work OK, but strange characters appear on the screen in random places and in text you write on the screen. This means that you have a bad memory chip. Bad news!
First things first. Before you open your RAM pack, check and make sure the ‘key’ is still in the third slot from the left. If not, replace it with a small piece of plastic cut to size (I found that the plastic AC receptacle plugs for guarding against children’s fingers happen to be the right thickness and width – just cut to length).
To open the RAM pack, remove the four screws, two in the front and two in the rear. Carefully remove the back cover and then work the connector through the opening in the front to separate the circuit board from the front cover.
□nee apart, carefully spread the two boards apart. Watch the flexible connector as it can break off from either end quite easily. This is the first thing to check as I have opened several to find this was the only problem. The easiest way to repair this is to add another wire to replace the broken one and be sure to cut the broken one back so that the frayed ends won’t short against anything.
Electrically, the power supply is the first thing to check. The RAM pack has its own switching power supply and derives +12 volts and -5 volts from the +9 volt computer supply. To check for proper voltages, plug the RAM pack in (cover still off) and turn on the computer and measure the voltage on pins 1, 8, 9, and 16 on the 4116 RAM IC chips with respect to ground. Ground can be found on pins 4 5 on the edge connector underside. Pin 1 on the 4116 RAM IC chip should read -5 volts, pin 8 should read about +12 volts, pin 9 should read +5 volts and pin 16 should read 0 volts.
If these voltages are off, check the diodes and the transistor. The transistor can be replaced by a 2N3906. I have seen the 5.1 volt zener diode short out in several packs, so if you don’t have -5 volts, this most likely is your problem (look at the schematic included in this issue).
There were three issues of the RAM pack with each one laid out differently as far as components are concerned, otherwise they basically work the same. The RAM IC chips are on the board farthest away from the edge connector. There will be 8 of them all marked with the number 4116. Each chip is 16K by 1 bit. The Z-80 microprocessor is an 8-bit machine, ie, 8 chips. All the memory chips are wired in parallel so it is difficult to determine which is/are defective.
I have found that sometimes you can determine which one is bad by freezing the chips, one at a time, until it works. You can find ‘freeze mist’ at your local Radio Shack or electronics store. To find the faulty chip, simply freeze one chip and turn on the computer with RAM pack. Do this for each one until you freeze one and find it works. It will probably only work for only a few seconds if you do it this way.
Though freezing the chips sometime works, you may find that you wasted a $3 can of freeze mist. If you do find a chip that you think is bad, replacing it is another story. You need a very fine soldering iron and some method of solder removal — I would suggest the vacuum method. These boards have very thin solder lands and were not designed to be repaired.
The other chips on the boards rarely go out so I would not worry about them. Your best bet is to buy several RAM packs as back ups. I have seen them for as little as $10 uncased. Syntax is selling them for $15 (see address last issue).
As a final note, when removing the screws from any of the computers, be sure to remember where they came from. The screws are not all the same size and could strip out their plastic counterparts when replaced in the wrong position. Also, don’t over torque the screws.
Next month I will talk about adding and changing the rear edge connector on the 2068 to run Spectrum hardware (like microdrives) .