Sunday 20 August 2023

ZX Spectrum Issue 2 Future Proofing

I was expecting this to be a repair, an issue two ZX Spectrum that the owner bought on ebay with the ominous status of "working when last used, but no longer able to be tested". That normally means guaranteed broken, possibly raided for parts.

Rather surprisingly, when the owner tried it, it worked.

Obviously, this is an issue 2 Spectrum, so the screen is either going to be greeny-yellow or bluey-yellow, but that can be fixed.

The Spectrum was sent to me for some further testing, and selected preventative maintenance.

Inside, it looks clean and not messed around with. There are various mods, but I think those are all factory changes.

The "mandatory" mod to (try to) improve the switching power supply circuitry. The design changed with every revision, and it was always pretty ropey.

The usual "transistor on top of the Z80" mod which limits the IO decoding to "A0 is low" like the ZX80 and ZX81, rather than all addresses as the Spectrum originally did. This transistor was added to the PCB on later revisions of the board.

Also seen there is the 1K pullup on the clock line, and not shown, some further component substitutions on the left of the board. 

More about these mods -

And finally this one is a bit unusual. Not seen it implemented this way before.

The upper 32K bank of RAM is provided by 8 64K RAM chips, only half of which is used. These chips were rejects with faults in one half or the other, and marked by the manufacturer as 32K chips. They came in two types, one with the bottom half usable, and one with the top half usable.

Jumper links were used to select which half to use, so all chips had to be the low or high type.

This is unusual as neither link has not been fitted, and instead a leg of one of the 74LS157s has been lifted and wired to 5V, and on the back of the board, two pins are solder blobbed to send an address line to that pin instead of 0V or 5V from the jumper.

The 64K chips have addressing as 8x8, 8 rows are latched, then 8 columns, giving a 16 bit address for 64K. Two 74LS157s are used to switch between A0-A7 and A8-A14 plus a dummy A15, which is hard wired by a jumper link either high or low depending on which half of the chips is good.

This was designed for Texas Instruments TMS4532 chips, which appear to be TMS4164 chips where one half has failed testing. (-L3 for lower half, -L4 for upper half).

This board has OKI chips, which has the dummy line at A7 rather than A15 (or it may be the other way around, my head hurts already).

It seems to have been a factory 48K board, so I guess they were using these with the extra mod on some of the later issue 2 boards.

Issue 3 and later boards had an extra jumper for this option to allow these OKI chips to be used as well as the Texas Instruments ones, and this modification appears to be the same as selecting the OKI jumper on later boards.

7805 Replacement

Now for the first change.

The standard Spectrum has a 7805 5V regulator. This is good, solid, reliable linear regulator, they rarely fail, and when they do, it is usually a no output situation.

The are, however, very inefficient. Given a 1A load and a 9V supply, 5W (1A x 5V) is used by the load, and 4W (9v-5v x 1A) is burned up as heat, hence the big heatsink.

Modern switching regulators can be over 90% efficient, so waste less energy as heat, so it is a worthwhile replacement to reduce the amount of heat in the tight case.

Later Spectrums had the regulator at the back of the board, so vertical switching regulators could be fitted, but the issue 2 has the regulator lower down in the case.

You can fit vertical ones on their side, with some header pins or wire links, but I like to use the right angled ones, such as this RS K7805-2000R3L, which is rated up to 2A, more than enough for the Spectrum and add ons.

Capacitor Replacement ?

Next, the thorny question of capacitors. There seems to be a modern obsession that "all capacitors should be replaced", and I don't agree with that. There are situations where that is the case, like cheap modern electronics, and particular things from the 2000s where there were lots of bad capacitors around, or wax and paper type capacitors from the 1950s and earlier found in valve equipment etc.

These ones from the 1980s tend to still be good, and quite honestly may be better than the modern ones they would be replaced with. Makes me sad to see people cutting out perfectly good 40 year old Nichicon caps and replacing then with random ebay tat that probably won't last 4 years never mind 40.

I asked in the well curated echo chambers of my Twitter and Discord, and almost everyone agreed with me that they should not be replaced unless they are actually bad.

I did due diligence and tested several of them (lifting on leg so they can be tested out of circuit. They all passed. 

That is a 100uF capacitor. The original specification would have been -20/+80%, so it is well within range.

I compared against a few modern capacitors I would have used to replace them, and the values were similar, other than the capacitance being lower on the modern ones.

