How-To: Modify Commodore 1902A to accept CGA Analog RGB

[MonitorGuru]

I posted this about 2 years ago on usenet, and realized it might be better appreciated/useful here.

This is NOT a hack like others posted that are very dangerous (like directly wiring inputs into the guns on the neckboard).  It really is less dangerous than a cap kit, and when the instructions are followed, you will end up with a good source for a really nice Arcade Monitor replacement (and one that is totally encased and has it's own isolation built in, so great for bench work--no open frame and no need to drag a heavy iso xformer with!)

Here's the original post:

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Yes, it *IS* possible to convert a Philips/Magnavox made Commodore
1902A to accept ANALOG RGB instead of DIGITAL RGB.

It's well known that the 1902 (non "A") model, the one with rounded
face and corners instead of square either directly supports Analog by
hacking the case to expose the 3rd position of the RGB switch, or that
and adding a handful of components missing on some boards.

Though I searched Google/Usenet high and low, not finding anything
[other than for the original 1902], it IS possible to modify the 1902A
series (and presumably its cousins in the Magnavox line) to accept
analog RGB for use by either an Amiga computer or arcade game boards!

The procedure:

0a) I take NO responsibility if you do not know what you are doing and
proceede to shock, maim and/or kill yourself, or any equipment you may
choose to attach to this modification.

0b) I have tested this out to ensure it is safe to connect to these
points are they ARE isolated, they are not part of a "HOT" chassis.
It's totally safe to touch these points and connect them, if done
correctly.

1) Open your 1902A. Remove the 2 screws recessed at the top back, and
2 on the bottom back, slide the cover off partway.

2) Disconnect the speaker from the connector inside, finish removing
the cover.

3) Desolder the back edge of the shield ground. Be careful--use a hot
enough iron (more than 35 watts) and don't tug, you may end up ripping
off some of the ground traces like I did.

4) Bend the shield back.

5) Locate on the top of the board the vertical riser card. This card
has a couple 74LS04 chips on it and a load of resistors. This is the
digital to analog converter board.

6) Towards the back of the monitor where the card attaches, there is a
pin labeled +5V.  Locate the pin on top, then find it on the bottom
side.

7) Using a sharp knife, scrape OFF the trace just before it connects
to the pin on the card going thru the main board. We want to depower
the card, we no longer need it for analog. Test that there is no
continuity with an ohmmeter. We cannot have any voltage running into
that pin.

Looking on the top again, towards the front of the monitor the card
connects with 3 pins labeled R, G, B.  They are NOT the set of 4 pins
labeled RGBI that were closer to the back and the +5 pin you just cut
the trace on.

9) Locate the front 3 connectors and solder RED/GREEN/BLUE wires to
these 3 points on the bottom. This is the RGB Analog out from the
converter card that we have just disabled. This is where you will feed
in your own analog RGB

10) Locate the left 8-PIN din connector on the left back edge.

11) Locate pin 6. It is down and to the left of pin 1, and down and to
the right of pin 4. Solder a BLACK wire here. This is VIDEO GROUND.

12) On the same connector, locate pin 8. It's the lowest pin in the
group. Follow the trace down til it ends. Solder a YELLOW wire here.
This is VERTICAL SYNC.

13) One the same connector, locate pin 7. It's down and to the left of
5 and directly left of 2. Follow the trace down til it ends. Solder a
BROWN wire here. This is HORIZONTAL SYNC.

14) Wrap the wires up and lead them out from under the metal shield.

15) Resolder the metal shield on the bottom.

16) Run the wires out the back around the power cord and up.

17) Reconnect the speaker in the back case and reattach the case with
4 screws after aligning the board when closing it up.

You are now done with the modification.  If you used the wire colors I
suggested above, your wire connections are as follows:

RED/GREEN/BLUE: Analog RGB signal wires
BLACK: Video Ground
YELLOW: Vertical Sync
BROWN: Horizontal Sync.

Note: If connecting to something like an arcade game board that only
outputs composite sync, you can simply tie together the brown and
yellow wires and attach to the composite sync lead. Nothing special
(usually) needs to be done.


Also note: You may need to place a hex inverter (7404) inline with the
sync (and/or video) signals if your video source doesn't have the same
positive/negative signals as the monitor expects.  Diagram here:

1 - U - 8
2 - x - 9
3 - x - 10
4 - x - 11
5 - x - 12
6 - x - 13
7 - x - 14

Pin 1: Sync from video source
Pin 2: Sync out to monitor
Pin 7: Ground
Pin 8: VCC (+5 volts DC)

Use pin pairs 3/4, 5/6, 9/10, 11/12, 13/14 if you need to invert any
other lines such as RGB.  The odd # is the input, the even # is the
output.



Final Note: You may find that your signal is very strong and you have
to turn down the brightness and contrast and still not get an exact
picture. I am working on solving this problem yet. It may be as easy
as adjusting the DRIVE or CUTOFF or SCREEN controls.

The original daughter board seemed to output 1.0 volts on the analog
outs, so that may be the range it's expecting.

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End of original post.

Note: This hack works best with computer outputs of around .7 volts (e.g. Advance Mame or ArcadeVGA) since it wanted 1.0 volts.  Arcade boards output 4-5 volts so that's why there was so much brightness noted above.