Author Topic: Dirty video on 6128 Plus RGB Scart  (Read 8134 times)

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Offline endangermice

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #25 on: 20:02, 03 August 12 »
Well considering that previously he was making cheap crappy HIFI systems the fact that the CPC and its successors were excellent machines has to be considered something of an achievement!
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Offline Bryce

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #26 on: 21:56, 03 August 12 »
I though that all your custom RGB video cables already got capacitors inside. :p

My version has one capacitor to hold the RGB switching voltage, but no capacitors for the colours, otherwise it would mess up the colours on a standard CPC.

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Offline endangermice

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #27 on: 01:13, 04 August 12 »
Yeah mine is exactly the same as Bryce's one capacitor to hold the RGB switching voltage but nothing else. If I'm going to solve a problem I'd much rather do it on the motherboard, it's neater and means the thing is outputting something correctly in the first instance.
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Offline Badstarr

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #28 on: 01:18, 04 August 12 »
Curious... My plus doesn't seem to suffer from this problem, this confirmed by me plugging it into several TVs since I bought it a year ago. No evidence of this problem on a GT65 either, although the horizontal picture is shifted slightly to the left. The previous owner had hacked a simple filter onto the power input to solve a slight bit of interference from the PSU (that I believe came from a flat bed scanne the, PSU not the interference  ;) ) perhaps this filter is somehow preventing the dirty video effect on my plus. I do plan on removing the filter and using a different PSU that I recently bought from eBay, however the PSU gets crazy hot so it's either a crap regulator inside or it's not properly heat sinked this makes me a little worried about using it with my plus for now until I crack open the case and see what's what.
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Offline endangermice

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #29 on: 01:53, 04 August 12 »
Interesting, maybe it only affects some machines though that really doesn't make any sense unless there were some late revisions to the motherboard....


I think you're very wise pulling the power supply apart before you do anything with it. A lot of the less scrupulous companies hide a multitude of sins beneath the casing!


I'm running mine on an excellent little switched power supply I bought a few years ago. It's meant to power a shuttle mini atx board but works very nicely with both my CPC and Plus! We used the same supply at work to power a few video playback machines which were run 24/7 for several years without issue which hopefully is a good sign!
« Last Edit: 11:09, 04 August 12 by endangermice »
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Offline TotO

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #30 on: 13:51, 04 August 12 »
Most quality videogames RGB cable get capacitors on the RGB lines.
Sure, if the system already got them, the colours may look a bit washed.
But w/o, you get worst... (like on the picture)
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Offline Badstarr

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #31 on: 13:52, 04 August 12 »
I have tried the PSU on a recently acquired, sold as faulty 464, which is working ok by the looks of things apart from the big hole in the case which apparently happened while it was on route to me. Kinda annoying since I'm using the case for my ├╝ber CPC project. Just sat idle with no peripherals attached the PSU was very hot but even with no load the unit gets really hot too but it's output seems rock solid stable. At least it's worked well enough to confirm the 464 main board works fine so I have a spare to add to the pile lol!
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Offline arnoldemu

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #32 on: 17:35, 04 August 12 »
Ah, ok, then that's an easy one - Yes, every Plus does that (except mine, because I've modded that bug out ages ago :) ). The streaks are due to the fact, that the cost cutting idiots at Amstrad in their infinite wisdom decided to delete the RGB output capacitors on the Plus. Add a 220pf capacitor to each colour signal (between colour signal and ground) and the streaks will disappear. Any higher than 220pf and the colours will run into each other and make the picture blurred.

Bryce.

Edit: Here you can see the difference:
wiki wiki wiki ;)
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Offline endangermice

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #33 on: 01:41, 05 August 12 »
Yeah this should definitely be a wiki article, once Bryce has got back with value of the caps!
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Offline TotO

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #34 on: 10:31, 05 August 12 »
Yeah this should definitely be a wiki article, once Bryce has got back with value of the caps!
The schematic show 220pF capacitors, as usually on RGB cables or inside videogame systems.
Now, if AMSTRAD remove them it may because the CPC+ Monitor get inputs capacitors instead...
« Last Edit: 11:07, 05 August 12 by TotO »
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Offline Bryce

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #35 on: 13:01, 05 August 12 »
I just checked. I used 220pf capacitors, but I've never connected my Plus to a SCART connection, it has an S-Video output. I also power it from a Spectrum +3 power supply and have extra filtering added, so it may be the result of all these factors, that I have no streaks. Adding the 220pf capacitors can't hurt though, so I'd recommend adding them anyway.

