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Converting the Samtron 5" CGA monitor to work with Jamma boards
---------------------------------------------------------------

The Samtron 5" monitor makes a nice workbench monitor because of it's small size 
and the ease with which it can be rotated for vertical games.

I got mine for $49 from TimeLine, Inc.

See http://www.digisys.net/timeline/blowout.html for details.

I do not work for them or have any affiliation with them at all.

Furthermore, I take no responsibility if you injure yourself or destroy your
equipment. Don't attempt this modification if you don't already have experience
with soldering and know the dangers involved with high voltage devices such as
CRT's.

The Samtron is a CGA monitor and thus differs from what is required to work with
arcade boards in several ways. First, it accepts TTL (digital) inputs for the
Red, Green and Blue video signals, where arcade boards output an analog RGB
signal. Second, it requires a separate horizontal and vertical sync, where
arcade boards output a composite sync. Lastly, it expects a positive sync, where
arcade boards output a negative sync.

A short list of the parts you'll need to do the conversion:

1. A sync splitter. I used the EL4583C from Elantec, but if you've got something
else that will do the trick, use it. I like the EL4583C because is strips the
vertical sync information out of the horizontal sync. Some splitters don't do this
and this can cause problems.

2. A hex inverter. I used the 74LS04.

3. A 22K potentiometer. (Doesn't have to be exactly 22K. You can use 20-50K)

4. A .1mf capacitor.

5. Some thin wire.

6. De-soldering tools: Solder wick, de-soldering gun, whatever works.


The first thing to do is to feed your analog RGB signal into the board right were
it would normally come out of the TTL->analog conversion circuitry. Do this by
locating the chip marked 74LS38N and removing it. It shouldn't be hard to find,
there's only 5 chips on the board. You'll want to pull up pins 6, 8 and 11 from
the chip and re-solder the chip to the board. Now solder the green input from the
arcade board where pin 6 was, the blue input where pin 8 was and the red input
where pin 11 was. Here's a diagram:


Pin 1         o     v     o        Pin 14
              
              o           o
              
              o           o
              
              o  74LS38N  o--Red---Pin 11
              
              o           o
              
Pin 6--Green--o           o
              
              o           o--Blue--Pin 8
              
Do not solder your inputs to the pins themselves. Solder to the contact points
on the circuit board where the pins used to be.

Next, you'll need to split your composite sync into separate horizontal and
vertical syncs. Here's a diagram to use the EL4583C to do it:


Pin 1     +-------------o     v     o              Pin 16
          |        
          |             o           o----------H Sync Out
          /                EL4583C
          \             o           o------------- +5v in
22K Pot-->/                   
          \             o---+       o
          /                 |
          | V Sync Out--o   |       o
          |                 |
Ground----+-------------o   |       o
                            |
                        o   |       o
                            |
                        o---+       o               
                            |
                            |
Composite sync in-----|(----+
                   .1mf cap
                     
                     
The pot between pin 1 and pin 6 (ground) is for adjusting the vertical hold.
The cap on the composite sync input is to smooth out any spikes on the line.
Composite sync goes into pins 4 and 8. +5v goes into pin 14. Horizontal sync
comes out pin 15. Vertical sync comes out pin 5.

Now that you have split the sync, it's almost ready to go into the monitor. All
that's left is to invert the signal from negative to positive. This is where
we use the 74LS04 going by the following diagram:


Pin 1    H sync input----o     v     o---- +5v in      Pin 14
              
         H sync output---o           o
              
         V sync input----o           o
              
         V sync output---o   74LS04  o 
              
                         o           o
              
                         o           o
              
         Ground----------o           o 


Pretty simple stuff, really. You can put your syncs into the monitor at the
normal place  the 10 pin .100" spacing dual row connector. Here's the pinout for that
just in case it didn't come with your monitor.


Pin 1  H sync input--o   o
                     
                     o   o--V sync input  Pin 4
                     
                     o   o
                     
                     o   o--Contrast 1    Pin 8
                     
Pin 9        Ground--o   o--Contrast 2    Pin 10

Tie pin 8 to pin 10 if you don't want to hook up a pot to adjust contrast.

That's it!

If you need help, email me at: powerjaw@crosswinds.net 

Tim

 


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