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ZX Spectrum repair, keyboard not working

P

powerlot

Experienced Member
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Hi all

I came across a ZX Spectrum and as I have never used a British microcomputer I wanted to try it out (the history of early homecomputers in Britain is vastly different than the rest of Europe / the world).

The computer had a voltage regulator that fell off when I removed the heatsink, due to rust or stress? I replaced it. Then there was also no RF output, just some very faint bars.

As it looks like to be "standard procedure" to composite mod it, I decided to forsake the RF modulator and implement the mod


I was happy to see video output and the boot screen, even though there appears to have been a lot of modifications on the board done (many sockets where added, and the soldering job is mediocre with all of the flux left on the board)

When I tried to type nothing happened and upon inspecting the flex cables, I noticed that one was broken so I fixed that with some tape and a flex circuit repair pen.

Now I only have half of the keyboard working. Is the keyboard on this computer problematic? How do I disassemble it? It looks like I have to pry off the plate and that will surely bend it.

Additionally, I received it with the wrong PSU! It has an audio plug style connector instead of the barrel jack. Are there adapters available and what are the keywords to look for? I don't want to butcher the original supply.
 

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The keyboard on every Sinclair computer was 'problematic' :)!

I would use a four-letter word to describe them, but the mods may ban me...

I'll see what I can find out for you after breakfast.

Dave
 
The keypad membranes on ZX series computers are known for going faulty, the ribbon 'tails' which emerge from the keyboard membrane turn brittle and crack laterally, causing the printed tracks to go open circuit.

Sometimes, you can recover them by snipping off the last inch or two of ribbon end and peeling off some of the backing to expose 'new' ends to plug into the keyboard connectors, but if this does not work then there are enterprising individuals who have had new keyboard membranes manufactured and they are not too badly priced.

You can prove whether the problem is the keyboard membrane or not by disconnecting the keyboard ribbons and using a wire link to momentarily short keypad row and column connections together.
 
+1.

Yes, the keyboard is stuck to the faceplate by double-sided sticky tape (go figure - but cheap). Unfortunately, as you have guessed, you have to get a screwdriver and pry the two apart. The maintenance manual indicates that you will probably damage the faceplate in the process!

I would first test all of the keys out and report the ones that are working and the ones not. The keyboard is a 5 by 8 matrix, so it is possible to work out whether it is complete rows and/or columns that are faulty (indicating broken row/column PCB tracking) or random individual keys.

Yes, test out the electronics by using a piece of wire across the appropriate row and column wires to simulate the key itself. Download the ZX Spectrum maintenance manual to help with the key matrix. It also depends upon exactly which ZX Spectrum model you have.

I also wouldn't bother with an adapter for the power supply. One more thing to go wrong! You can purchase the correct connector and attach it in place of the incorrect one. How have you got it working at the moment? You just have to remember that the inner pole is negative whilst the outer barrel is positive (the opposite of convention). Getting this wrong will fry your ZX Spectrum...

You can purchase a replacement power connector from retroleum.co.uk. I am sure you can find these in Europe as well... Didn't we used to be part of Europe at one time?!

There is also a company in Gloucester UK selling replacement keyboard membranes, rubber keyboard mats and faceplates in different colours and styles. These are remanufacture units.

Dave
 
Thanks for your replies!
SiriusHardware said:
Sometimes, you can recover them by snipping off the last inch or two of ribbon end and peeling off some of the backing to expose 'new' ends to plug into the keyboard connectors, but if this does not work then there are enterprising individuals who have had new keyboard membranes manufactured and they are not too badly priced.
Click to expand...
I tried this and also more flex cable surgery, it's a bit better but still not completely working. I won't invest more time in fixing the broken membrane and order a new one!

daver2 said:
Yes, test out the electronics by using a piece of wire across the appropriate row and column wires to simulate the key itself. Download the ZX Spectrum maintenance manual to help with the key matrix. It also depends upon exactly which ZX Spectrum model you have.
Click to expand...
I have Issue 4B. I tried poking the connectors and most combinations result in some kind of feedback from the machine. I have no idea how to use a ZX so no idea if it behaves properly.

daver2 said:
I also wouldn't bother with an adapter for the power supply. One more thing to go wrong! You can purchase the correct connector and attach it in place of the incorrect one. How have you got it working at the moment? You just have to remember that the inner pole is negative whilst the outer barrel is positive (the opposite of convention). Getting this wrong will fry your ZX Spectrum...
Click to expand...
It's connected to my bench supply, I verified the polarity before applying power (else this thread wouldn't exist, or would be a bit sadder :))

daver2 said:
You can purchase a replacement power connector from retroleum.co.uk. I am sure you can find these in Europe as well... Didn't we used to be part of Europe at one time?!
Click to expand...
As someone who occasionally orders electronic parts from the UK, it's slightly annoying that it got more complicated now... if you mean Sintech, I'll order the replacement membrane from there, the prices seem fair.
 
There are plenty of YouTube videos (if that's your thing) showing how to replace a Spectrum keyboard membrane. With patience, heating (e.g. with a hair dryer), and a guitar pick or some similar kind of spudger, you can pull up the faceplate without damaging anything.
 
I just looked at it, on mine there are some brass tabs. If you straighten them well enough the faceplace comes off easy. I expected sticky tape and unstuck two of the tabs from the plate while prying, but it still looks like it can be reused again
 
Excellent work if you can reuse it. Perhaps the technical manual was being a little pessimistic in the hope that you will damage it and have to buy new ones...

