Video Issue

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papanillu
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Video Issue

Postby papanillu » Mon Mar 05, 2018 7:11 pm

Hello;

When I started my old QL after find in my loft, it had a video issue, colors were not "solid"

I changed all electrolitic capacitors, the MC1377 and the ZX8301 chips,
but the issue is still there

You can see a video here
https://youtu.be/BlkOk-rsFko

Any suggestion?

Thanks.


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Cristian
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Re: Video Issue

Postby Cristian » Mon Mar 05, 2018 8:00 pm

My experience is limited, and I've never seen an issue like this. May I suggest you to provide more information meanwhile? This might help to detect the problem.
Do you use some kind of upscaler?
Have you tried to connect via RGB-SCART or antenna cable?
Have you already tried other displays?


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Re: Video Issue

Postby RWAP » Mon Mar 05, 2018 8:07 pm

That is a new one on me - it suggests the RGB output is not working properly - if you are using a QL SCART lead - are you sure it is one made for the QL and not one made for a Spectrum (as there are differences between the cabling).

If it is just a connection via the antenna, it could be a tuning issue on the TV - remember the screen after log on should be red on the right hand side - not red & cyan stripes.


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papanillu
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Re: Video Issue

Postby papanillu » Mon Mar 05, 2018 8:38 pm

You're rigth.
I could have give a litle bit more info.
I use the pal composite output with a composite to vga converter. It works quite well with my other retros, including another ql bought from Rich the last week.

I can't test via the rf modulator because no one of my tellys has an analog input.

In my other monitor the result is the same.

Any sugestions?


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Pr0f
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Re: Video Issue

Postby Pr0f » Mon Mar 05, 2018 8:44 pm

to get red/blue stripes like that in what should be solid red suggests video is taken from the output of the MC1377, and that is not operating correctly - possibly +12v supply is not working correctly?

I think it's pin 14, but my circuit diagram I printed is tiny..


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Dave
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Re: Video Issue

Postby Dave » Mon Mar 05, 2018 8:47 pm

The chrominance subcarrier is losing synch for some reason. There are quite a few reasons why this could happen. One of my Issue 7 boards does this. I have never found the cause, but I never looked too hard either.


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Re: Video Issue

Postby martyn_hill » Mon Mar 05, 2018 10:02 pm

Hi Papanillu

If you want to validate the advice given thus far, you might want to see whether using the composite monochrome output fed to your VGA upscaler renders a solid - albeit grey-scale - image where you currently see the blue/red unstable stripes.

The upscaler should cope with this and it would simply involve temporarily disconnecting the 2 wires connected to pins 1 and 3 in your QL Monitor plug and reconnecting the wire from pin 1 (PAL colour video) to pin 3 (monochrome video).

If this produces a stable solid 'colour' (ok, grey) where the wavy red/blue were, then the components around the MC1377 could be at fault.

My guess? Either the 4.43MHz crystal oscillator X3, or the phase delay sub-cct involving the inductor (L3) and capacitors (C33 & C34).

Those crystal oscillators in particular can get mechanically damaged due to being raised from the cct board and are cheap and dead-easy to replace/re-solder.

Good luck!


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1024MAK
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Re: Video Issue

Postby 1024MAK » Mon Mar 05, 2018 10:25 pm

I agree, it looks like a problem with the PAL colour circuitry (the MC1377 and associated circuitry).

Well worthwhile seeing if the monochrome signal is okay, as suggested above.

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
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Dave
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Re: Video Issue

Postby Dave » Tue Mar 06, 2018 6:04 am

*grins*

Look at how the white area is perfect. Only the red area goes to green and back. This is interesting, because in the Y-UV model, the V axis is +red/green-. The red signal itself is intact, otherwise white areas would show cyan banding. So the signal path from the 8301 pin 30 to the red input on the 1377 (pin 3) is likely ok. There is no chrominance signal during white.

As Martyn stated, L3, C34 and C33 are a good place to start - that, with R55 also changes the impedance from 10 to 50 ohms. The B-Y and R-Y clamps on pins 12 and 11 (through C28 and C27, respectively) null any residual color subcarrier (I'm just quoting from the datasheet here 'cos it's fiddly!)

What would help us would be to divide the screen into a red, green and blue area. Or to have a video: type MODE 4 then do PAPER 1 then CLS then PAPER 2 then CLS then PAPER 4 then CLS. It helps to know if/which colors shift to which. This will tell us if one or both axes are toggling. This gives us a big clue if the problem is external (just one axis) or timing or internal (both axis)...

Are you in NTSC or PAL territory? This does make a difference. From the red/green inversion, it seems it would be PAL.

Hope this helps.


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papanillu
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Re: Video Issue

Postby papanillu » Tue Mar 06, 2018 6:32 am

Thanks,
I will start test things as soon i get the capacitors and resistors.
As currently i'm sharing my desk with my daughter i cannot leave open computers in a half of repairing status, so usualy i get spares, and later do the job.

As you said Dave,This is a PAL territory, and I'll record the video Dave sugested this evening, when i finish work.

Thanks again



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