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From: essayes@telerama.lm.com (David Shuman)
Newsgroups: rec.games.video.arcade.collecting
Subject: Sega XY on Atari monitor: it works!
Date: 3 Jan 1996 16:32:11 -0500
Organization: Telerama Public Access Internet, Pittsburgh, PA USA
Lines: 216
Message-ID: <4ceskr$ajn@asia.lm.com>
NNTP-Posting-Host: asia.lm.com

	The good news is, it can be done.  The bad news is, it isn't
perfect, at least not yet. 

SEGA-TO-ATARI VECTOR MONITOR ADAPTER
     by David Shuman
     mappy@virginia.edu, essayes@telerama.lm.com
     3 January, 1996

DISCLAIMER AND WARNING
	The author disclaims any and all responsibility for damage and
injury directly or indirectly caused by the use of the information
contained in this document.  Procedures for modifying game and monitor
circuitry must be performed carefully and at the reader's own risk. 

OVERVIEW
	This hack allows you to play Sega XY (vector) games (Eliminator,
Space Fury, Star Trek, Tac/Scan) on an Atari color XY monitor.
	Sega XY games originally came equipped with the Electrohome G08
color vector monitor.  Both the first incarnation of this monitor, the
G08-001, and its replacement, the G08-003, were notoriously unreliable,
and were the primary reason most Sega XY games quickly ended up in
landfills.  The records of the Video Arcade Preservation Society show that
these games are rare even among collectors, no doubt due mainly to the
scarcity of working monitors.  John Grigsby said it best when he remarked,
"Sega vector games are like British sports cars-- they're great when they
work."
	Though the Atari color XY monitors were also infamous among
operators for their frequent failures, these monitors perform adequately
well when not subjected to constant use.  Atari color vector monitors are
far more common than their Sega counterparts; it is likely that far more
Atari units were manufactured, and that fewer of them succumbed to the
catastrophic (often firey) failures that sent thousands of Sega XY games
to their graves.
	Compared to the Electrohome G08, the Wells-Gardner 19K6101
monitors are extremely reliable and easy to repair.  Zanen Electronics
sells a "Get Well Kit" that provides a quick fix for most problems with
the W-G, and Gregg Woodcock's Atari XY FAQ offers an exhaustive review of
failures and fixes.  By contrast, no commercial kits are available to
rehabilitate the G08, and no FAQ exists to aid in its repair (at least not
yet).  Complete schematics for the most common version of the of the
G08-003 are difficult to find.  Finally, unlike the W-G, the G08's
circuit boards are not connectorized, so even removing them from the
chassis for testing and repair can be extraordinarily frustrating.
	Owners and would-be owners of Sega XY hardware have always known
that these games would be much easier to acquire and keep running if they
could be interfaced to the Atari XY monitor, but, at least on
rec.games.video.arcade.collecting, no one has ever described exactly how
this feat might be accomplished.  It appears, however, that the mystery
may at last have been solved.
	This hack was developed and tested using a W-G 19K6101 monitor
with the 31X series boards supplemented by an original Atari Input
Protection Circuit PCB.  I see no reason why the adapter would not work
equally well without the IPC or on a 32X or 33X series monitor.

THEORY
	Many collectors have played Atari vector games on an oscilloscope
in XY mode, and know that a color vector monitor is just an o-scope with
three color guns.  Both the G08 and Atari XY monitors have inputs for X
deflection, Y deflection, red, green, blue, and ground.  How different
could they be?  Not surprisingly, they're very nearly alike, at least in
terms of their operating parameters.  The biggest difference is that the
W-G and Amplifone have different deflection ranges than the G08: 
according to the manufacturer's specifications, the Atari system uses -12
to +12 VDC for X deflection, and -8 to +8 VDC for Y deflection; the Sega
system uses -4 to +4 VDC for X and -3 to +3 VDC for Y.
	Why not just plug the X and Y outputs from the Sega game boards
into the X and Y inputs of the Atari monitor?  You could, if you could
live with the shrunken picture, but if you're using an original Sega G-80
power supply, you'll eventually blow the 7905 -5 VDC regulator.  The
reason for this failure is probably that the input impedances for the X
and Y signals are slightly different for the G08 than for the W-G, and the
vector generation circuits on the game boards wind up drawing too much
power.  (The G-80 power supply itself is flaky at best, and should be
replaced with a modern switching power supply.)

DESCRIPTION 
	The conversion circuit is constructed around the TL082 op-amp (the
dual version of the TL081).  This chip acts both as an amplifier, to boost
the voltage of the Sega video outputs, and as a buffer, to isolate the
game boards from the monitor and match impedances.  The device is
configured as a pair of non-inverting amplifiers (one for each axis), each
of which accepts the deflection signal from the game board, amplifies it,
and outputs it to the monitor at a voltage that produces a full-size
image.
	The conversion also requires a minor modification to the spot
killer circuitry of the W-G monitor.  Without this alteration, the spot
killer will partially activate during screens where few vectors are
displayed, distorting the picture.  Adding a pair of capacitors
effectively reduces the sensitivity of the spot killer so that it won't be
triggered when it shouldn't be.  A real failure condition, in which the X
and Y inputs fall to zero, still deactivates the beam immediately,
preventing damage to the phosphor coating.  The alterations to the spot
killer do not affect the monitor's ability to properly display Atari XY
games. 

