Story so far. Cheapo Spectrum+, replaced the Z80, replaced an upper RAM chip. It works. Keyboard works a bit but there is a large portion in the middle that doesn't work.
So, I have a brand new membrane, the one that was recommended on several websites. I should also point out that several websites also noted the potential difficulty of getting Spectrum+ keyboards working correctly.
Pah. How hard can it be?
On the standard ZX Spectrum keyboard, there are several key combinations that allow different modes to enter different commands. For example, 'Extended Mode' requires the pressing of SYMBOL SHIFT and CAPS SHIFT. This turns the cursor into a flashing 'E' indicating 'Extended Mode'. Simple.
For the 'upgrade' to the Spectrum+ the keyboard was improved by adding specific keys to cover these modes so instead of having to remember a key combination, all I need to do is now find the right key. In the example above, there is now an 'Extended Mode' key. Nice.
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| Extra special keys. Yum. |
To support this the keyboard membrane in the Plus is different to the standard rubber keyed models. It has an additional layer to cover those extra keys. Remember that the function is still the same but the membrane now has to mimic two keys being pressed at the same time. The membranes are bigger and, because of the extra layer, the tails need to be clamped together to make sure that the three layers are making good contact.
Dismantling the Plus keyboard is also much more involved than the standard model. There are two clamps which need to be removed. This frees up the tails. Then there are ten screws that fix a metal (or could be plastic) back-plate. Underneath this could be a cardboard spacer (if the back-plate is metal) followed by the membrane and then a 'bubble' mat which acts as the return spring for each key.
In my case, I have a metal plate and a very, very saggy bubble mat. More on that later.
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| So. Many. Parts. |
The old membrane tails didn't look in too bad condition but, with a bunch of keys failing to work, it seemed obvious a new membrane was needed. There are several reproduction membranes on the market but the one that kept being recommended was those supplied by RWAP, certainly a well respected supplier in the Retro community. Having read that the most excellent people at Mutant Caterpillar assisted with tweaks to its design, and also seeing that Retroleum recommended them too (notwithstanding he won't stock any '+' membranes due to issues with making Spectrum+ keyboards work properly), I ordered one of this type.
On arrival it's obvious that the membrane is made from very different materials to the original. The plastic is smooth and transparent and the overall membrane is slightly thinner than the Sinclair original.
A sidenote. If you are lucky enough to end up with an Issue 4S then you have a Spectrum+ manufactured by Samsung. Yes, Samsung. Unsurprisingly, the keyboard membranes on this variant of the Speccy do not suffer the same degradation problems that the Sinclair models do and most are reported to be still going strong.
Anyway, back to my Issue 4A. I have watched several videos on replacing the Spectrum+ membrane including the excellent Mark Fixes Stuff. I also read up as much as I could on Google given the apparent difficulties that people encounter when carrying this out.
Fitting the membrane is a reversal of dismantling the keyboard. Firstly, the bubble mat sits behind the keys. Then, the membrane is laid on top of the bubble mat, ensuring that the location posts protrude through the locating holes and that the screw posts line up correctly. Then, the cardboard is placed on the back to help keep the membrane flat. This also has a couple of location holes. Then, finally, the metal back plate is attached with many, many screws securing all of the layers to the keyboard.
When this is done, it's important to remember that the tails need to be re-fitted in the claps. This is make sure that all three layers of the membrane have their contacts pressed together properly.
Finally, the tails are inserted into the two connectors on the main board. And it just works! Right?
Wrong.
First major problem. The mid portion of my keyboard was sunken quite badly. I had a to order a new bubble mat which, fortunately, was in a much better condition than the one I had and was a very reasonable price from our friends at Retroleum.
Next problem. None of the keys actually seemed to work properly at all. Certain keys on the left side of the keyboard would produce numbers but that was about it. After re-installing the clamps and tightening them a little more the situation improved.
A lot of the keys now gave the correct response. Space, Enter and the two variants of Shift worked with most keys. But there was another big problem. The extra keys wouldn't work. Neither would certain key combinations, most notably the symbol shift + 'p' combo to give the " symbol. Any Speccy afficiandos will know that it's a bit tricky to load anything without the " symbol.