I don't think the value is particularly important. The basic range of capacitors available at the time were limited to 10, 22 and 47 with multipliers. So the designer would have looked at what they needed for the reservoir capacitors and decided 47uF was too low and 220uF was unnecessary and so went for 100uF.

The only one that was a little out of range was a 1uF capacitor in the reset circuit.

Seemed worthwhile to replace that and also the other 1uF capacitor, C46, as that was a critical part of the -5V circuit.

I tried a few modern ones, and they were generally worse, lower capacitance and higher ESR.

I decided to go for 1uF ceramic bead capacitors, which looked quite good, and I think they should be fine for that application.

The two 1uF capacitors are either side of the keyboard connector (where C46 is marked with the incorrect polarity, not that it is a problem with non-polarised ceramic caps). I raised the legs a little, as there were various traces below.

Transistor Replacement

Also in the power supply circuitry I replaced TR4 and TR5, as these are common points of failure in all Spectrums.

The ZTX650 was replaced with a ZTX651 which is the same, just with a higher voltage rating.

There was a BC308 fitted for TR5, which is a bit lower power than the ZTX213 it is meant to be. (Possibly a later repair, hence the power supply mod?)

I replaced with a ZTX751, which is both higher power and higher voltage rated.

Colour adjustment

To get rid of the yellow screen you need to tweak the two variable resistors. The official recommendation involves setting them to get a particular voltages (the inverse of each other).

I prefer to use the technique of tweaking each pot in turn to give the sharpest signal on the scope. -

It starts of quite fuzzy. The sharper that line is, the whiter the picture is. Issue 3 and later boards do this automatically, there are no pots to adjust.

As you tweak them, the line gets sharper and less yellow / blue.

And that is about as good as I can get it.

Composite video mod

The last step was the composite mod. I have tried various options on issue 2 boards, and tried some of them again here.

The simple 100uF capacitor version works, but not very well.

I went back to my normal way of adding a TFW8b composite video buffer board.

Does the job nicely,

That is all the modifications complete.


One further thing I would like to look at would be the ULA. Like most Spectrums, the ULA runs hot, and will over time cook itself and fail.

I normally recommend getting a replacement, such as the vLA82, and preserving the ULA in a working state (rather than waiting for it to fail and then installing a replacement anyway)

In the Spectrum+ case, there is enough height to install a heatsink on the ULA to help a bit. There is not room in the rubber key case.

Some people desolder the ULA socket and solder the ULA directly to the board, so that there is space for a heatsink. I am not really a fan of that, it risks damaging the board, and will need to be redone when the ULA eventually fails.

In this case, the owner does not plan to use it heavily, and wants to keep it original, so the fact the ULA runs hot is accepted.

I did some soak testing to make sure everything was running well, but made sure to do that with the case open and a heatsink on top.


All that's left is a cosmetic fix.

The membrane it not original, and has been replaced in the past, made by a company called Tesla (not that one).

In order to do that, the faceplate would have been removed and replaced, if it had not already fallen off and been stuck back on several times already.

As you can see, lots of residue of past applications of glue and double sided tape.

You can get replacement faceplates, but the owner wanted to keep this one original, and it's not in that bad condition, so time for the slow process of cleaning all that off.

I use an old credit card or plastic business card, and some label remover spray. Don't use pointy metal things as they can damage the metal faceplate.

All clean, time for some new tape. I put that on the Spectrum first. Seems easier that way.

And there we go. There is still some glue residue on the faceplate, but it is a bit risky trying to clean that as it can take the paint off.


Testing all went well, no problems.

Of course, I am using a TFW8b divMMC future to load the games.

It's a hard job having to play all these games, but I just have to force myself to do it.

All done and ready to go back to the owner.


I wasn't going to write this post up, didn't think I had much to say......

Didn't even get on to the hFE testing on the transistors.

Or sorting out my capacitor collection to try to find more capacitors to compare against.


Minstrel 2 and 3 (ZX80 and ZX81 compatible)

There are Minstrel 2 and 3 PCB, full kits and build versions available from my SellMyRetro store.

Minstrel 4D (Jupiter Ace compatible)

Minstrel 4D kits are currently on special discount at The Future Was 8 bit, in DIY kit and built and tested versions.

DivMMC Future

The DivMMC Future is available from The Future Was 8 bit


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