comp mod – Gatt's Retro Tech

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.

Well, I’ve finally managed to get my hands on a ZX Spectrum 48k with a view to repairing it as it was listed as not outputting any type of display.

It came with the RF lead and the original PSU, as well as a couple of cassettes.

The speccy itself is actually in rather decent condition with only a couple of minor dents on the front of it.
It is dirty, again not unexpected given its age.

Inside, we can see that it is an Issue 3 board and, again, it is in pretty decent condition with no evidence of any prior work being done to it.

Unfortunately, the keyboard membrane didn’t survive me taking it apart so this will need to be replaced in the event we get it up and running.

Now the fun starts as we try and troubleshoot what the issue is. First order of business will be to do a cursory check of all the capacitors and chips to ensure that there is nothing obviously leaking, damaged or missing.

My rough list of objectives will be to:

Test the 7805 to ensure it is supplying the correct voltage, as I understand this is a common issue
Check the ULA – as I don’t have a diagnostic cart, initially this will probably be with a multimeter and oscilloscope
Do a composite mod. That way I don’t need to worry about having a TV that takes the old AV input.

Tag: comp mod

Spectrum 48K Repair – Part 1

My first ZX Spectrum 48K Repair Project