From: rls@cbnewsc.cb.att.com (richard.l.schieve)
Newsgroups: rec.games.video.arcade
Subject: Tech_tip: Typical Atari Color Vector Failures
Keywords: Monitor used in Tempest, Gravitar, Space Duel, etc...
Date: 30 Jul 92 15:38:36 GMT
Article-I.D.: cbnewsc.1992Jul30.153836.1783
Distribution: usa
Organization: AT&T


Well, feedback from previous "Tech Tips" has been good so I thought
I'd try another one.  I'm NOT an absolute expert on this stuff, I've
just fooled around with the insides of arcade games a fair amount
and this is from my personal experience.  Constructive comments and
questions are welcome as maybe I can learn a few things too.

This Tech Tip covers typical failures of the Wells-Gardner
Quadrascan Color X-Y Monitor (Atari Part No. 92-053).  This is the
monitor used by Atari for Tempest, Gravitar, Space Duel, Black
Widow, Major Havoc, and I believe Quantum (but I've never seen a
Quantum).

I'm starting out here by reposting some general vector monitor info
that I've posted before so we all start out at about the same point.

--------------------

Vector monitors are available in black and white or color.  A black
and white picture tube has one electron gun that lights just one
type of phosphor (usually, but not always white).  Color tubes have
3 electron guns that when the yoke and neck magnets are aligned
properly, each hit their own phosphors only, either red, green, or
blue (RGB).  Something called a shadow mask is used so each gun hits
only one set of phosphors.  This is just basic TV stuff and holds
true for raster monitors too.

The electron guns in the neck of the tube emit a stream of electrons
that bombard the face of the tube that would hit dead center if not
for the deflection magnets on the neck of the tube.  There are two
deflection coils.  One for horizontal deflection (X) and one for
vertical deflection (Y) of the electron beam.  Consider the center
of the screen to be (0,0) volts to the deflection magnets.  If you
want to move the beam to the right you put a positive voltage on the
horizontal deflection "X" coil (+,0).  A negative voltage moves it
to the left.  Up and down are accomplished with positive or negative
voltages to the vertical deflection (Y) coil.

The deflection coils are driven with the same kind of circuitry some
audio amplifiers use.  Imagine that the game puts out pre-amp levels
and that the monitor amplifies and displays the output.

The third section is what (at least by Atari) is called the "Z"
amplifier which controls the brightness.  There is a "Z" amplifier
for each electron gun so that means a black and white vector has
only one "Z" amp while color has three "Z" amps.

So to draw an asteroid the game shuts off the Z amp or amps and
applies the correct vector information to the X and Y amplifiers
driving the deflection coils to move the beam to the desired
location.  Then the appropriate Z amp or amps is turned on to
illuminate the screen and the vectors are modified to draw an
outlined asteroid.

On some monitors you can turn the brightness up to the point where
the Z amps don't completely shut down and you can see the full path
of the electron beam as it flys around.  I don't think I care to
write the program that does this in real time!

What I have described so far is basically on the Wells-Gardner or
Electrohome monitors used by Atari and at least in one Midway vector
game (Omega Race).  Stuff deleted...

Knowing how these things work helps greatly in trouble-shooting.  For
instance deflection of the beam to the edges of the screen puts the
greatest strain on the X/Y deflection circuits so if you monitor has
problems at the edges, something is weak.  The monitors make their
own positive and negative DC from AC inputs so a reasonable thing to
check would be the power supplies.

Vector monitors are also fussy about the quality of certain transistors.
The X and Y deflection circuits are very much like audio amplifiers
and tend to be hard on the big transistors used in the final stages
of amplification.  The Atari vectors use a push/pull rearrangement with
NPN and PNP transistors for both the horizontal and vertical amps.
If you loose one of these transistors you loose deflection in 1 of 4
directions depending on which transistor goes out.

There is another circuit in the Atari stuff that is very important
called the spot killer.  What the spot killer does is to shut down
the Z amps if the X or Y circuits get bad and the beam won't move
around the screen enough to keep from burning the phosphors.  The
phosphors will become permanently damaged if the beam stays in one
place for too long.  When the spot killer is active a red LED on the
monitor main board lights.  The spot killer also lights if the logic
board does not supply the low level X and Y signals for the monitor
to amplify.

----------------------

OK, so that was I said before and now I'll talk about specifics and
what devices I often find bad in the Wells-Gardner color vector
monitor.  If you don't have a manual for this monitor, get one, as
it does a good job of explaining how each section works and also how
to adjust it.

