French ZX81, mod on video out before the modulator

Discussions about Sinclair ZX80 and ZX81 Hardware
Post Reply
rpaugusto
Posts: 4
Joined: Wed Nov 06, 2019 7:43 pm

French ZX81, mod on video out before the modulator

Post by rpaugusto »

Hi!

Bought a couple ZX81, one of them from France. Decided to open this one and it's an Issue 3 with 184E ULA. What's noteworthy is this mod between the board and the modulator input, check the red box on the diagram I made and the pics on Imgur. Just a begginer in electronics but seems that its purpose might be to increase video signal? Those a lot more knowledgeable than I am surely will let me know what it is.
Fr_ZX81_mod.png
Fr_ZX81_mod.png (8.24 KiB) Viewed 740 times
Pics of the mod on an external site (Imgur album, if external linking is not allowed let me know immediately): https://imgur.com/a/h3zDM8Z

Anyway, in a few days I will undo this mod to install a ZX8-CCB and have a proper composite video out.

User avatar
1024MAK
Posts: 2897
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: French ZX81, mod on video out before the modulator

Post by 1024MAK »

Compare to the schematic of a French Zeddy here ;)

Mark

rpaugusto
Posts: 4
Joined: Wed Nov 06, 2019 7:43 pm

Re: French ZX81, mod on video out before the modulator

Post by rpaugusto »

Mark, thanks for the reply and the link for the french schematics. So it's factory fitted, not a mod.

Still, one thing seems odd to me. According to the Ferranti ZTX310 series datasheet I find online, the collector and emitter seem to be switched. If I follow the datasheet, it would seem that the emitter is the one connected to the +5V and the collector is connected to the 150ohm resistor. That's why in my small schematic the transistor is that way. But it didn't feel right to me, that is also one of the reasons I asked about it.

Anyway, can't tune this right on a fairly modern LCD (that can be forced to SECAM) due to the lack of back porch. All I get is a fleeting distorted black background with small white edges on the right and bottom.

So it seems that to turn this into a standard UK Issue 3, just need to disconnect D9' from the ULA pin16 line, remove the ZTX313 and both resistors, and connect the modulator input to UK2 trace. Is this correct?

User avatar
1024MAK
Posts: 2897
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: French ZX81, mod on video out before the modulator

Post by 1024MAK »

French video uses an opposite polarity video signal. So it will only work with a French TV or one that accepts a SECAM video signal. That’s the purpose of the transistor, to invert the video signal. Note this does not on it’s own change the video from black on white to white on black, because it is also changing the polarity of the synchronisation signals.

Transistors connections / pin-outs are normally viewed from below, whereas ICs/chips are normally viewed from above.
Also the Ferranti E-line plastic packages have the front with the printing on and the curved corners, unlike the more common TO92 cased transistors where the front with the printing is the flat side. So it can be a bit confusing.

Mark

User avatar
1024MAK
Posts: 2897
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: French ZX81, mod on video out before the modulator

Post by 1024MAK »

The U.K. ZX81s take the signal from pin 16 directly to the modulator input. No diode, resistors, transistors or capacitors are used for the video signal. There is however a connection to the Mic socket using some resistors and capacitors.

Image

Mark

rpaugusto
Posts: 4
Joined: Wed Nov 06, 2019 7:43 pm

Re: French ZX81, mod on video out before the modulator

Post by rpaugusto »

1024MAK wrote:
Sun Nov 24, 2019 10:55 pm
Transistors connections / pin-outs are normally viewed from below, whereas ICs/chips are normally viewed from above.
Also the Ferranti E-line plastic packages have the front with the printing on and the curved corners, unlike the more TO92 cased transistors where the front with the printing is the flat side. So it can be a bit confusing.
Thank you, two things I didn't know!
1024MAK wrote:
Sun Nov 24, 2019 11:11 pm
The U.K. ZX81s take the signal from pin 16 directly to the modulator input. No diode, resistors, transistors or capacitors are used for the video signal. There is however a connection to the Mic socket using some resistors and capacitors.
R31 isn't present and there isn't continuity there, so that takes care of it. So doing what I said before is enough then. Thank you! :)

User avatar
mahjongg
Posts: 170
Joined: Tue Nov 24, 2015 10:25 pm

Re: French ZX81, mod on video out before the modulator

Post by mahjongg »

It seems the patch schematic is drawn wrong, the emitter should be on the 150 Ohm resistor side:

The emitter can never be on the V+ side, unless the transistor is a PNP, but a ZTX313 is an NPN device.

Here is a corrected schematic, also drawn in a more conventional way, with ground at the bottom.
Fr_ZX81_mod.png
Fr_ZX81_mod.png (8.09 KiB) Viewed 659 times

Post Reply