I think it may have been Sintech (if they are in Gloucester).

The 'trick' with the Spectrum is that most keys have five (5) separate functions... I would suggest downloading the user manual and having a read of the basics about entering a simple program. What you want to enter is something like 10 PRINT "the quick brown fox jumps over the lazy dog" 20 PRINT "0123456789" 30 PRINT "any remaining symbol keys not used in lines 10 or 20".

The PRINT key is (of course) a function on the 'P' key.

Dave
 
Thanks, I'll read into it while the membrane arrives. Maybe I'll also check the capacitors...
 
Be mindful that many ZX Spectrum's often don't need the capacitors to be replaced. There are a few larger electrolytics in the voltage generator section that are under stress with low to medium ripple currents, but the remainder are all decoupling capacitors and if the video is OK ( it's a bit noisy at the best of times ) then there is also probably no need to replace the capacitors in the video section either.

If the capacitors aren't under load, then even if the dielectic liquid dries out, the paper dielectric is still going to function at low current. And the less you replace the lower the chances of breaking on old PCB. I have a few I bought from Ebay where people had destroyed the plating by changing capacitors, and some had even had tracks lifted.

Generally, having seen both sides of the argument now, I would recommend not replacing ZX spectrum capacitors over replacing them.

The keyboard membranes can't be repaired - While it's sometimes possible to patch them up and get them electrically connecting again, the plastic in them is deteriorating so quickly that even if you have a fullly working membrane, its going to fail so if it's original, get rid of it now and put in a replacement. The plastic will continue to deteriorate and is super-brittle. The replacements will last a lot longer as materials have changed since then.

Retroradionics are the cheapest keyboard membranes internationally, and can sell a new bezel cheaply as well - it's all aftermarket, but the quality I've received so far is very good. Be prepared for a long wait though - reviewing the other posts in this forum will tell that a couple of months is normal for delays around this time of year. Ebay is the quickest and most reliable, but you'll pay several times what you should.

When you get the keyboard working, use this program to check the upper memory clears the ram test -

PRINT PEEK 23732 + PEEK 23733 * 256

It will tell you the last memory location that was working when it did the ram test, and will show whether the upper 32K banks are working.

Regards
David
 
Alright thanks for the additional tips, I only replaced the capacitor closest to the modulator, the wavy lines are still there but it's much better now. I'll leave it at that regarding capacitors if everything else is working.

Instead of hacking up the original Sinclair power supply I found a generic one with similar output and connector and reversed the polarity, it also works fine.

I will update the thread once the membrane arrives and also check the RAM with the suggested program. I'm curious if the tape player I downloaded on my phone can load games successfully.
 
Hi all

Happy new year and good news:

The membrane arrived, I fitted it and it works perfectly, all keys respond now :)

cj7hawk said:
PRINT PEEK 23732 + PEEK 23733 * 256
Click to expand...

This results in 65535, I assume the RAM is good?

I managed to load a tape game from my phone with the jack on the phone side slightly unplugged (the sounds remind me of faxing) and looks like the Spectrum loads with no problems... thanks everyone for your help :)
 

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It depends a little on your tastes, for example many would nominate MANIC MINER but I really can't stand that game or any of its follow ups.

If you can find them, JETPAC, Sabre Wulf, ATIC ATAC are all nice games, or STARQUAKE.

We can probably advise you better if we know your tastes, but 95% of all Spectrum software (and a lot of other materials such as programming books) can be found on World Of Spectrum

(www.worldofspectrum.org)

Nearly all of the rights holders of original Spectrum software gave permission for WOS to host downloadable copies of their software there, I think only one or two of the original rights holders declined to do so.
 
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Hey, thanks for the recommendations.. Just wanted to dip my toes into what most Spectrum owners would do with it. I'll check the mentioned games out.

Not sure I want to program on this keyboard, I almost got a headache typing that one statement above... Pressing BOTH shifts changes the behaviour?!
 
Oh yes, welcome to the Spectrum keyboard :)...

It was a vast improvement on the ZX80/ZX81 keyboard though (and the wobbly RAMPACK)!

Dave
 
The Spectrum 'Plus' machines which came slighly later had nicer keyboards and the later still Spectrum 128K models had nice keyboards and an alternative version of BASIC which allowed you to type the BASIC code into the machine in a more conventional manner, rather than having to play TWISTER with your fingers. They still also have the original 48K tokenised basic for compatibility purposes.

As you have the original machine with rubber keys (commonly known as the 'Dead Flesh' keyboard) your gaming experience will be much enhanced if you can find a joystick interface for it - there are two main types, the 'Kempston' single joystick interface and the Sinclair two-port one which was built into the optonal 'Interface 1'. There were also third-party interfaces such as the RAM TURBO which actually has nothing to do with RAM despite the name - it just provides a twin joystick port, one of which can be used in either Kempston or Sinclair compatibility mode.

If you know a bit of electronics it's not too difficult to make your own Kempston joystick interface (just a couple of TTL ICs and the relevant connectors for the Spectrum rear edge connector and the joystick).
 
Incidentally, if you want to try out a British computer which is also a practical, well made and usable machine then you should try to find yourself a BBC microcomputer model B. They have proper keyboards, use more or less conventional 6502 architecture and are much easier to work on. They were also well supported with all sorts of software including many games, including the original wireframe version of ELITE.
 
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