Amstrad CM14 CRT computer monitor - blank screen

[PenroseHouse]

Hello all, I am new here. I'm not a hardware guy, I'm a programmer, so plain English please!

NOTE: My father is an electrician and can help me discharge the monitor. I have also watched several YouTube videos on this.

I just bought an Amstrad CM14 CRT monitor on eBay. New old stock, immaculate.

...

I plug in a VHS video player, or my MiSTer Multisystem computer, either way using a special RGB SCART lead I bought from a website called Cool Novelties, designed for plugging modern computers into a CM14.

I get perfect stereo sound.

The screen remains blank, doesn't really look like black pixels, just ... nothingness. However, when I switch it off, I get that shrinking white oblong flash you expect when you switch off a CRT.

...

I haven't opened it up yet. Could it be a loose connection, maybe something came loose when it was shipped? If not is it worth changing the capacitors?

Two questions that may help:

1 Should there be any kind of test card screen? eg my 1980s Commodore shows red, green and blue and NO SIGNAL if nothing is plugged in.

2 There are three leads: the power lead, the image input lead that the SCART lead plugs into, with several pins ... and a third lead, very small round end - what is that third lead for? I think keyboard ... if so, does the monitor refuse to work without one?

Thank you for any help!



Aiden Smith

[Maarten]

Make sure that the MiSTer computer puts out an RGB signal. The VHS player doesn't so with that the screen should remain blank (dark).

[hamid_1]

Your monitor requires separate RGB (red,green.blue) signals. A VHS player will supply a composite video signal. This will not work without additional conversion.

Most DVD players and digital TV set-top boxes can provide RGB signals on their SCART sockets. Try one of these instead of the VHS player. Sometimes you first need to enable RGB output in the settings menu. This will mean connecting it to a TV that accepts both RGB and composite video over SCART, then going into the setup menu and selecting TV SCART OUT = RGB instead of CVBS or S-video. You must also use the TV SCART socket on a set-top box, not the VCR SCART socket if there is one.

The MiSTer Multisystem does support RGB over SCART according to the manual here: https://rmcretro.com/mmsguidev2

See pages 40-41 : You must switch off the VGA output in the settings, otherwise the RGB SCART output will be at VGA frequency which the Amstrad monitor may be unable to display. You can use a TV or monitor with HDMI connected to the MiSTer Multisystem to make the changes.

I tyhink the third lead from the Amstrad monitor was to provide power to the computer keyboard part of the system. It is not required here.

[Graham G3ZVT]

As has been said, the VHS machine is a red herring because they don't output RGB, so test with a satellite or terrestrial STB configured for RGB.

As for your emulator, I can't help, other than to point you to section 7.2 HERE .

My feeling is there is nothing wrong with your CM14.

[ben]

Test with a DVD player or freeview box which has RGB output

[PenroseHouse]

Let's forget the VHS, ultimately all I really care about is the MiSTer.

hamid_1, yes, you are certainly right, the MiSTer puts out a "European RGB SCART" signal.

I've been through the entire MiSTer Multisystem manual, and I'm sure I have the correct settings for RGB rather than VGA. I'm now getting a highly corrupted picture, hopefully there's a photo attached.

Purple and black, which doesn't have anything to do with the expected image.

It's constantly changing, though has a consistent black/purple banding, so maybe it's getting SOME kinda signal.

I want to be pretty sure the monitor itself is fine before I try emailing the manufacturer of the Multisystem, who may say it's a MiSTer problem, or post on a MiSTer forum and be told that it's a Multisystem problem - the sytem consists of two separate products functioning together.

Thanks again.

[marceljack]

Your picture is a classical "out of sync" picture (picture does not synchronise horizontally but it is synchronised vertically).
This is probably due to the fact that the line frequency of your signal is not 15625 Hz (frequency of 625 lines TV) but a different one, may be in the order of 32 kHz or so (VGA output at 640 x 480)
Did you try it with a good TV RGB signal (SCART output from a DVD player or a satellite or terrestrial decoder) ?

[marceljack]

In fact I ralise that it's not synchronised vertically either, which is normal if the signal is VGA compliant (vertical frequency 60 Hz and not 50 Hz as for a normal 625 lines signal).
Another possibility for no sync is a bad contact at pin 20 of the scart connector (composite video input also used to synchronise the CRT scanning).
Or the absence of +12 V at pin 8 (slow switch) which is maybe required by this monitor to enable the composite input.

[Maarten]

I can't tell for sure whether it's synchronised vertically. The black bar might be a photografy artefact. Before anything else, I would indeed find a DVD player or DVB receiver that is known to and set to output RGB signals.

[PenroseHouse]

Thanks for that. At the moment I can't get hold of a device that definitely outputs RGB (I think the MiSTer does but has complicated settings).

