|transistors (TR4) tends to blow up in
response to sudden voltage fluctuations.
This results in the loss of the -5 volt and
+12 volt supplies to the RAM and the
appearance of little black squares over
the screen when switched on. The only
solution is to change the transistor (of
course, if the machine is under guarantee,
then send it back to Sinclair Research). |
The change in ULA on the Mark 3s also brought to light another problem, because some software houses had not been following the procedures set out in the Spectrum manual. The keyboard input port also reads in a value from the EAR socket and until now this has been Binary 1. On Mark 3 Spectrums, this value was not maintained because, to reduce power consumption, the values of the pull-up resistors were altered.
The result is that the EAR bit now floats until the ULA has warmed up, and no value has been given for it by Sinclair Research in the manual. Games and other software which check the whole byte, and not just the keyboard bits, can no longer be used and anyone owning a Mark 3 is strongly advised to check before handing over the 'greenies' that any new purchase is not going to suffer this problem.
It also turned out that Mark 1's design caused problems with the Interface 1 - and thus its compatibility with the ZX Microdrive. A transistor modification has had to be made inside the interface to cut down the time taken to switch on the unit from power-up.
|SEXING YOUR |
S P E C T R U M
An amusing divertissement for any known collection of true Spectrumists is to enquire innocently whether someone can identify a particular ZX model - or ask how A's machine differs from that of B. It's a guaranteed crowd-puller! Stephen Adams fancies he can supply the answers and, taking the Spectrum by the chips, he details the various stages of development since its April 1982 launch.
After three issues of the ZX Spectrum
and countless modifications, many of
you must be wondering just what goes
on inside that little box of tricks - and
just how you're affected by which model
you have. Well, wonder no longer -
here for your delectation is a comprehensive guide to the Spectrum's innards. |
Perhaps the most obvious distinguishing factor of the Mark 1 is that it sported a grey keyboard. The Mark 2s and 3s both have a blue background to the keys.
Internally, the issues have been
mucked about a bit. But, for details, check
the annotated photos for the complete
picture. Apart from the RAM changes
and the re-positioning of various major
components, Sinclair Research made
their most radical alterations to the
Spectrum when introducing a redesigned
PCB and a new ULA to their final
When Sinclair Research began work on the issue 3 Spectrum, a low-powered version of the ULA was employed. Even so, a problem with the Spectrum internal power supply is that one of the
|Click on the thumbnail for the BIG picture.|
|SPECTRUM ISSUE 1 (16K) |
|SPECTRUM ISSUE 2 (48K) |
|SPECTRUM ISSUE 3 (48K) |