Some words of caution on the manual.  I've got 2 versions TM-183 1st
and 2nd printings both from 1981.  Neither show all the different
versions of this monitor.  The largest circuit board is called the
deflection board.  Most deflection boards are just one board but
I've seen versions with a second daughter board suspended above the
big board by various resistors, etc.  The books only show one
version.  Also the transistor types for Q100 and Q101 are not always
the same but the book only shows one type for each.

The neck board comes in 2 varieties too.  One with a brightness
adjustment in one corner and the one shown in the manual that does
not have a brightness control in the corner.

Some High Voltage (HV) supply boards have an LED and a HV limit pot
but the one shown in the manual does not.  If someone has any other
manuals that reflect some of these changes I'd sure like to have a
copy.

The good part of all this is that the 3 basic units, deflection
board, neck board, and HV board are all interchangeable as units
and they are all connectorized.  In other words if you have one
working monitor you can try the boards from your bad monitor (one at
a time) even if the components are not exactly the same.  Also, the
most often failing components are common to the different versions
of the boards.

The only other components (besides the tube itself) are the six
transistors mounted to the chassis.  The 3 NPNs are 2N3716s and the
3 PNPs are 2N3792s which are all in the final stages of the
deflection amps or the power supplies.  The deflection amps are like
an audio push-pull amplifier and to power these amps the monitor
takes AC in and produces plus and minus DC voltages.

2N3792 (PNP) Q706  X in minus direction (left)
	     Q606  Y in minus direction (down)
	     Q103  output negative power supply

2N3716 (NPN) Q705  X in plus direction (right)
	     Q605  Y in plus direction (up)
	     Q102  output positive power supply

Up, down, left, and right are all referenced with the monitor
mounted horizontally like in Space Duel.  For Tempest the monitor is
rotated 90 degrees.

These transistors often go bad and here is a quick lesson on how to
check a transistor with an Ohmmeter.  Unplug the red plugs from the
deflection board to isolate the transistor from the circuit.  From
the bottom the transistor configuration is (Oh boy! time for a
picture!): 
               _
              / \
       base- /o o\ -emitter
            |     |
             \   /    the case is the collector
              \_/	

Pretty crude but you get the idea I hope.  Anyway, with your ohmmeter
on a low scale (RX1) there are 6 possible measurements; B to E, B
to C, reverse probes and repeat.  With the probes one way you should
get a 10 to 20 ohm reading and when you reverse the probes you
should see an open circuit.  The last 2 reading are C to E and then
reverse the probes and repeat.  These should both be open.  Only 2 of
the 6 possible reading should show resistance.

So what symptoms go with what.  Well if Q705 (X+) goes the top half
of the pictures is gone (actually compressed into a line).  If Q605
goes (Y+) the right half disappears.  If more than one goes or
either of the power transistors goes (Q103 or Q102) you get no
picture as the spot killer turns on (the LED on the deflection board
lights) and shuts down the electron beam,

Checking these transistors is one of the first things you should do.
Exact replacements are nice but I've substituted others with some
success especially if you put the nonstandard transistors in the
place of Q102 & Q103 as those two are for the + & - power supplies
and not as critical as the ones that drive the deflection coils.

Also, when replacing these transistors, don't forget the little
clear plastic insulator and make sure the socket is centered.  If
any part of the transistor is touching the chassis you are asking
for trouble.  Any place that has transistors should also have some
of the special grease which helps transfer the heat from the
transistor to the chassis so the transistors last longer.

On the deflection board, the most common failures are Q100 and Q101
and when these go they take R100 and R101 with them.  These
transistors are part of the + & - power supply circuit and are often
bad with the resistors really burnt.  Even in-circuit and not
isolated from other components you can still get a pretty good idea
with an ohmmeter if the transistors are bad as transistors tend to
fail catastrophically.  In other words they usually completely short
(0 ohms) or open.  If you see 0 ohms where there should be an open
circuit or 10 to 20 ohms the transistor is probably shorted.  If you
see greater than 10 to 20 ohms when the reading should be in that
range the transistor is probably open.

Replace the resistors with the same type but it is not uncommon to
see Q100 & Q101 replaced with a larger transistor that will handle
more current.

Once in a while you will see D105 or D104 open or shorted also.

Watch for broken solder joints at the base of the connector pins for
all the major connectors.  You tend to rock the plugs back and forth
when you pull the connectors and this often cracks the solder joints
to the circuit boards.

These are the most common failures with the deflection board.
Usually, the electrolytic capacitors are still OK (though always be
suspicious of electrolytics drying up and loosing micro Farads) but
if the transistors in the chassis are OK most distorted pictures are
due to problems with this board.

The neck board very seldom has problems.  The few I've seen are from
mishandling where someone has broken some of the pots that control
the RGB drives.  Check the pots if you are missing a color.

