This spectrum 48k seems to work fine except for the 6,7,8,9 and 0 keys. This is almost certainly going to be the keyboard membrane and I just happen to have a spare one (newly manufactured membranes available from sintech.co.uk). While I’ve got it open I’ll do the well-known composite mod to give a better picture by modifying the RF output to give a composite signal instead.
The composite mod is very easy to perform and only requires basic soldering skills. First I disconnect the resistor from the output connector shown circled in the picture above. Next desolder the two wires from the PCB connected to the RF modulator on the left side (also circled above). Once all these are disconnected I cover with heat shrink-wrap so it can easily be restored at a later date.
The next step is to connect the top most hole on the where I disconnected the wire, to the output connector inside the modulator. This can be done with a simple piece of wire but supposedly a better picture can be achieved by soldering a 100uf capacitor between them (positive side to the PCB), so this is what I’ve always done:
With the composite mod complete I move on to the keyboard. First I need to confirm that all keys work from the motherboard side using a piece of wire to connect pins from KB1 to pins on KB2 using a lookup I found on Google to test the 6,7,8,9 and 0 keys. ALl keys work this way including the offending numbers. This means I need to replace the membrane.
Using a heat gun on a low setting so as not to melt the rubber keys I gently heat the metal faceplate to melt the glue a little and allow me to free up the faceplate from the case. With this removed it’s an easy task to switch membranes, I take the opportunity to clean the rubber keys while I’ve got it open with some isopropyl alcohol and an old toothbrush. The second picture above shows the old (top) and new (bottom) membrane, the old membranes get so brittle after 30 years they break incredibly easy and it’s almost always going to be this when you have keyboard issues.
After putting everything back together using a hot glue gun to re-adhere the faceplate. I plug in and turn on the computer and try to type the problematic numbers again and it’s a success!! Look at that picture quality over composite, well worth the little bit of effort.