ZX Spectrum 48k r2 composite video problems

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
paf
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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Fri Nov 23, 2018 10:25 am

Hey, nobody can help? :shock:

In the meantime, I have found a video of somebody repairing a ZX Spectrum 48K with a 6C001 ULA. A some point, an oscilloscope trace of the ULA's Y output is shown, and the signal levels are the same as my 5C112 ULA produces and not at all what is described in the ULA book. Unfortunately, it was measured as an AC signal, so I cannot know how the DC component was.

So, either the information in the ULA book is wrong, or I understand them wrongly. :(

Still, it would be great if somebody could confirm me the voltages on the ULA's Y output, on the input to the RF modulator and tell me if he sees colors burst during the vertical sync too! If possible with a 5C112 ULA, but also with another type it would be much better than nothing.

Regards
Patrick

PS: by tweaking VR1 and VR2 I can always convince the TV to lock on the vertical sync and display a good picture. The problem is that it only works a few days, then I have to tweak again. As already said, it doesn't seem to depend on the temperature of the components.
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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1024MAK
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Re: ZX Spectrum 48k r2 composite video problems

Post by 1024MAK » Mon Nov 26, 2018 6:37 pm

Various waveforms here ;-)

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1024MAK
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Re: ZX Spectrum 48k r2 composite video problems

Post by 1024MAK » Tue Nov 27, 2018 11:11 am

Patrick, I’ve added some more ‘scope traces, use the link I posted earlier ;-)

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paf
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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Tue Nov 27, 2018 8:49 pm

Thousand thanks to artVan an 1024MAK for your scope traces. Now I have all the information I need.

For those who might be interested:
a) The amplitude of sync/black/white on the Y signal is similar between 5C102 (issue 1), 5C112 (issue 2) and 6C001 (issue >2): 3.4-3.6V for sync, 2.2-2.5V for black, 1.25-1.6V for pale white.
b) The composite signal into the RF modulator shows even less differences between the different ULAs.
c) The voltage between black and pale white level is smaller than between sync and black: about 50-90% whereas the PAL norm suggests 230% for the brightest white.
d) The vertical sync pulse is exactly the same on 5C102 and 5C112: 256us, much shorter than the PAL norm says and with none of the pulses which the norm specifies.
e) The color bursts all 64us continue during vertical sync pulse.

I suspect that d) or e) are the cause for my synchronisation problems with the Samsung LCD TV. The fact that it depends on the color settings (VR1 and VR2) suggests rather e).

c) is a likely cause for the bleak colors on my TV.

I have created an EXCEL sheet with all the information (in french), if you want it just send me a private message.

Regards, Patrick
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

paf
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Re: ZX Spectrum 48k r2 composite video problems

Post by paf » Thu Dec 06, 2018 7:03 am

Last update...

I have bought the following composite video to HDMI converter for about 10 USD: https://www.aliexpress.com/item/Mini-10 ... 504eOIkcBs

It works, there are no synchronisation problems but the colors are very bad. It's interesting that when I hook my Spectrum to my Samsung LCD TV (with the same internal Spectrum settings), they are OK on the TV.

I have tried the converter with another video source (DV camera) and the picture quality is OK.

As long as my TV works well with the composite video from the Spectrum, I won't tamper with VR1/VR2/TC1 so I don't know yet if I could get a good picture with the converter.

Regards, Patrick
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My computers: Sharp PC1211, PC1500, Sinclair ZX Spectrum 48K, QL, Commodore Amiga 500, PC, PC, PC, PC :-(

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