Getting a useable video signal out of a PCW

Started by JonB, 17:13, 05 January 17

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JonB

While I wait to fix my 9512 power supply, I have decided to hook the processor board up to a composite monitor (well, a TV to be precise). I have video, GND and sync on the CPU card. The picture is being generated from Video & GND (via a composite input), but it flickers all over the place and the TV is unable to lock on. Probably due to no synch! Anyway, what's the easiest way to connect it? I could combine for composite, or use a SCART input..? If so, how?

1024MAK

If the video signal and the sync signals are at normal standard levels (check with a 'scope), feed the video to the green video channel on a SCART input, the sync to the composite/sync input, and ground the red and blue video inputs.

That should give you a green screen display  :P

Mark

Looking forward to summer in Somerset :-)

JonB

#2
Well, Mark.. Seek and ye shall find: http://www.amstrad.es/forum/viewtopic.php?f=35&t=2480

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A close up of the "board". Just a 7400, resistor and variable resistor. Plus a bunch of jumpers (and two resistors that aren't needed).

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Gotta love that breadboarding kit..


Now, the signal is a little bit noisy. Not sure why, but I guess it is due to not having shielded signal cables between the PCW and the board.

1024MAK

#3
To reduce noise, you may need to do a number of things:

       
  • As you say, used screened cable, braided is best,
  • Add some decoupling capacitors (say a 10uF to 47uF and a 100nF) near the chip,
  • Add a filter (resistor, say 10 to 68 ohms and a RF choke / inductor) in series with the +5V supply to the chip,
There may be some other things, but I have not studied the circuit.

Mark
Looking forward to summer in Somerset :-)

Bryce

The breadboard will be the main source of most of that noise. On a real PCB the noise would be greatly reduced.

Bryce.

JonB

I'm gonna put it on stripboard then.


Just glad it worked (first time, heh!). I'm running it off a nasty little Chinese power supply though. It's only temporary, I hope to fix the 9512's power supply board soon...


Cheers
JonB

Bryce

Then cut through all the over-length copper strips after it's soldered up and as 1024MAK said, decouple the IC with a 100nf cap as close to the power pins as possible, a 100µf cap across the power lines where they enter the circuit and coax type screened cable for the signal connections (or at least twist the cables).

Bryce.

JonB

Something like this then..


[attachimg=1]


JP1 needs to be connected to +5v on the 9256 PCB.
JP2 plugs directly into the video pins of the 9256 PCB.
Yellow is Video, Green is Sync, Red is +5v, black is GND/0v.
VR1 is 10k (bigger than the schematic, but works).
R1 is 1k.


It's about as small as I could make it, to keep signal paths short.

Bryce

Yes, except C2 should be a ceramic 100nf capacitor.

Bryce.

JonB


Bryce

It's a bit on the high side but it's better than the 47µf.

Bryce.

JonB

#11
Eventually....


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I managed to put the connector block on the wrong side of the board, so had to turn it upside down, and build a spacer for the block.. duh..

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Even so, picture quality is acceptably good.


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Now just the serial port to go (Audronic is sending one  8) ) and I can begin IDE driver development. Then the PCW can get some love.

Bryce


JonB

#13
Yes, mostly.  :)


Thank you both for your advice!




1024MAK

No problem  :D

Glad you got it working to your satisfaction  ;D

Mark
Looking forward to summer in Somerset :-)

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