ZX/TS Repair Guide – Part II

This is the second in a series of articles on how to repair your ZX81 and TS1000. I will also try to cover some of the problems found in the 1500 and 2068 as well in future articles.

From my experience with the 2X81 & TS1000, I would say that there are four common problems with this series. They are the infamous ‘ RAM pack wobble’, keyboard ribbon cable cracking, loose ground strap, and 5 volt regulator over heating and failing. I have seen other problems crop up, but these are by far the most prevalent.

Probably the most aggravating thing that can happen is to have a real long program that you have been writing for the last six hours sitting there on the screen, you reach down, press a button and the computer goes wild, scrolling unknown characters up the screen, flashing and then finally, the terrorizing blank screen. You have gone beyond the realm of the computer’s comprehension and into the twilight zone! In other words, the computer lost connection with its memory through a bad contact.

The problem stems from a design flaw. The computer has rubber feet on the bottom while the RAM pack sits directly on the table top. You press a key from the top, the rubber feet give a little, the RAM pack does not and the connector between the two takes up the slack. Connectors, when they slide against each other have never been known to maintain a good connection, especially if there is dust or any other contaminant on the contacts.

There have been dozens of cures for solving this problem, the best that I have found are to clean the rear edqe connector, put larger (taller) feet on the computer to raise the RAM pack off the table, and place a piece of velcro or double sided tape between the RAM pack and the rear of the computer.

When cleaning the rear edge connector, make sure the computer is turned off. Use cotton swabs and denatured alcohol, clean both top and bottom and dry off excess alcohol with a dry cotton swab. Also, try not to leave any cotton hairs behind on the connector. It is best not to try and clean the RAM pack connector so as to not damage any of the ‘pins’.

If you have lost responce from the computer when you press a key, you more than likely have a cracked ribbon cable. The most common keys to lose are keys 1-5. The laminate that the cable is made of becomes dry and brittle and eventually cracks. To repair this, you must go inside the computer and repair the break.

To disassemble the computer, disconnct everything from it, turn it over and remove the three rubber feet that are not near a screw hole. This should reveal five screws. Remove screws and bottom cover. You now should see the bottom (foil) side of the computer. Set the computer so that the power and tape jacks are on the right and unscrew the two small Phillips head screws. Now carefully and slowly lift up the board and rotate it towards you to expose the top side of the board and keyboard cable. The cable is short so the side closest to you will not come up very far, so rotate so that this side acts as the hinge.

Once you can get to the ribbon cables, you will note that there are actually two. Very carefully pull these one at a time out of the connectors on the board. The top should now separate from the circuit board.

The cracks should be very apparent. Generally, the crack is near the now loose end and can be repared by simply cutting off the bad part with a pair of scissors. If you will have to cut off more than an inch, use copper or nickel print instead. Most electronics stores carry this for repairing cracked circuit boards and it works very well for repairing cracked ribbon cables. It comes in a small jar with an applicator brush in the lid. It’s like paint. Simply dab a small amount across the crack in the metal part of the cable taking care not to let it spread and short to an adjacent metal strip. This stuff also works well repairing your rear window defroster strips in your car!

Once you have done the repair, take some cellaphane tape and wrap around the ribbon cable about 1/4″ from the end to help aid in reinserting the cables into their connectors. Now position the board so that you can grasp both sides of the ribbon cable where the tape is and slide back into their respective connectors. Reassemble the computer and check out.

The grounding strap on the foil side of the computer comes loose sometimes at the rear edge connector end and causes problems with interference on the screen and loading. Simply solder it back in place taking care not to let the strap touch any other solder lands.

The 5 volt regulator is mounted on a square flat piece of aluminum, and to the circuit board. This can sometimes overheat and cause the computer to crash after 15-30 min- utes of use. If the screw and nut that holds this all together was not tightened well at the factory, tightening it may cure your problem, otherwise, replace it.

Sometimes, the power supply is putting out to much for the regulator to handle and replacing it will help. I have found that the Atari 2600 game replacement power supply works fine as long as you don’t have any more than one or two peripherals connected to the rear. You will know when you have loaded it down two much when thick bands (hum bars) travel up the screen when you turn it on.

The only thing left to go wrong are the integrated circuit chips (ICs). With the computer opened up, you can see four IC’s (five in the ZX81). The only two chips that I have seen go bad are the ULA and the RAM IC. If the ULA is bad, it usually gets very hot and no control is seen from the computer. If the RAM IC is bad, generally the cursor won’t come up but the screen steadies. If you think that the RAM chip is bad and you have a 16K RAM pack, just remove the 2K RAM IC from the computer and try it with the 16K RAM pack. The computer does not need the 2K RAM chip when using the 16K RAM pack.

There are three different issues of the TS 1000 out there, so the layout may not be the same as that shown in the picture. To tell the chips apart, the ULA says ULA or Ferranti on it. The Z80 microprocessor chip is the other IC the same size as the ULA. The ROM is socketed and the RAM is soldered directly to the board. To obtain parts for your computer, write to: Timex Computer Corp. Little Rock Product Service Center Little Rock, AR 72203. You can also buy whole computers with minor defects from SYNTAX magazine, RD 2 Box 457, Harvard, MA 01451 (617)456-3661 for $15 each and from Electronics Supermarket, PO Box 988, Lynnfield, MA 01940 (617)532-2323 for $18.88 each. This way you have all the replacement parts you may need.

Remember that you are dealing with sensitive components and make sure the computer is off before pulling the RAM pack or any other peripheral from the rear of the computer. Also, if you don’t feel confident about soldering, ask friend who can do it for you. This would be a difficult first project to tackle.