CPC464 keyboard repair

[grobda]

Heya's

Recently got my first CPC464 offof ebay (untested).

It had a fault where some keys would result in other characters appearing and others didn't work at all. My unit has the 'twin membrane' keyboard IE two layers separated by thin white lines of insulation/adhesive.

I googled a bit and found some mentions of cleaning, with or without fluids, and some about repairing traces.

I was a bit reluctant to start introducing fluids to such a keyboard so after taking the keyboard apart I dusted everything off, then carefully peeled apart the two membrane layers; halfway from one end, then let them seal together again, then repeated the process from the other end.

This solved the problem, I can only guess that the two membranes had become partially stuck together over time due to spilled fluid or being pushed together for an extended period. 

I thought I'd post it here in the hope it may help someone else with a similar problem. Apologies If i've missed this method being discussed previously. 

Was expecting more problems from an untested ebay unit but none have made themselves known, even the cassette belt is still ok 

[Bryce]

Hi Grobda,
    yeah, that's pretty much all that's required on most 464 keyboard issues of this sort. There are further issues that effect other CPC's: On 664's the keyboard membranes get brittle and need to be completely replaced, on CPC+ the resistance of the pads increases and the pull up resistors need to be removed and on occasions (all models) will have a failed AY chip. You got the easier issue. Congratulations

Bryce.  

[martin464]

I had the same luck. Got 2 464's, the more broken looking one had stuck keys and all it was, was the membranes sticking together. Just taking it apart and putting it back together fixed it. This 464 is a 1984 one and works perfectly

The other, apparently better looking one seems to be a 1990 one with the ASIC and it's not too happy
Yes it fires up but there is a buzzing or fizzling sound and it randomly resets if you leave it on. It can take 30 minutes or an hour but it just does a complete reboot after a while
I was going to make this one a project CPC for some nefarious upgrades but this problem is a bit worrying...

[eto]

"untested" CPCs are often a very good deal. Worst case so far was a 464 with fried ROMs due to being connected to a 19V power adapter.  

And just yesterday I finally received a 464 with low keys. I was looking for that one for quite some time. It was sold as "partially working" and the only fix required was to resolder the screen connector. 

[Bryce]

I had the same luck. Got 2 464's, the more broken looking one had stuck keys and all it was, was the membranes sticking together. Just taking it apart and putting it back together fixed it. This 464 is a 1984 one and works perfectly

The other, apparently better looking one seems to be a 1990 one with the ASIC and it's not too happy
Yes it fires up but there is a buzzing or fizzling sound and it randomly resets if you leave it on. It can take 30 minutes or an hour but it just does a complete reboot after a while
I was going to make this one a project CPC for some nefarious upgrades but this problem is a bit worrying...

That is most likely something simple like dry joints on the power connector or even dirt in the connector.

Bryce.

[martin464]

Bryce how do you do it. Yes the power socket was visibly corroded. A broken up emery board poked inside seems to have done the trick for now. Thankyou!

Can I ask, if I remove the tape deck is there enough power to run a 3.5" floppy disk instead or does it need external power?

[Bryce]

Bryce how do you do it.

Years of experience repairing hundreds of CPC's and other retro computers.

The Amstrad disk drive requires 12V which the 464 doesn't supply. You also need a disk controller and DOS ROM to connect and use a disk drive and the 464 also doesn't have this.

Bryce.

[martin464]

or the stormlord fairy is feeding you inside information!
but about the disk, a 3.5" floppy only needs 5v am I right? No 12v needed for these

So I could power the DDI-1 interface off the motherboard which I understand normally is powered off the DDI-1 itself through the ribbon cable, but I could wire it to be powered off the motherboard and have the disk powered off it as well, I was thinking

To begin with I'm just trying to get the 3.5" floppy working as a drive B: but if that works I wanted to make it the drive A: and burn a ROMDOS EPROM, the DDI-1 I have already has a socket for the ROM. I could just burn a new EPROM and replace it

[Bryce]

Oh no, the secret of the Stormlord Fairy is out!!

Yes, 3.5in drives only need 5V, that's why I specified "The Amstrad disk drive requires 12V". As for powering it all from the CPC, that shouldn't be an issue, but I would recommend adding an inductor (in series) and an electrolytic capacitor (in parallel) on the 5V rail between the drive and the mainboard. Some drive motors, especially if they are a bit older can cause voltage spikes on the 5V rail which could cause the CPC to crash or even damage the mainboard electronics. If you look at the Plus schematics you'll see that Amstrad did this too (C39 and L04).

Bryce.

[martin464]

Yep as I suspected she has you on the payroll i knew it!

[xyo]

It had a fault where some keys would result in other characters appearing and others didn't work at all

After resurrecting my 6128 I had the same problem - though in my case most keys "worked", but apart from ESC and RETURN all returned all kinds of different symbols:



I took apart the keyboard and was surprised there was acutally lots of thick dust inbetween.
At one place on the membrane, there was some kind of dirt on it. I also wasn't sure if and what liquid to clean it with and decided to use just some wet cloth which worked perfectly.
Also after putting everything back together, all keys produced their intended character again

Just wanted to let everyone know, since this was the only post I could find where the keyboard technically worked, but resulted in output of wrong characters and strange symbols.