keyboard – Gatt's Retro Tech

Welcome to part 2 of this series where I try and repair my first ZX Spectrum.

So far, we’ve installed the Composite Mod and managed to get it boot, but not been able to get much else due to a broken keyboard membrane

Well that finally arrived today – and here it is:

(Excuse the dirty mat underneath – I really should clean that some day)

This came from ZX Renew and is about £8 + P&P so not bad really

Installing is a bit tricky – the first task was to remove the face plate – being careful not to bend it too much

Once done, it was time to clean off all the glue and give the rubber key mat a good wash and a scrub to get rid of the 40+ years worth of grime that had built up – lovely!

And whilst they are drying all that was needed was to slide out the old and knackered membrane and put the new one in its place.

Super easy – so easy in fact I forgot to take pictures – ah well!

Anyway with that in place and the mat dried and put back on it was time to stick the faceplate back on – some double sided tape should do the trick here

And here we go – I powered it on and was able to type out the first line of a program

Lovely, ok lets press enter and get the next line in..

Oh dear.. that isn’t supposed to happen..

This doesn’t look good at all – time for some more trouble shooting.

First up the 7805 voltage regulator – this should read 12V on the left leg, and 5V on the right leg
Results: 10.2V on the left and 5 on the right – that looks good to me – next – the voltage pins on the lower ram modules

The pins we are interested in are the 4 corner pins and should read -5V on the top left, 0V on the Top Right, 5V on the bottom right and 12V on the bottom left

And here we have a couple of problems
I was reading 0V on the -5V pins, and 9.5V on the 12V pins

I reached out on the MFMI Discord server and was told that the chances are that TR4 or TR5 are a goner

So, I’ve ordered one of each and will probably just replace both of them and report back when done
Thanks to Royal Mail – this will take a few days so see you then!

So after purchasing a faulty ZX Spectrum 48k, it’s time to start trying to get it up and running.

Task 1 – Composite Mod

From my previous post, we know that it needs at least a composite mod so we’ll do that first. This is actually fairly simple to do and there are a ton of videos on YouTube that walk you through the process

I would recommend heading over to the likes of More Fun Making It or Happy Little Diodes who both have a great selection of videos on speccy’s and the composite mod.

All you are really doing is disabling the current RF signal and putting a capacitor in it’s place. It’s well worth doing as most modern tellies don’t usually have the old analogue input anymore.

Once done it should look something like this

Anyway with the mod in place, it was time to put it all back together and power it up.

And this is what happened:

Well that was a lot more than I was expecting.

This is good news, it should mean that the majority of the components are good – specifically, the ULA, the ROM, the Z80 and the lower ram chips.

I won’t know any more until I can replace the keyboard membrane.

Tag: keyboard

Spectrum 48K Repair – Part 2

Spectrum 48K Repair – Part 1