Also, and more annoying, most of the middle row of letters would trigger an additional character. For example, pressing 'g' would give a 'g' (or appropriate keyword) on the downstroke of the key, but on releasing the key, an unexpected 'f' would appear. In fact, all of the affected keys generated an extra 'f' except for 'f' which generated an extra 'g'.
This extra character problem wasn't limited to the middle row either. Other random keys suffered but the middle row was by far the worst.
By now I was getting frustrated and contacted the seller. Their response was almost instant and very helpful and detailed. I tried a lot of the suggestions but some I knew wouldn't make any difference as I was having the problems even before the keyboard assembly was installed back onto the case.
I ranted a bit on the Spectrum FB group too where the seller also happens to hang out (not surprising really!). Even more helpful suggestions came my way to try and I tried them all. Nothing worked.
Eventually, I tried stripping the membrane out of the keyboard assembly and plugging it into the main board but leaving it flat in front of the machine. By pressing my thumb and finger against the tails on my desk to replicate the clamps I could then see if I had a (rare) faulty membrane.
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| And...stretch, 2, 3, 4 and.. reach 2, 3, 4.. |
The results initially seemed to indicate the membrane was fine. I could press the 'keys' on the middle row and no extra characters appeared. I could hit the extra keys and they would 'just work'. Except one. The " key. This would still not work, nor would the symbol shift + 'p' combo.
One of the FB group referred me to the Spectrum service manual. An addendum to manual detailed instructions on how to upgrade a 48K rubber keyed spectrum to a Spectrum+. One of the steps listed was to check that 'STOP' worked on the keyboard i.e. symbol shift + 'a'. On my membrane, this also didn't work even when presented naked on my desk. The suggestion to improve this was to add a 22K ohm resistor across R68 . This has the effect of reducing the resistance from 10K ohm to 6.8K ohm. This I duly did.
Annnnd, it made no difference to the 'STOP' or " symbol issues.
At this point I decide to re-install the old membrane, just to see if I could make the 'STOP' work at least. The 'a' key was not in the group of faulty keys.
Having read so much and done so much fiddling with the new membrane I was a dab hand at re-installing the old one. Imagine my surprise when, after completing the re-installation I discovered that, not only did 'STOP' work but so did the " symbol and every other key too, even those in the previously faulty area. (Well, I say 'all' but one key didn't work - the 'Inv Video' key but who uses that? And more importantly, the 48K rubber keyed combo for 'Inv Video' still worked if I needed it.)
From this I can only assume that a previous owner had been fiddling, buggered up the clamps when putting everything back together and assumed that the membrane was dead. It's not. At least not yet.
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| Big clamp.... |
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| Little clamp... |
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| This is getting a bit wearing... |
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| Tails, tails, tails, you can swing them to and fro... |
But then the awesome Mutant Caterpillar offered to test the membrane for me, on behalf of RWAP.
They found that the membrane was fine. They trimmed the ends of the tails very slightly to get rid of the worn ends and it just worked. I wasn't surprised by this given my finger stretching exercise, but I was very puzzled as to why it didn't work in my Spectrum+.
So I received the membrane back and decided it give it another try. And.....
It.
Just.
Worked.
I have no idea what I did differently, but the thing just worked. The only problem is the 'STOP' keyword doesn't work (which is the SymbolShift + 'a' key combo) but this is a known issue that I can probably solve with tuning the resistor across R68 - as per the ZX Spectrum service manual.
Perhaps I wore away one of the contacts on the tails on the very first install resulting in the erratic behaviour I saw all the way through, or maybe I just didn't hit the sweet spot before in the Molex connectors on the main board. Or maybe the screws on the backplate were uneven or too loose or too tight. Who knows. At this point the membrane now works. I'd highly recommend them despite the problems I had which were obviously not down to the actual membrane. You can get them here.
So, after all that the only things I have left to do are add a proper composite mod and, if possible, try and sort the STOP keyword. :)
