Most of us have “cussed” and “discussed” the pressure sensitive keyboard on Uncle Clive Sinclair’s little monster. To keep the price of the computer below $100, and subsequently earn a niche in the Smithsonian Institute, Washington, the keyboard had to be functional as well as inexpensive. Visiting a MacDonalds’ restaurant, we see pressure sensitive keyboards on cash registers being used to keep track of customer orders. This application has proved to be quite successful and apparantly holds up under constant use.
Just what does the T/S 1000 keyboard consist of? It is rectangular in shape (6 3/16 in. long by 2 1/2 in. wide). There are four rows of 10 keys per row, making a total of 40 keys. Three plastic sheets are glued together to make the keyboard sandwich assembly. The top layer has the keyboard characters printed on the top side and printed circuits are underside of this first sheet. One quarter inch printed discs under each key are integral with the circuits. The middle layer is like a template with (40) 7/16 in. holes located under the keys. The bottom plastic layer has the corresponding printed discs and circuits. The middle template sheets acts as an insulator but allows the upper discs to make contact with the correspnding lower discs to complete a circuit when a key is pressed. The resiliency of the top layer (printed keyboard sheet) in conjunction with air entrapped in the cutout space between layers, allows for the contact action of each of the keys. This entire sandwich assembly comes with peel off tape on the lower surface. For installation purposes, the double ribbon cable can be slipped through a slot in the upper right hand corner of the keyboard mounting area. The keyboard assembly is positioned and secured. The ribbon cables are then plugged into their respective connectors on the circuit board.
To digress a bit, it must be admitted that when I bought my first T/S 1000 computer back in August, 1982, I had never seen the computer and was shocked to see the so-named “greasy kid” keyboard. After that, a great deal has been learned about it! For the first 6 months, the keys were pressed a bit too hard. Instinct seemed to tell me that the harder a key was pressed, the better the contact was made! Nothing was further from the truth. A fellow SINCUS club member saw me bearing down on the keyboard and gave a well deserved critique on the technique. As he said, “There is a sensitive spot on the key outline and a surprisingly gentle touch is all that is needed to bring the 1/4 in. internal printed discs together to make the contact.” My mistreatment of the keyboard plus 1 year of steady use seemed to have caused a failure of the “ENTER” key in the second row plus the four keys to the left. The wiring diagram showed that the keys are connected on the same circuit (printed) line. My keyboard failure seemed to occur after storing the computer in a plastic cover in a damp environment for a month. A slight amount of oxide may have formed at the connector contacts.
The writer remembered back in December 1982 (first meeting of SINCUS club) when it was the consensus of a number of users that the keyboard was quite rugged and no failures had been reported at that time. Since then, there have been a number of reported keyboard failures.
The keyboard assembly is connected to the circuit board by dual ribbon cables containing five and eight printed flat wires respectively. These cables are plugged into connectors on the circuit board.
As you recall, a connector that plugs the 16K rampack to the computer circuit board has to be unplugged and plugged-in periodically to clean the contact points (ED. note- that is excellent and often overlooked advice!). Again, because of cost consideration, oxide free metals were not used. However, it is not practical to repeatedly unplug and plug-in the keyboard ribbon cables into their connectors for cleaning purposes.
If a group of 5 keys fail, carefully open up the computer case and unplug (pull out) the ribbon cable containing the suspected defective printed circuit. Apply a small amount of contact cleaner to the ends of the printed lines and wipe dry. It may even be necessary to cut off a small amount of material from the end of the cables(s). Replugging of the ribbon cables requires patience and extreme care. Trying to plug them in a number of times may result in worn, creased, and unusable printed circuit ends. Also, failure may have occurred after someone has carelessly opened the covers and caused a tear to develop in the ribbon cables at the board connectors.
It should be remembered that a new keyboard can be ordered directly from the TIMEX service center in Little Rock, Arkansas. The charge is $7.50 plus $2.50 for shipping.
Another source of a keyboard assembly is to find a computer owner who has installed a regular positive-action keyboard and has no use for the top cover of his/her T/S 1000. If the cover is not a gift, the going price is about $5.
If you happen to own an early ZX-81 computer, trimming off the ribbon cables could present complications. It seems that the printed lines are encased in clear plastic and bare at the ends on one side only. It is very difficult to scrape off the plastic if the cable has been trimmed back as the printed lines are usually damaged in the process.
A reference to a ZX-81 keyboard failure was reported in the TIMEX-Sinclair User magazine, Vol. 1, No. 3 issue. It stated that, “A keyboard went bad in part after 1 year of extensive use. The ENTER FUNCTION key gradually failed so that first the FUNCTION key failed the the LET key, and finally the ENTER key. No further explanation was given for the cause of failure. Apparantly, heavy usage combined with a vigorous touch caused the printed circuit to pull apart within the keys themselves.
In conclusion, it would be interesting to hear from other users and compare notes on this KEY subject!!
Bill Stemples
SINCUS