SPECTRUM FINDS ITSELF AT FULL STRETCH
Sinclair has been overtaken by other suppliers of upgrades. Stephen Adams reports.
The Spectrum is in four basic forms. Model 1s which have two IC-type sockets into which a printed circuit board is plugged and model 2s which have all the RAM fitted on to the main printed circuit board. There are, of course, two types in each category, the 48K and the 16K.
The 48K cannot be expanded, as all the existing memory space is covered with either ROM or RAM - all 64K of it. The ROM can be switched-out externally to add different ROMs using the ROMCS but the RAM cannot as there is no equivalent RAMCS.
Therefore all memory expansions can be done only to the 16K models. As the computer already contains 16K of RAM, the top 32K of memory area - 32K-64K - is the only space to put it. Most memory expansions fill the space with 32K worth of RAM chips but the East London Robotics 64K add-on - the SP80 - has two sets of 32K RAM which can be switched in and out under a program instruction or from the keyboard.
The kits consist of four memory-decoding chips, except model 1 versions where all the chips are soldered to a PCB, and eight 32K by one-bit chips. Those RAM chips are very sensitive to static electricity and warnings are included in all the instruction sheets. The way to handle them properly is not to remove them from their protective packaging until needed and to keep touching an earthed object such as a radiator or gas pipe occasionally to release the static charge. The static can be caused by nylon or wool in your clothing being rubbed.
Putting-in the chips can be a little difficult and it is recommended that you check that the pins are straight before you insert them. The Fox Electronics kit has the pins already straightened. Also check that none of the pins is bent outside the socket or underneath the chip when you have finished.
The best technique to use is to put all the pins on one side into the socket and then to pull the pins on the other side over the holes in the other side of the socket. Once both sets of pins are resting in the socket holes you can push down gently on the chip to push it into its socket.
The instructions vary from a four-page, step-by-step meticulous description by Delta Research to one page just listing where the chips go by Fuller. Apart from the Fuller instructions all provided sufficient detail to allow you to know where each chip goes and what precautions to take.
As part of its instructions Fox Electronics also includes a sheet showing how to tune the colour on a Spectrum to give better results. It also shows you how to tell whether it is the Spectrum or the TV set causing a problem. Sheets normally cost £1 from Fountain Electronics but are free with its RAM kit, along with a small Basic program to demonstrate the usefulness of 48K.
The fitting of the Fox kit was the easiest but if you have problems you can send back the kit with the details of the Spectrum at any time in the following 12 months. East London Robotics will fit the chips at an extra cost of £7 by post or £3 by personal visit.
Delta sends a memory test tape which checks all the memory by using a machine code program going through at least all the manufacturer's specifications. The tape can be used to keep an eye on suspect memory faults as it stops with an error which can then be sent back to Delta or Sinclair. The tape normally would cost £3.50 and is a really comprehensive test taking at least eight minutes.
East London Robotics and Delta were the only model 1 boards we could test and both could be fitted easily with less trouble than the model 2.
East London Robotics also does an SP80 kit which can provide 64K of memory in the same sockets as the SP48 (48K) fitted. That is achieved by having two separate banks of 32K, switched by an OUT instruction.
That is possible only because of the 64K RAM chips which are used instead of the 32K RAMs and a massive modification of the decoding chips which plug into the Spectrum. No soldering is required but some care needs to be taken inserting the chips, as three of them are wired together using twisted insulated wire.
A LED is also soldered on to one of the chips to indicate which bank is being used. The only problem is that you can see it only with the top off or looking through the edge-connector hole. The LED lights when it is in bank two and when the machine is turned on the light shows on bank one.
The only problem with the SP80 is that the stack and any program running in it must be in the 16K of memory provided by Sinclair. If it was allowed to go into the top 32K it would be switched-out on the first OUT instruction and the program would have nowhere to go.
Also the machine code stack for return addresses would have to be in the lower 16K area for the same reason. That leaves the user to develop a program which will be able to use banks of memory without having the convenience of Sinclair Basic to cope with it.
Prices of the various kits are detailed, along with an indication of how good the instructions were, guarantee period and number of tests performed on memory after fitting.
It is also possible to add a RAM pack on the back of the Spectrum. A Spectrum-type 32K RAM pack has been produced by Cheetah Marketing Ltd. It fits very snugly on the back of the Spectrum and because it is outside the case it does not matter if it is model 1 or model 2.
It also has an extension piece on the hack so that you can plug in Microdrives when they arrive. The RAM pack has no known wobble in use and can be fitted very easily by plugging it into the expansion port. It costs £39.95.
The alternative is to use a ZX-81-type RAM pack with an adaptor to give you another 16K or 32K. To use it you will require an adaptor.
EPROM Services and myself both make adaptors for the 16K RAM pack. Mine can also cope with a 64K RAM pack to give a full 32K by changing a soldered strap on the board or ordering a 64K version. I also produce an Adam II which lets you have a 16K RAM pack on the back of the Spectrum but which also allows devices which used to work in the popular 8K-16K region to work in the 56-64K region of the Spectrum. All the adaptors mentioned cost £9.
Of the RAM packs and kits reviewed. Fox Electronics for kits and Cheetah for RAM packs stand out as good value for money.
Fox Electronics. 141 Abbey Road, Basingstoke, Hampshire. Tel: 0256-20671
Cheetah Marketing, 359 The Strand, London WC2 0HS. Tel. 01-240-7939.
East London Robotics, No. 11 Gate, Royal Albert Docks, London E16. Tel: 01-471-3308.
Delta Research - cheques to Servodata Ltd - 15 Church Street, Basingstoke, Hampshire. Tel: 0635-45373.
Fuller Micro System, 71 Dale Street. Liverpool 2.
Eprom Services, 3 Wedgewood Drive, Leeds LS8 1EF Tel: 0532 667183
Stephen Adams, 1 Leswin Road, London N16 7NL.
All information in this page is provided by ZXSR instead of ZXDB