Last is the HV supply.  I've worked on lots of these and have only
seen one of the infamous bad HV transformers.  Normally HV failures
are due to a semiconductor or capacitor failure.

Before you work on this beast, discharge the tube as it can really
zap you even when turned off.  Connect a clip lead between the
chassis and the shaft of a long narrow plastic-handled screwdriver.
Work the end of the screwdriver under the big suction cup on the top
of the tube until you hit metal.  There will often be a snap (from
the spark) as the HV runs at around 20 thousand volts.  Just go
slowly and just use one hand.  It won't bite as long as you are
careful.  I discharge the tube with my HV probe so I can watch the
voltage go down and the internal resistance of the probe bleeds the
voltage off slowly.  If the snap bothers you, put a resistor in
series with your clip lead to drain off the voltage more slowly.

Let's do this one by symptoms.

No High Voltage (HV), you don't hear the crackling sound when you
first turn the monitor on:  Check the transistors as I described
before. The ones I've seen fail most often are Q903, Q902, and Q901
though they are all suspect.

Low HV:  Most of you don't have a HV probe but the most common
symptom of this in this monitor is the screen looks like you are
looking at the center through a magnifying glass.  I've seen several
time where ZD902 (150 volt zener diode) goes bad and the HV drops
from 19.5 kilovolts to around 10 kV.  It's kind of like the
electron beam moves slower with less HV giving the deflection
magnets on the yoke more time to deflect the beam.  A new ZD902 and
everything is better.

The picture seem to shimmer or kind of sparkle, all the lines seem
to be unstable:  Replace all the electrolytic capacitors in the HV
supply.  Make sure the replacements are rated at as least as many
"working volts DC" WVDC and have as least as many micro-Farads.  It
doesn't hurt to replace a 22 uF @ 50 volt capacitor with a 50 uf @
100 volts if that is all you have around.  More is equal or better. 

Adjustments:  R918 is the HV adjust and my advice is that unless you
have a HV probe, don't mess with it.  If you do set the HV for 19.5
kV.  Some HV supplies have a HV limit pot with an associated LED.
This is not discussed in the manuals and is not on all HV supplies.
I'm not sure what to do with this one.  On the outside of the case
are focus and brightness adjustments.  Adjust the focus until the
picture is sharp and adjust the brightness just under the point
where the dot in the center starts to show.  Be careful to not get
carried away with the brightness as you can do permanent damage to
the phosphors.

Now some things besides the monitor itself.  Tempest is harder on
this monitor than the other Atari vectors.  The attract mode that
displays "TEMPEST" (often burned right into the phosphors) really
stresses the monitor.  For Tempest, I like to do what Atari did when
they offered the Major Havoc conversion.  Add a fan to the back door
of the game.  I try to find a small cooling fan that just moves a
small amount of air (not one that howls).  You have to cut a hole in
the back door and position it so it directs air at the deflection
board.  I usually connect the power for the fan to the wires that
head up to the florescent light and put a connector in so that you
can still remove the back door (with the fan mounted on it) without
it hanging on the wires to the fan.  

All of this has assumed that you had a good logic board in the game
and the monitor was receiving the vector info.  If the spot
killer stays on and the monitor seems OK verify the presence of the
X and Y signals by measuring between ground and pin 7 of the big
white connector for the X signal and pin 8 for the Y signal.  This
is an AC signal and if either is missing the spot killer circuit is
just doing it's job and saving the tube's phosphors.

Here are some neat parts places to get the big transistors (and
other things) that I've posted before:

			2N3716	2N3792

Newark Electronics	$1.55	$1.49
Chicago 312-792-8233

Mouser Electronics	$1.59	$2.38
1-800-346-6873

Allied Electronics	$1.43	$1.24
1-800-433-5700

Digi_Key
1-800-344-4539

Here are a few surplus type places that I've ordered from that have
great prices on things like electrolytic capacitors that will be
happy to send you a catalog:

All Electronics 1-818-904-0524

Marlin P. Jones 1-407-848-8236

As far as adjusting purity (red gun hits red phosphors only, green
gun hits green...) and convergence (red, green, and blue guns hit
adjacent dots to make white instead of separate colors), that is a
whole different subject and the manual does a good job of walking
you through the procedure.

Obviously, there are lots of possible failures with these monitors.
The ones I have discussed I've seen in quite a few and probably
account for about 90% of the failures I've seen.

Well, if you have read this far you must have found this article
interesting and I hope useful.  I'll be glad to try an answer any
questions.

				Rick Schieve
				rls@ihlpb.att.com
Newsgroups: rec.games.video.arcade