@Badstarr: A GT65 definitely won't display this bug, as it doesn't use the RGB signals at all, only the LUM signal is used to produce the Green screen picture.

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Offline Badstarr

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #36 on: 17:36, 05 August 12 »

@Badstarr: A GT65 definitely won't display this bug, as it doesn't use the RGB signals at all, only the LUM signal is used to produce the Green screen picture.

Bryce.


I thought that might be the case, although I'm a little unclear (probably as I haven't given it much thought) how the GT65 uses the signal from the CPC to create the image, I had an instance where one of the RGB signals didn't reach the GT65 due to a dry joint and I was presented with a poor contrast monochrome image. So am I correct in assuming that the RGB signals are summed together within the GT65? Or are they simply not relevant at all?
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Offline arnoldemu

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #37 on: 17:42, 05 August 12 »

I thought that might be the case, although I'm a little unclear (probably as I haven't given it much thought) how the GT65 uses the signal from the CPC to create the image, I had an instance where one of the RGB signals didn't reach the GT65 due to a dry joint and I was presented with a poor contrast monochrome image. So am I correct in assuming that the RGB signals are summed together within the GT65? Or are they simply not relevant at all?
they are summed within the cpc/plus and appear as lum on the monitor connector.
I think the gt65 only uses lum,gnd and sync.
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Offline Badstarr

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #38 on: 17:52, 05 August 12 »
I see, so I wonder why loosing one of the RGB signals resulted in the image I described? I don't fancy cutting the signals again on my GT65 screen to experiment but I wonder if it works like a balanced XLR cable anyone with a bit of studio processing experience will be familiar with this and when they go "one legged" and you get a noisy low gain signal. Just a theory anyways.
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Offline arnoldemu

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #39 on: 11:10, 06 August 12 »
I see, so I wonder why loosing one of the RGB signals resulted in the image I described? I don't fancy cutting the signals again on my GT65 screen to experiment but I wonder if it works like a balanced XLR cable anyone with a bit of studio processing experience will be familiar with this and when they go "one legged" and you get a noisy low gain signal. Just a theory anyways.
I don't understand what you are talking about. But do you mean that the gt65 could be sinking/draining the r,g,b signals although it doesn't use them, this produces a load on the signals that modifies their appearance?
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Offline Badstarr

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #40 on: 16:32, 06 August 12 »
This link might explain my thinking ... Balanced Audio Cable


I'm not saying this is actually what is happening, it's just an idea really. I'm sure Bryce knows better than me. I'm just wondering if my GT65 (one of the last to be made) has a different method of pulling the signal from the CPC? There are definitely RGB connections present on the connector and they do seem to effect the image on the screen. I used to do a little trick years ago as a quick fix when the original plug dropped off the video cable on the GT65, basically I used to tie the RGB connections together and connect them to the sync pin (as far as I remember) and I would get a decent image, if a little saturated, on the monitor.


Without the RGB signals the picture on screen is washed out. I haven't got access to my older GT65 so I can't check if it also happens on that one.
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Offline Bryce

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #41 on: 16:46, 06 August 12 »
According to the GT65 schematics, RG and B are terminated, ie: they are all connected to ground through a 100R resistor. The reason for this is that they would effectively be an antenna if they weren't terminated, which would interfere with the signals required - LUM and Sync, but the RGB signals definitely aren't used to produce the any part of the picture on the screen. If you had a break in one of the RGB wires, it could still have a negative effect on the picture quality due to the interference it could cause.

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Offline Badstarr

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Re: Dirty video on 6128 Plus RGB Scart
« Reply #42 on: 17:33, 06 August 12 »
I see, thanks Bryce, now it makes sense  ;D
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