Refurbishing a 48K Rubber Keyed Sinclair ZX Spectrum – Preparation

Recently, I was lucky enough to ‘win’ a pair of 48K Sinclair Spectrums. The original rubber keyed variety. This model was one of the first I ever owned (the actual first being a Sinclair ZX81).

Rubber Key Sinclair ZX Spectrums

Rubber Key Sinclair ZX Spectrums

Here, I have managed to obtain issue 3 and an issue 3b models. The one with the better case is the 3b so I chose that to refurb first. In preparation for this, I tested both machines. I was surprised to find they both worked, at least to the copyright message, and both the membranes were intact. In fact, the issue 3b seems to have an aftermarket replacement membrane.

The one thing I did notice is the speed at which they both got to the copyright message, it was very fast, a telltale sign of upper RAM issues. This was confirmed when I put a Retroleum Smart Card on the back configured with Brendan Alford’s diagnostic software. Retroleum also supplies diagnostic software with their card but I have grown used to Brandan’s so I use that.

The test revealed that several of the upper RAM ICs on the Spectrum were not reading back correctly. Rather than replace the individual chips which are not the easiest to find, I decided to use a Retroleum Upper RAM module instead. This replaces all the upper RAM with a single module, relieving the old machine of lots of it’s legacy ICs.

Another vital thing to do is a re-cap. The capacitors in old machines often fail and in the Sinclair ZX Spectrum, can lead to faults in other parts of the machine. Again, Retroleum to the rescue! They do a complete re-capping kit that includes a Composite Mod too (although in this instance I will be using my own composite mod solution).

Further to the above, I want to replace the inefficient 7805 5V Regulator. There are two easy solutions; one is the Murata Switch Mode 5V Regulator that can supply 1.5A of power (also available from Retroleum), or the smaller TRACO TSR 1-2450. Since this machine will be seldom used and part of my personal collection, I decided on a TRACO unit.

The last thing to consider is the general look of the machine. The issue 3b’s case is in good condition with only one missing foot. One thing that is dissapointing is the state of the front fascia. It has several dents and a few small scratches. Luckily, all these spares are available nowadays and I chose ZX-Renew to supply a new black fascia.

Sinclair ZX Spectrum - Showing Dents

Sinclair ZX Spectrum – Showing Dents

Once all these spares have arrived, I will post updates in parts to show each step in the process that I take.

Series Index: Refurbishing a 48K Rubber Keyed Sinclair ZX Spectrum


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