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 (8.24 KiB) Viewed 2222 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.
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There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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?
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.
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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.
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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!
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