Amstrad Gt64/65 Monitors

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Lusciousthelock
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Amstrad Gt64/65 Monitors

Post by Lusciousthelock » Mon Jul 30, 2018 8:26 pm

Is it possible to modify one of these cheap old GT64/65 monitor's for my BBC B? I believe there is a dangerous 5v connection somewhere in that cable that doesn't effect the colour versions. I'm just after a good quality mono display. Can a basic composite connection be made?

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IanS
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Re: Amstrad Gt64/65 Monitors

Post by IanS » Mon Jul 30, 2018 8:58 pm

The GT64/5 only uses two signals from the CPC, pin 4 is sync, pin 6 is luminance. Pin 6 on a beeb is 5V.

I've not tried it, but you can probably connect a beeb to a GT64 with a small adapter cable.
The video signals (RGB) from the beeb are TTL level, so will need some resistors in-line to connect to the GT64. You need to sum the RGB signals from the beeb with suitable weighted resistors and feed that into pin 6. The sync signal from the beeb can be fed via a resistor into pin 4.
Obviously connect the GND connections together, pin 5.

dominicbeesley
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Re: Amstrad Gt64/65 Monitors

Post by dominicbeesley » Mon Jul 30, 2018 9:36 pm

I seem to recall that the composite out of the bbc could be fed to both the luminance and sync pins to get a picture on these or am I wrong?

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IanS
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Re: Amstrad Gt64/65 Monitors

Post by IanS » Mon Jul 30, 2018 9:40 pm

dominicbeesley wrote:
Mon Jul 30, 2018 9:36 pm
I seem to recall that the composite out of the bbc could be fed to both the luminance and sync pins to get a picture on these or am I wrong?
Sounds possible, at least the composite signal will be about the right voltage level for the GT64, so no resistors required.

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1024MAK
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Re: Amstrad Gt64/65 Monitors

Post by 1024MAK » Mon Jul 30, 2018 9:57 pm

I believe the +5V from the monitor is on a separate 2.1mm 'barrel' power connection. Just make sure it can't short out on anything.

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Lusciousthelock
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Re: Amstrad Gt64/65 Monitors

Post by Lusciousthelock » Tue Jul 31, 2018 4:40 am

I remember reading somewhere that the colour models work fine (They do, used to run one, although the colours always seemed to bright) but the monochrome models were a no-no. There is a video I found somewhere on Youtube of somebody trying to get one to work but that luminance pin stopped the picture from being stable. I was just wondering if anybody had produced an adapter cable or mod as these monitors are nice and cheap :D

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1024MAK
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Re: Amstrad Gt64/65 Monitors

Post by 1024MAK » Tue Jul 31, 2018 7:32 am

The luminance is the video signal...

A composite monochrome TV signal is made up of combining the video signal (the luminance signal) and the horizontal and vertical synchronisation signals. The Beeb has available a monochrome composite video signal on the BNC connector. And a combined (or composite) sync. signal on the RGB DIN socket. So it should be possible to get one of these monitors working with a Beeb.

In the past I have used a RGB display to show a monochrome video picture by only using a single RGB channel (the green channel).

If a GT64 or GT65 monitor was available near me, I would get it and have a go.

Mark

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IanS
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Re: Amstrad Gt64/65 Monitors

Post by IanS » Wed Aug 01, 2018 7:58 pm

dominicbeesley wrote:
Mon Jul 30, 2018 9:36 pm
I seem to recall that the composite out of the bbc could be fed to both the luminance and sync pins to get a picture on these or am I wrong?
It works, but it's not great. Not a lot of adjustment, the pictures is with the lowest brightness set on the monitor. It's difficult to get a black background.
Using the seperate sync and summing the outputs of the RGB signals, would probably be better and would allow some adjustment.
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dominicbeesley
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Re: Amstrad Gt64/65 Monitors

Post by dominicbeesley » Wed Aug 01, 2018 8:18 pm

That's a DC offset problem, try putting a capacitor (1uf or greater - if polarised +ve end towards the beeb). The beeb doesn't include a blocking capacitor...and nor does the amstrad by the looks of things....

At least the sync seems to work!

D

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1024MAK
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Re: Amstrad Gt64/65 Monitors

Post by 1024MAK » Thu Aug 02, 2018 8:30 am

For a composite video signal, the typical values for DC blocking capacitors are 47uF or 100uF, both 16V.

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Re: Amstrad Gt64/65 Monitors

Post by dominicbeesley » Thu Aug 02, 2018 10:55 am

Oops yes, brain fade from carrying too many boxes...

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jonb
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Re: Amstrad Gt64/65 Monitors

Post by jonb » Thu Aug 02, 2018 1:09 pm

The best use for a GT65 is to re-screen an 80 column PET:

http://www.vcfed.org/forum/showthread.p ... post398553

(As originally demonstrated by Nightfall Crew but I couldn't find a link on their blog).

Edit: Got it! http://www.nightfallcrew.com/08/08/2014 ... placement/

IMHO, of course!
Last edited by jonb on Thu Aug 02, 2018 1:26 pm, edited 1 time in total.

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Re: Amstrad Gt64/65 Monitors

Post by johnkenyon » Thu Aug 02, 2018 4:02 pm

IanS wrote:
Wed Aug 01, 2018 7:58 pm
dominicbeesley wrote:
Mon Jul 30, 2018 9:36 pm
I seem to recall that the composite out of the bbc could be fed to both the luminance and sync pins to get a picture on these or am I wrong?
It works, but it's not great. Not a lot of adjustment, the pictures is with the lowest brightness set on the monitor. It's difficult to get a black background.
Using the seperate sync and summing the outputs of the RGB signals, would probably be better and would allow some adjustment.
I would also suggest a fifth resistor feeding the sync input.
The CPC has sync mixed into the luma output, but it is a a lower voltage level than the sync output.

I reckon if you feed the composite signal the sync pin, you'll get a stable picture but the reference black level will be wrong. A quick way to check would be to wire the monitor sync input to the RGB sync output via a 220R resistor.

The service manuals are at http://www.cpcwiki.eu/index.php/Service_Manuals.

/John

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