Fixing Keypad Issues on an Issue 6K 128K Toastrack

When the Sinclair 128K ‘Toastrack’ machine was released in early 1986, one of the peripherals advertised with it was a separate numeric keypad. This was advertised in the 128 brochure as offering “greater control when using the full screen editor, and can also be used as a calculator or a 128K games controller”.

However, despite being bundled with the Spanish version of the 128, they were never offered for sale in the UK, and only a couple of units are known to exist today.

To speculate why this happened, first we need to take a look at the hardware. The first UK 128 PCB revision was the Issue 6K, and in this version there was an incorrect resistor value in the circuit that supplies the -Vcc line to the MC1488 line driver IC, which handles the serial and keypad ports on a 128. This meant that the MC1488 was not getting proper supply voltage, preventing a keypad from communicating properly with the machine.

R137 is outlined in red in this Issue 6K 128. It has already been replaced with a suitable 15 ohm resistor.

This was fixed in the Issue 6U revision, which is by far the most common version of the 128. However, for Sinclair to market the keypad successfully they would have had to recall all 6K machines for remediation, and presumably the precarious financial state of Sinclair Research at the time meant that the easiest course of action was to simply not offer the keypad for sale in the UK.

Knowing what the issue is, and how it was fixed in the Issue 6U, it is simply a matter of replacing R137 on the 6K PCB with a 15 ohm resistor, and any available Spanish keypads should work without problems.

(Thanks to Ian Gledhill for identifying the issue, deriving the fix and publicising the details)

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