CONSTRUCTION
	Using two 1/4 W 2.2K resistors, two 1/4 W 3.3K resistors, and two
10K potentiometers, construct two non-inverting amplifier circuits using
the schematic crudely reproduced below.  The drawing shows the
configuration for the X axis; the Y axis circuit is identical. 

               +Vcc
                |
             ...|
       2.2K  |  \
Xin ---RRR---|+  \
             |    :----+------ Xout
          +--|-  /     |
          |  |  /      |
          |  |..|      |
          |     |      |
          |    -Vcc    R
          +----------->R 10K pot
                       R
                       |
                       |
                       R
                       R 3.3K
                       R
                       |
                       |
                      gnd

	The adapter works reasonably well using either two 741 op-amps or
one TL082 op-amp.  Other chips may work better; please experiment if you
have the opportunity. 

Pinout of the TL082:
	1  out #1		5  in(+) #2
	2  in(+) #1		6  in(-) #2
	3  in(-) #1		7  out #2
	4  -12 VDC		8  +12 VDC

Pinout of the 741:
	1			5
	2  in(-)		6  out
	3  in(+)		7  +12 VDC
	4  -12 VDC		8

	During operation, the TL082 draws approximately 5 mA from both the
+Vcc and -Vcc lines.  It should be possible, then, to provide power for
the adapter directly from the +12 VDC and -12 VDC lines of the game's
power supply.  In my test fixture, I have been using a switching power
supply to provide the requisite voltages to the game boards, and the G-80
power supply (which remains connected as a source of 6 VAC and audio
amplification) to feed the adapter circuit.
	To modify the spot killer circuit of the W-G monitor, you will
need two 2.2 uF, 50V electrolytic capacitors.  Remove the deflection board
and solder these capacitors to the underside of the board in parallel with
C802 and C803.  Be sure to install the new capacitors with the correct
polarity.
	The Wells-Gardner 19K6101 monitor takes power from a center-tapped
50 VAC winding on the main transformer (the "25-0-25" scheme).  Note that
this is a different arrangement than that used by the Electrohome G08,
which draws power from a center-tapped 100 VAC winding (the "50-0-50"
scheme).  If you are mounting the W-G monitor in a Sega cabinet, you will
need to install an extra transformer to power the monitor.
	If you are installing the Sega game boards in an Atari color
vector cabinet, use the standard "edge connectors and fingerboard" method,
incorporating the video adapter and a suitable power supply.  Sega XY
games use different voltages than Atari color XY games, so it may be a
good idea to attach the Sega boards to a separate power supply.  A small
switching power supply capable of +5, +12, -5, and -12 VDC will be
adequate.  The +5 VDC supply should be rated at 10 A or more.  Note that
the original Sega G-80 power supply also produces 6 VAC for the sound
board; I don't know whether the 10.3 VAC found in Atari color XY games is
an adequate substitute.  Experiment at your own risk. 

ROOM FOR IMPROVEMENT
	The adapter circuit works reasonably well, but not perfectly. 
When many vectors are displayed at once, some of them appear in the wrong
place, and images become slightly distorted.  Fortunately, the effect is
not so great as to render the games unplayable, or even unsightly, as the
distortion is often unnoticeable.
	If the potentiometers are adjusted so that the picture is fairly
small, all vectors are displayed normally.  This effect suggests that the
bad vectors might result from a reduction in bandwidth as the gain is
increased.
	The problem persists if the TL082 is reconfigured as a pair of
inverting amplifiers.  Though this design provides a higher slew rate than
the non-inverting circuit, the source of the distortion is apparently not
the configuration of the amps.
	The picture is no better or worse if a pair of 741's is
substituted for the TL082.  It may be that neither of these op-amps is
entirely suitable for the task, and that some more modern device might be
necessary to completely eliminate the garbled vectors.

FOOD FOR THOUGHT
	A Space Duel cabinet would make a perfect home for Eliminator and
Space Fury, as it has the same controls and accomodates two players at
once.  Space Fury would drop nicely into a Gravitar cabinet; so would
Eliminator if you didn't mind being stuck with one-player mode only.
	Tac/Scan and Tempest are both vertically-oriented color XY games
that use two buttons and an encoder wheel.  It shouldn't be too hard (and
might be very easy) to adapt a Tempest encoder wheel to play Tac/Scan, or
vice versa.
	Maybe Tac/Scan would be easier to play with a trackball or a
"roller controller" like you find in a dedicated Major Havoc.
	As for Star Trek, well, how about drilling a few extra holes in
your Tempest control panel and dropping more buttons in there?  Or adding
an encoder wheel to your Gravitar control panel?  Hmm.
	We know you can build a digital-to-analog adapter for joysticks. 
If you could build an analog-to-digital adapter, you could rewire a Star
Wars flight controller to emulate rotate left, rotate right, thrust, and
fire, and use it to play Eliminator or Space Fury. 

ACKNOWLEDGEMENTS
	Thanks to:	Doug Jefferys		Duncan Brown
			Gregg Woodcock		Bryan
			Bazil D'Oliviero

(c) 1996 David Shuman


-- 
								--Dave--

 


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