P.C. Boards are identified as issue 1 or issue 3. Both versions are found in ZX81s/only issue 3 is used in the TS1000. Electrically, they are almost identical. Issue 3 is an improved layout with a neater appearance. ZX81s have ICs mounted in sockets. The TS versions have their RAM chips hard soldered in place. All ZX81s were built in England or sold in kit form. Most TS were built in Portugal, but some were built in France. Quality control was highest on ZX81s. Poor quality control was observed on TS units, particularly those manufactured in Portugal. Fabrication defects most observed were faulty installation of the flexible PC “fingers” from the keyboard into the special sockets. This defect has been observed in almost 100% of TS units manufactured in Portugal. Unsoldered or incompletely soldered ground busses were observed in both Portuguese and French units.
ICs(Integrated Circuits)
All units use a SCL (Sinclair Computer Logic) special purpose chip manufactured by Ferranti-ULA 2C184E. (Ferranti will not even acknowledge letters requesting info or cost).
All units use a D2364C ROM.
CPUs vary. ZX81s have NEC P1Z308-151 or D780c-1. TS uses Zilog chips. All are Z80A chips in one form or another.
RAMs, INTERNAL
Most variation is seen with the RAM. In the ZX81, only 1K of RAM is built in. The PC board was cleverly designed to use either 2 @ 2114s or a single 4118. The 2114 is a 1K x 4 static RAM, whereas the 4118 is a 1K x 8 dynamic RAM. TS internal RAMs are all 2K in various versions from different manufactors, i.e., Toshiba 2016P-1, Motorola 2CM38818C, NEC D4016D-1,Toshiba TMM 2016P-1.
TRANSISTORS
ZTX-313s are normally used I have found uMPS-2369s mounted in one computer. Also, I have found that an MPS-3563 works well as a substitute.
LOAD/SAVE Modifications
Change R-27 to 27ohms and C-11 to 0.015 mfd.
RAMs, EXTERNAL (16K variety)
PC boards are identified as issue 1, 2, or 3. Issue 1 and 2 are composed of 2 small PC boards that are folded inside the case. The differences between 1 and 2 are minimal-an additional diode and decoupling capacitor. Issue 3 combines many logic functions in a single Ferranti ULAIHO35E chip to reduce the chip count by 5 and a single PC board is used, mounted in the same case. (Ferranti won’t answer any questions on this chip either)
Transistor is either a ZTX-750 or ZTX-752. Recent French and Portuguese units used either a 2N6727 or MPS6727.
Wobble is often a problem-varies with units. The French units utilize a PC connector that has the tightest fit and consequently, the least wobble of all units observed. (Unfortunately, ZX80 and ZX81 were chosen as names of Sir Clyde’s(sic)first two computers. There is a tendency to confuse these designations with Z-80 and Z-80A, which are the CPUs. Therefore, remember that Z-80A is a Zilog designator for their 4MHz CPU.)
POWER SUPPLY PROBLEMS
Once in a great while you get a noisy power supply. In such cases,the bridge rectifier is first suspect. (A power supply can still partially function with 1 or 2 diodes bad- but it will be noisy!) You must crack open the power supply case and find the faulty diode(s) and replace them with 1N002 diodes. I’ve never seen a capacitor fail, but it could and the replacement is a 1000 mfd /16V capacitor.
I recommend cracking open the power supply whether there is a problem or not. I can put a miniature SPST switch in series with the output so that I can kill power at the power supply,rather than pulling the plug at the computer.
To avoid drop-outs due to looseness of the power supply plug, I removed the power jack completely. (Desolder it and remove.) I then hard wire the power wires in place, tack them securely with some silicone rubber and the power drop-out problem is completely solved. For an even neater job, one should consider putting a small male and female coonector near the power supply to disconnect the system. Watch out you don’t reverse polarity!
HEAT AND THE ZX-81/TS1000Where I live heat is not a problem. However based on the vast amount of letters and complaints seen in SYNC and SYNTAX, heat is a problem for many and the logical solution- the only one I’ve not seen presented is to get the primary heat generator outside of the computer case, like so:
1. The primary heat generator is the 7805 3-terminal 5 volt regulator. The higher the input voltage applied to the this device the more energy which must be dissipated in heat by this device to reduce the output voltage to +5VDC.
2. The input voltage (from the external power supply)varies from a high of 11+ volts down to 7 volts. The variation comes about primarily due to the amount of add-ons which are connected to the computer. Each device added pulls more current- which causes voltage to drop. (Should you add too many external items, say with a 750ma power supply, the response would be too high a current drain, voltage would drop too low and the computer would quit functioning.)
3. Desolder the 7805 regulator and remove it along with the aluminum heat sink. Now, combining with (step 3 above)connect a 3-wire input to the computer. Three wires are now needed because we not only need a +5 volt and ground(return) line, but we need the un-regulated line which provides between 7 and 11 volts- for use by the external 16K RAM.
4. Mount the 7805 regulator- with a good heat sink to the external power supply. Use silicone rubber to mount the heatsinked 7805. Now rig 3 lines via a 3 wire plug and jack to provide variable DC (7-11 v),regulated +5VDC and a ground. Don’t forget to put a SPST miniature switch on the external power supply.
____________/____( 7-11 vdc
| ______ (
_______ | ( ) (
)____|__(7805 )__/____(5vdc reg.
ext. ) (______) ( power ) ( ZX81 or supply) ( TS1000 computer) (
)_________________/____( ground
_______) (_________________(Editor’s note)
*** This article is the opinion of the author,
SINCUS is not responsible for the results of any changes that you make in your computer as a result of reading this article.*************