ULA ZX8301 - TV Picture Capabilities

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Martin_Head
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Re: ULA ZX8301 - TV Picture Capabilities

Postby Martin_Head » Wed Sep 13, 2017 11:04 am

When you said
tcat wrote:My TV is quite a modern Thomson make, it gives excellent picture with ZX Speccy and ATARI ST, all in TV mode, no flair in white.
QL Picture over RGB-SCART cable is also perfect.
was that cable RGB only, or was that using composite video? If the picture using composite video is OK. Then it looks like the MC1377 is working Ok. And I would just replace the UHF modulator before worrying about anything else.


Nasta
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Re: ULA ZX8301 - TV Picture Capabilities

Postby Nasta » Wed Sep 13, 2017 11:22 am

tcat wrote:Hi Nasta,
Thank you, I was hoping for and awaiting your comment, too :-)
Now again studying `QL Service Manual' and Issue#6 diagram. It says composite PAL signal is output/divided to UHF Modulator, capturing in the picture, when you say lowering amplitude, you mean changing values of the resistors R85/R86, how?
PAL-UHF-feed.png
Could it also be some of the modulators' transistors being degraded, worth replacing?
Many thanks so far.
Tomas


I suppose there could be a problem with the transistors but I would not say it is highly probable.
Regarding the resistors, perhaps increasing R85 from 100 ohms to 120 or 150.
The general problem with QL screen output is that it is internally routed from the 8301 which is all the way on the left-hand side, to nearly all the way on the right-hand side, through long lines that are handled as if they were digital, but in the end get to be interpreted as analog. This means that both when they are low and high, the signal has a whole lot of hash superimposed on it. So a lot of stuff can interfere.


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Re: ULA ZX8301 - TV Picture Capabilities

Postby tcat » Wed Sep 13, 2017 5:19 pm

Hi Martin,

ZX/ATARI over UHF modulated TV cable, picture is of good quality, this may exclude TV from fault finding, I should think.
I did not try QL composite output, that would require feeding the signal to a yellow input socket on my TV, also bypassing modulator is needed for that, to get composite signal, I guess. Tried only RGB over SCART that is good, though.

Hi Nasta,

I will toy with R85 value as suggested, perhaps I may bridge it in parallel with the same value, that's an easy test for me. White flair appears as if of some kind of digital pattern. I cannot therefore help thinking about PENCL signal, could it be the cause of picking noise in modulated video?

Many thanks all so far.
Tomas
Last edited by tcat on Thu Sep 14, 2017 8:19 am, edited 1 time in total.


Nasta
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Re: ULA ZX8301 - TV Picture Capabilities

Postby Nasta » Wed Sep 13, 2017 7:38 pm

Putting a resistor in parallel with any other resistor results in a resistance value which is ALWAYS LESS than either of the two in parallel. So in this case you might try it with R86.


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Re: ULA ZX8301 - TV Picture Capabilities

Postby Martin_Head » Thu Sep 14, 2017 10:48 am

tcat wrote:Hi Martin,

ZX/ATARI over UHF modulated TV cable, picture is of good quality, this may exclude TV from fault finding, I should think.
I did not try QL composite output, that would require feeding the signal to a yellow input socket on my TV, also bypassing modulator is needed for that, to get composite signal, I guess. Tried only RGB over SCART that is good, though.

The SCART socket on the TV should have a video in pin 20, use pin 18 as ground. The composite signal is available on the RGB socket on the QL (pin 1, and pin 2 for ground http://www.dilwyn.me.uk/docs/hardware/monitor%20connections.jpg) You should just need a piece of screened cable between the two. There is no need to bypass the modulator, or even open the QL.



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