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The broken keyboard was an easy fix, after a few measurements with my multimeter one broken trace was found. Vtech used a nice pcb for the first layer, but a kind of printed graphite for the second layer... and over time the graphite layer became non-existent.
I soldered a small wire in => keyboard works!
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Still i only got a b/w display and not the nice green one :-)
The "color-part" is located on a small extra pcb, after i got hold of a shematic i quickly found, that the color-circuit needs 12V to operate, which where not present...
So i started with the simplest things: I measured (and changed) the electrolytic capacitors, mesaured all diodes and even checked the transistors => no use
The circuit uses an oscillator-coil and two transistors to generate 12V from the 5V input.
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I checked the coil at last, because i thought coils do not easily fail (and i did not know anything about the inards of an osczillator coil, i even thought they could not be opened).
But, after i desoldered the 2 outer pins i could remove the top!
And what did i find on the inside?
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After looking very closely i recognized, that one wire was broken!
You can see it in the picture (left bottom).
So coils do fail!
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After very carefully lifting the wire i tied a knot around it with one strand of wire, soldered it on and connected it to the pin at the bottom. |
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That's how it looks... |
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So after putting everything back together... |
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I got a nice colourfull Display!
In the picture you see a little color demo running.
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