ZX80 Jumper A - B or A - C, purpose?

Discussions about Sinclair ZX80 and ZX81 Hardware
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Erm
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ZX80 Jumper A - B or A - C, purpose?

Post by Erm »

Hello,

I cannot find a definitive answer on this. I know it has something to do with the video output.

On this ZX80 I got today, the PCB connection that is normally present between the A and C positions has been cut and instead there is a new connection from A to B. I can see that it selects between pins 7 and 9 of IC9.

Also, there is a transistor (a ZTX108) with a resistor under the board across the modulator +5v and video in.

I get no picture on the usual channel but there is something in the way of video there as when I press keys there is a flicker and the pattern changes.
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I'm Chris, a Brit in Germany (Chr$ on the QL forum). https:\\QXL.WIN < QL related collection.
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1024MAK
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Re: ZX80 Jumper A - B or A - C, purpose?

Post by 1024MAK »

Did it come from France?

Mark
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Erm
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Re: ZX80 Jumper A - B or A - C, purpose?

Post by Erm »

I did wonder about the French TV standards, because it doesn't seem to match the norms for UK/Europe or USA (it has no D11). It came from Austria, but who knows where it came from originally.
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1024MAK
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Re: ZX80 Jumper A - B or A - C, purpose?

Post by 1024MAK »

The A-B / A-C link selects between the normal ZX80 black on white and an inverted white on black display. This is ahead of the mixer that adds the sync.

French analogue TV has a different signal polarity compared to PAL and NTSC, hence a transistor is used to invert the video signal after the sync signal has been added.

I've not confirmed if this is the case with your ZX80 yet.

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

:!: Standby alert :!:
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Erm
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Re: ZX80 Jumper A - B or A - C, purpose?

Post by Erm »

I suppose it won't do any harm to remove the transistor and resistor (or disconnect but leave in place) so that I can at least see if it's fully working. I'll have to see how I feel about the inverted video - I've just got another Lambda as part of a lot and they are 'inverted' from new and it looks cool but not sure I could live with it on a ZX81.
I'm Chris, a Brit in Germany (Chr$ on the QL forum). https:\\QXL.WIN < QL related collection.
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1024MAK
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Re: ZX80 Jumper A - B or A - C, purpose?

Post by 1024MAK »

Take some good quality photos from various angles before you disconnect or remove it.

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

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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Erm
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Re: ZX80 Jumper A - B or A - C, purpose?

Post by Erm »

1024MAK wrote: Sat May 30, 2020 11:08 pm Take some good quality photos from various angles before you disconnect or remove it.

Mark
Certainly will.
I'm Chris, a Brit in Germany (Chr$ on the QL forum). https:\\QXL.WIN < QL related collection.
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