Friday, June 08, 2018

ZX Spectrum - My Little Buddy

In 1984 I remember coming home and seeing my Dad reading a book about the ZX Spectrum + and my brother and I both asked "Are we getting one then?" to which my Dad replied, "Go and look in the other room."

Sat on the table was a shiny new ZX Spectrum +. To cut a long story short, my brother and I played many, many games for many hours. My Dad spent many hours tinkering (much to my Mum's annoyance) and, at one point, managed to break the copy protection on Jet Set Willy II, purely because of the stupidly huge copy protection foldout card...


As time went on I became interested in other computers (the Amiga by any chance?) but I always remember the happy clicks that emanated from the keyboard whenever any key was pressed.

So in 2018 I went and bought a ZX Spectrum 48K with rubber keys because, why the heck not? As with most eBay purchases, the unit was sold as 'untested' but did come in its original polystyrene but no outer cardboard box. It also has the original manuals - which, to be honest, are far superior to the Spectrum + documentation - and the original power supply.

Before I did anything I went and watched the videos by the excellent Joulespercoulomb. In particular, he has produced a video that details the tests to undertake before plugging in the Spectrum. There is also a very useful video on power supplies.

Original Power Supply 
This supply is original and marked as 'UK 1400' which means that it should deliver 1.4amps. Other supplies might be marked 'UK 700' which, as you might guess, delivers up to 700mA.

Power Supply Opened Up for Testing
Opening up the supply reveals just how simple (and cheap) these supplies are. It's basically a transformer with four diodes and a couple of smoothing capacitors. That's it. The DC 5V that they produce is notoriously 'lumpy' and in my unit, generates an almighty coil whistle. But I'm getting ahead of myself.

I checked the diodes in circuit and they all seemed OK. I checked the transformers windings to check for short circuits etc. Also, no problems. This supply should be OK to use. And one final point. The Spectrum expects the centre pin to be NEGATIVE. The modern supply I have has detachable barrel connectors so I have to make sure when I plug it in that I have it the right way around or I may release magic smoke.

To the main event. There are five screws in the bottom of the Spectrum's case. Once taken out the top lifts off but remains connected to the main board by two connectors from the keyboard membrane. In my machine, the membrane was intact but in a sorry state.

Left Connection - Bent and Broken
Right Connection - Also Bent and Borked
I suspected that the keyboard wouldn't work even if it booted correctly. This is fairly standard for these machines but new membranes are available from various places. No biggie.

Can you hear me, Mother(board)?

The main board is an Issue 2 which is easily identified by the location of the voltage regulator heatsink being on the bottom right. Later boards moved this to the top, near the edge connector. There is also a factory mod on the CPU with a transistor mounted across it. This was added due to a 'bug' (or feature) in the ULA - Uncommitted Logic Array which meant that the system would be too aggressive when it thought that RAM was being contended between the CPU and ULA, causing speed and timing issues. Later boards incorporated this update onto the actual board.

Before I could do anything I had to modify the TV Modulator. No-one uses RF these days but, as luck would have it, by disconnecting two wires and soldering the end of one of them to the centre pin of the modulator's connector, the Spectrum will output composite which most modern TVs (and my hacked display) will accept. This is actually a quick and dirty way to do this and I will be re-visting the display quality in a later post.

Anyway, the moment of truth. Does it boot and display the (C) 1982 Sinclair Research message?


Broken. Sad Face.


Fortunately, I have a neat device called a Smart Card V2 from (a site well worth visiting if you like the Spectrum). This device allows you to load games from Micro-SD cards but also includes a diagnostic ROM. This was what it said about my little buddy:

Tell me what's wrong Doc...

So, U19 is suspect. This is one of the upper RAM chips. Fortunately, our friends at had some in stock so, for the bargain basement price of £1.25 I ordered another RAM chip. Time to get the soldering iron out.

Cheap but Effective
 First, I had to remove the old chip. This involved using the iron and a desoldering tool. Loads of videos and tips exist for this online so I won't go into too much detail. The tracks are more delicate than I'm used to with other systems such as the Amiga A500 but, with a bit of patience, I managed to get it off without issue.

The new chip went on without any problems too. I used leaded solder as supplied by my Dad. The solder must be 30 years old but is great quality and works a treat.

New RAM Loveliness

The more observant may notice that I mounted the new RAM chip directly to the PCB like the others. I'd like to say that I didn't put it in a socket because it would spoil the aesthetic appeal of the original circuit board design. Actually, I didn't have any sockets.

Old Broken Chip - Boo!

So, does it work now?

Anxious Wait

Now that is a result. I can't do much yet because of the knackered keyboard membrane but the motherboard is healthy now. More next time!

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