After the success of his two previous computers, the ZX-80 and particularly the ZX-81, in April 1982 Sir Clive Sinclair presents the ZX-Spectrum. For the first time, a computer with high-resolution graphics, colour, sound and 48k RAM was sold under £200. Once more Sinclair revolutionized the microcomputer industry with new standards.
Again based on the Z80A CPU, two models were launched: one with 16K and one with 48K RAM.
There were 8 revisions of the motherboard (revisions 3 and 6 also had 3A & 6A versions) with revision 1 machines much sought after now by collectors. An original issue 1 machine had light fawn coloured keys. All the other versions had the familiar darkish blue/grey keys.
A much improved keyboard over the ZX81, it was still a low cost, multifunction affair, but with a rubber key mat rather than a flat plate. It probably held the world record for the number of functions on one key - as many as six!
The best feature of the Spectrum was its graphic possibilities. With 256 x 192 pixels, it was one of the highest resolutions for a microcomputer of its range in 1982. Unfortunately, colours were limited to 1 ink colour and 1 paper colour per 8 x 8 pixels squares. The greatest drawback was the sound possibilities - a poor beeper controlled by the BEEP statement – that was it !...
Problems and Issues.
To keep the prices down Sinclair used faulty 64K chips (internally 2 X 32K). All the chips in the 32K bank of RAM had to have the same half of the 64K chips working. A link was fitted on the pcb in order to choose the first half or the second half. It was possible with a few logic chips for an experimenter to have access to the faulty 32K bank with all sorts of odd results possible !
Issue 1 ULA Problems
Issue 1 Spectrums had problems with the ULA so that INKEY$ (gives the currently pressed key) only worked in 50% of the cases. This was solved by adding a small PCB board (the spider), piggybacked on the ULA.
In issue 2 there still were some problems with the ULA, but it was improved by adding a transistor on top of the CPU.
Tuning & Colour
Issue 1 & 2 boards were famous for poor background colour and poor tuning. Yellows which should be white, dot crawl and difficulties getting the colour to “lock” are very common. On some TV’s it is impossible to even achieve colour.
There are two preset capacitors and two variable resistors all in a row which can be adjusted to help with these difficulties - this is covered in the REPAIRS section of the website. Click here for the specific link.
Issue 3 boards & onwards have improved ULA circuitry which is self adjusting so these presets are not fitted.
Production was switched to Taiwan from this model onwards.
48K Machines working as 16K
I have seen this unusual problem five times now - I have had fully populated boards which should have been 48K spectrums only working as 16K machines.
Sinclair sold a number of 48K machines that had memory faults as 16K machines (labelling them 16K of course). The extra 32K of RAM was soldered in place however, even though it was not working. At the time this caused a major problem for people who had bought a 16K machine because of cost constrainst and then saved enough dosh for an upgrade pack - imagine the surprise when they opened up their machine to add the extra chips! Through the years, people have swapped motherboards around trying to get working machines, or the 16K labels have been lost or removed giving a nasty shock the moment anyone tries to load a 48K program and can't understand what is wrong.
As a check, enter the follow code directly:
PRINT PEEK 23733 and then [ENTER]
For a 48K machine the result will be 255
For a 16K machine the result will be 128
Flashing Coloured Blocks
One of the most common serious Spectrum faults.
The outputs of the voltage regulator circuitry are taken to the edge connector and an oscillator in the circuit is extremely sensitive to any extra load. This extra load is usually generated when a joystick or other peripheral is added or removed with the computer turned on.
The other culprit is one or more of the lower 16K memory chips shorting. These 4116 chips can still be found and faulty ones replaced if you are good at de-soldering, but finding the faulty memory can be very hit and miss unless you have a good record on the lottery or football pools. Sometimes you might be lucky and it will be obvious as a particular chip getting hot much more quickly than the rest.
Higher Memory Faults
These problems often go un-noticed until a certain memory address is accessed which results in a system crash or “Out of Memory” message.
To check that the entire memory is operational enter the following line:
PRINT PEEK 23732 + 256 * PEEK 23733 and then [ENTER]
The result should be 65535 for a 48K spectrum or 32767 for a 16K model. Any shortfall (lower number) indicates a memory fault.
Permanent Black Raster and White Border
The cause of this is a faulty ULA chip – these are usually available in the shop. On some models the chips are not soldered onto the boards and replacement is very easy. Carefully lever the chip out of its socket cradle with a flat screwdriver blade (or chip puller if you’re posh), levering evenly at each end to avoid bending the pins. Push the replacement evenly into the socket ensuring that it the right way around (the notch on the ULA chip goes towards the back of the board).
Keyboard problems
Groups of keys mysteriously stop working - probably the most common Spectrum faults and fortunately one of the easiest to put right. This is almost always a worn out membrane, although splits in the keyboard mats can also cause similar problems.
New keyboard membranes and rubber keyboard mats are usually available in my shop and fairly easy to fit provided you have strong fingernails and lots of patience. The keyboard plate has to be removed and is usually stuck down pretty solidly on "rubber key" Spectrums.
For instructions on how to replace a "rubber keyboard" membrane, see the REPAIRS section of the site, or click here.
For the two Spectrum+ models, again it's the REPAIRS section, or click here.
© DataServe 2006
