Sinclair ZX80 / ZX81 / Z88 Forums

Hi,

I finally worked out the 48k+ issue 4B that had the fried transistor.
I bought a replacement for the keyboard membrane, but once mounted I have a strange issue.
The special keys aren't recognized. All keys seems to work fine, but the extended keyboards don't. I see they seems to be seen, but they do nothing.
For example I can press Symbol Shift (left one) and change the token for the keys, I can press Symbol shift plus "extended" and see the cursor change to E, but Extended mode alone don't do anything. the special keys " and ; do nothing, except change to Extended mode when pressed with Symbol shift, and so on.
For delete I need to press Caps shift and 0 like in the 16/48k (rubber keyboard).
Is this a rom issue or a membrane issue?

Thank you kindly


F:

That’s an issue with the different layers of the membrane not making good enough contact with one another where the clamps are.

The ROM and the rest of the circuitry have no idea of the existence of any of the ‘extra’ keys. The system just sees what it thinks is two keys being pressed simultaneously, because under the ‘extra’ keys, the membrane has two sets of contacts wired in parallel with the equivalent individual keys that perform the same functions.

Mark

From the instruction for Spectrum+ upgrade:
If the word STOP does not appear on the screen, the following modification will be necessary:
locate R68 on the circuit-board. Solder a 22K resistor across it, being extremely careful to avoid solder splashes and short circuits.

Uh, ok, I soldered the resistor but it didn't work.
After checking VERY carefully at some image I found on the internet, I found that my clamps were mounted in the wrong way, the throat of the hole must not be on the TOP side but in the bottom side.
It may be a bit illogic, but if they must push on the membrane, they must be correctly mounted.
Many thanks guys!
Now the restoration of my Plus is complete! it works like a charm.

Kind regards


Federico

Good to know that you now have it working

Mark

Yeah, planning to play The Hobbit this weekend! Thanks!