+2 (grey) SCART glitches

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FelisSapien
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+2 (grey) SCART glitches

Post by FelisSapien »

Hi all,

Looking for suggestions as to where I go next with some video issues I have...

Connecting via RGB/SCART (not tried compostie yet) I'm seeing the screen blank briefly every couple of seconds. Otherwise the image quality is very good. Whatever is happening to cause the blank is also sending the TV a wobble as it resets it's channel settings and dropping back to it's automatic aspect ratio. Also, over 10 mins or so I see the image slowly become more yellow. Likely not related but, who knows?

My board is Z70500 Iss 3. Based on some browsing I tried a few things...

moved the jumper from LK4 to LK2 and changed resistor R9 (1k to 680): This made the image more stable but the picture quality became MUCH worse.
flipped transistor TR4: Didn't have a noticeable impact.

I've reverted back to using LK2 and the SCART glitching issues returned.

Previous to this I've also made modifications to balance out the audio. These didn't seem to affect the image at all. Just mentioning to provide a fuller picture of where my dabbling has taken me.

Other things worth mentioning(?): I've a second +2 that I've not touched yet. It displays the same SCART glitches. Maybe this is fingering the SCART cable? Might be worth buying/building another. The one I have currently is labelled COOLNOVELTIES. I could also try to use another screen. My 'retro' LCD is generally pretty good. I have a CRT set also but there's no SCART input on that,only svideo. Not sure I can prove much there.

Any thoughts?

Thanks,
Carl
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1024MAK
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Re: +2 (grey) SCART glitches

Post by 1024MAK »

The first step with a display issue like the one you describe, is always (where possible) try using as many different TV’s that you can.

Keep in mind that when the ZX Spectrum was designed, there were not many SCART equipments TV’s available in the U.K. and those that were, were all CRT (Cathode Ray Tube) types without any automatic input selection switching. You the user had to manually select if you wanted composite (the default for SCART) or RGB.

With any TV’s that you do have / try, see if there is a remote control button or menu setting to switch the SCART manually to RGB. Also where possible select PAL I as the colour standard.

With automatic switching SCART sockets, the TV tries to workout what type of signal the sending device is sending. There are a couple of control lines to do this. For a TV to correctly recognise a valid RGB signal, the voltages on the control lines have to be in the correct range. If the voltages are not correct, some TV’s will keep switching between composite video and RGB video.

LCD TV’s work completely differently to CRT types. LCD TV’s sample the incoming signal and then try to make sense of it. So as the ZX Spectrum computers don’t actually produce video signals to the full broadcast standard, some TV’s may be confused and then will not stay in the correct ‘mode’.

So, please do try with another TV if you can, and see if your TV(s) have any manual user controls.

The second thing to note is that the +2 and the 128k RGB video out DIN sockets by default don’t have a proper +12V output suitable for the automatic function switching signals for the SCART sockets on modern TVs. So a ‘hack’ is often employed. This may, or may work...

I will post further information and help later.

Please do post back here.

Mark
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FelisSapien
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Re: +2 (grey) SCART glitches

Post by FelisSapien »

Thank you!

Good points, all makes a lot of sense. If I remember correctly I needed some kind of expansion to get RGB output with my rubber keyboard Speccy back in the day.

Will test more without further hardware hacking and report back soon.
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1024MAK
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Re: +2 (grey) SCART glitches

Post by 1024MAK »

it would also help if you could work out the connections and wiring in your Cool-Novelties cable, as I don’t think I have one for the grey +2 from them. I modified the cable I got from Retro Computer Shack.

I use SCART RGB cables from both Cool-Novelties (cool-novelties) and Retro Computer Shack (retrocomputershack) for a variety of 1980s computers.

If you do work out the connections and wiring, please post up the details.

Mark
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FelisSapien
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Re: +2 (grey) SCART glitches

Post by FelisSapien »

Some progress...

Based on measurement between the pins it seems I've got a cable designed for a +2A.

I'm basing this on
Pin 4 of the DIN(?) mapping to SCART 20 (VIDEO IN)
Pin 1 is mapping to SCART 16 (BLANKING)

Although this very nearly works with my setup it doesn't tie up with a +2 SCART doc I found.

Not had a chance to try more TVs yet (time is very short at the moment) but it seems like tweaking the cable might move things forward.
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Ast A. Moore
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Re: +2 (grey) SCART glitches

Post by Ast A. Moore »

FelisSapien wrote: Tue Apr 09, 2019 8:33 am Based on measurement between the pins it seems I've got a cable designed for a +2A.

I'm basing this on
Pin 4 of the DIN(?) mapping to SCART 20 (VIDEO IN)
Pin 1 is mapping to SCART 16 (BLANKING)

Although this very nearly works with my setup it doesn't tie up with a +2 SCART doc I found.
This will work on a standard +2A. On a +2, this will work, provided the jumpers LK2/4, LK1/3, and LK7/8 are configured appropriately. The value of R9 will have to be lowered, too (680 Ohms seems a tad high; I’d go for around 470 Ohms).

LK1/3 is best left unconnected
LK2/4 set to LK2 (to R9)
LK7/8 set to LK8 (audio output)
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FelisSapien
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Re: +2 (grey) SCART glitches

Post by FelisSapien »

Need to test further but now have things set as above.

Initially, things seemed improved. Picture is stable but there remains an issue where after a period of time the image quality degrades quite dramatically.

Thanks for all the help so far. Will test further and report back.
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