Lambda 8300

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Re: Lambda 8300

Post by 1024MAK »

I have just been doing some work on the Lambda 8300 that I got a while ago as a non-working unit.

The PCB pads on the main board for the connections to the keyboard have been damaged (lifted pads and tracks). So after cleaning the board to see how bad this problem was (and to check for any short circuits there), I decided it was time to try to power it up.

So, first I tried a regulated PSU set to 9V DC. I got a completely white screen with no letters, no start up message, and no curser :(. Out came the multimeter, yep, the +5V supply was good. Now what about this seperate supply to the "ULA". Strange, on the schematic, it looks like a 9V supply, fed via a Zener and single transistor. The power pin of the "ULA" had about 5.4V on it. I checked some pins of the Z80 CPU with the logic probe while I was there, nope, the signals did not make sense. Something wrong.

So, next I disconnected the regulated PSU, and changed the output to 12V DC. WOW, that made a difference...
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Now I had a READY start up message in the top left on a black screen AND a flashing curser in the bottom left of the screen. I should point out that the iPad is not good at taking screenshots like this, it is much clearer in real life, and not blurred like in the photo. But not shown in this photo, there are some vertical "white" lines.

This photo shows the lines:-
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Anyway, here are the power supply details and voltages:-
Input voltage: 12V DC regulated, centre pin is +, outside is negative. Actual input voltage is 11.98V
Zener voltage is 8.1V
After the transistor, this gives a "ULA" supply voltage of 7.85V

Right, I'm off to figure out how to wire the keyboard back up...

ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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