I will buy something if I need to, but maybe below proves that it's NOT a hardware problem but MiSTer settings - please tell me if I'm wrong.

...

I now get a pretty decent picture if I run the MiSTer as the early 80s arcade machine Asteroids (the MiSTer is an FPGA-based Linux computer that can be reconfigured to run as various computers at a hardware level, reconfigures the flips and flops!!).

But...

The image is present twice horizontally.

The left-hand side of the screen shows the image once, and the right-hand side of the screen shows the exact same image again.

Does this prove that the SCART leads work... leaving only that it's a problem with the MiSTer settings?

If so, I'll probably have to move on to a MiSTer forum, but I want to be sure the monitor is essentially working.

[Maarten]

An image shown twice horizontally is quite typical for an SD (480i/576i) monitor that somehow manages to sync to a VGA signal.

[marceljack]

Originally Posted by Maarten

An image shown twice horizontally is quite typical for an SD (480i/576i) monitor that somehow manages to sync to a VGA signal.

+1
This is the most likely explanation.

[PenroseHouse]

OK, so you're saying it's likely the MiSTer is outputting an VGA signal (which the monitor just happens to sync to, kind of a fluke) - when it needs to be outputting an RGB signal?

In which case it IS the MiSTer settings that's the problem - the monitor seems to essentially work?

[John123]

According to this, 'For analog RGB output, you need to change the settings in the mister.ini file'.

Detailed instructions can be found here. Hope that helps!

[marceljack]

Originally Posted by PenroseHouse

OK, so you're saying it's likely the MiSTer is outputting an VGA signal (which the monitor just happens to sync to, kind of a fluke) - when it needs to be outputting an RGB signal?

In which case it IS the MiSTer settings that's the problem - the monitor seems to essentially work?

In fact VGA at 640 x 480 resolution is an RGB signal but at twice the line frequency of an (NTSC) TV signal (so 31,5 kHz line / 60Hz frame frequencies).
Basically VGA is a non-interlaced version of an NTSC TV signal in RGB format with separate line and frame syncs.

[nigelr2000]

Basically this was a monitor designed to produce a watchable picture for a home computer and also provide the power supply for the main unit, it is not and never was a high quality monitor with a fine shadow mask suitable for high definition large pixel displays. It's basically a ZX3000 portable telly with no tuner IF or sound circuitry. I think you would be much better laying your hands on an old SVGA monitor that was designed for various computer based signals

[Maarten]

That depends on what you want to display on it. It should be as good or better than any RGB input TV set in displaying pictures from homecomputers, old consoles, etc.

Most VGA and SVGA monitors will not sync below 30kHz or so, so they're incompatible with homecomputers. You'll need a converter, and the resulting picture can be slightly better but probably not so much better that it will be worth the trouble.

[PenroseHouse]

Thanks for this, John123, marceljack, nigelr2000 and Maarten.

I'm ill at the moment, hence haven't been able to look at it, but I'm REALLY grateful for these responses and will look into all of this the minute I am able.

If the thread is still open when I solve the problem I'll post here so anyone with the same problem who finds this page will hopefully get their answer.

[Oldmadham]

Originally Posted by PenroseHouse

OK, so you're saying it's likely the MiSTer is outputting an VGA signal (which the monitor just happens to sync to, kind of a fluke) - when it needs to be outputting an RGB signal?

In which case it IS the MiSTer settings that's the problem - the monitor seems to essentially work?

Not really a fluke, analog monitors are quite forgiving when it comes to synching to anything approaching the correct signal.

PAL/SECAM receivers will usually sync to NTSC & vice versa.
If the VGA line rate is about 2x that of NTSC, it will be quite likely to sync to that as well, as it only needs to lock to each alternate line.
.
That said, you can get two "side by side" pictures with the normal PAL/SECAM or NTSC line rate, if you adjust the horizontal hold grossly outside its correct setting.

[PenroseHouse]

Thank you for all of these responses, I'm finally back to trying to get this to work, and have tried all of these combinations, some of them out of desparation pretty much knowing they would work, and whatever I try I'm just getting those random horizontal lines.

I'm going to email the company who make the MiSTer Multisystem to see if they've tried it with this monitor, but thought I'd also post the combinations I've tried (based on link provided above) here in caes I'm ALMOST there:


forced_scandoubler=1
composite_sync=1
ypbpr=0
direct_video=0

forced_scandoubler=1
composite_sync=1
ypbpr=1
direct_video=0

forced_scandoubler=1
composite_sync=1
ypbpr=0
direct_video=1

forced_scandoubler=1
composite_sync=1
ypbpr=1
direct_video=1

"Using the Analog IO Board"
vga_scaler=0
composite_sync=1
vga_mode=svideo Note: cvbs can also be used but is not recommended
ntsc_mode=0