vrijdag 28 april 2017

ZX Spectrum

The ZX Spectrum (or 'Speccy') is the successor to the cheap, but underpowered ZX-81.
Though still not a real powerhouse, the 16 or 48K RAM and slightly better keyboard than the ZX80 and ZX81 made this a moderate leap forward for Sinclair. Well, you can Read the full story on the Register. And definitely look at the Flickr page of Rick Dickinson with all the images showing how the Spectrum was designed.
This computer was one I already owned before starting my collection. It was given to me years ago by my father in law, and was stored somewhere in a dark closet ever since. After bringing it back to the light, one of the first things to do was modifying it for composite video output. There's an extensive description on the retrogamescollector website that I followed. But instead of connecting straight to the video signal I used the same setup as used to modify my ZX81, with a transistor and a 100 ohm resistor.
Transistor on the left. 100 ohm resistor from centre pin to the metal shield.

Unfortunately the Spectrum suffers from the same problem as the ZX81: the plastic of the membrane keyboard has become brittle, and it almost immediately broke when I removed the cover.
Crack.. Oh No.. Not again...
And this is a lot worse than the broken cable on the ZX81 which could be fixed using a pair of scissors and some patience. The ribbon cable broke at the edge of the membrane foil itself. So I tried to connect some bare wires to the remaining traces using a soldering iron, but this is useless. The plastic and the metal trace just melt and there is no connection between the wire and the keyboard. Probably something could be done using conductive glue, but it's way easier (and probably even cheaper) to buy a completely new membrane. I got mine from RWAP Software, through the SellMyRetro site.
It's easy to remove the keyboard
Removing the metal frontplate and the rubber key-pad was easy. Just bending up the copper notches and carefully lifting the metal plate using a flat screwdriver worked for me. But I've read in several places that the metal plate may also be glued so be careful !
New Membrane
Then it's just a matter of replacing the broken membrane and re-assembling the keyboard.

DC- 2.1x5.5mm Socket
Since the unit was open now anyway I decided to replace the DC-input. During the first tests I already noticed this was really bad, and just moving the Spectrum often caused a reset because the power was interrupted. The connector on the board is a very standard 2.1 mm DC power socket, so getting one of these was easy. Replaced it, connected the keyboard and the power supply, and it worked !. For 30 seconds...
Then the screen went blank, and a terrible smell came from the power supply. After opening the PSU housing I found I could barely touch the transformer since it was really hot. and the rectifier diodes did not look good either.
Closer examination of the Spectrum mainboard revealed a short circuit that was caused by some solder that I dropped when removing the power socket. And since it seems that there is no protective fuse anywhere, this caused a total burn-out of the PSU. While looking for a replacement, or maybe even just a new transformer I found the Spectrum repair Guide, which saved my day.
Here I learned that the transformer actually is protected by a thermal fuse. And even though this fuse is not replaceable it is possible to mount a 160mA fuse in parallel so the transformer will work again.
Now that's not as easy as it is shown in the repair guide. You cannot really just 'solder a fuse'. If you use a standard 5x20mm glass fuse it will break as soon as you try to solder something to the metal caps.  So I used a fuse-holder, which will just fit next to the diodes, and connected it to the transformer using two wires. Then I just replaced all the diodes and the PSU was up and running again.
Next I incorrectly assumed the centre pin of the PSU was positive voltage and the outside was negative, so the Spectrum still did not work. But after correcting this it finally came back to life.

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