Another area that has been modified on the MM is the
This is commonly done by replacing the plywood panel with a transparent
acrylic one with a copy of the original artwork attached to
The assembly is then backlit to give it an effect similar to the
playfield. For example, this unit
Altenbach is about $150
shipped. I decided to build my own due to cost and
to make the light circuit more sophisticated.
At the start of the project, I had only a vague idea of how to
construct the backboard, but I knew that I would need the artwork along
the way. I started by dismounting the backboard for scanning
Photoshop work. It removes rather easily. Once the
connectors are demated, and the screws at the bottom of the playfield
are removed, the backboard can be wiggled out. Only the
plastic (which is suspended inside the castle tower window) provides a
little bit of interference, but if you are gentle, it will come out
I then scanned the backboard in two halves at 200 dpi with my lie-flat
HP4600 scanner and stitched the two halves in Photoshop. I
continued the editing by using "Select by Color" to select the
lightning bolts, and
copying them onto a new layer. The white color was replaced
perfect white (RGB=255,255,255) for the brightest flash. I
disatisfied with the way the blue-black transition scanned, so I added
a solid blue background with a vertical black gradient. I
decided to retrieve my scan of the NOS plastics that I performed
previously, and added the damsel plastic onto the artwork (the mounting
holes were removed). This
allows me to decide later if I want to use the actual
The result is below. Note that the dots on the edge of the
lightning bolts due to the silkscreen process have been preserved.
The finished artwork that was used for the electronic backboard.
The big black circle in
is for the ball gate cutout, but it turned out that I did not need to
cut open the circle. For registration, I also included the
location of the screwholes that fasten the backboard to the playfield
Artwork printed onto two sheets of paper.
I printed the artwork on
types of media to find the best result. Clear transparent
produced too many bright spots from the LEDs and did not diffuse the
enough. Color photo paper was too thick, and blocked too much
light. Translucent craft paper had poor color
Finally, I found that regular white paper produced the best
image. The lightning bolts were bright and evenly lit in
There are four lightning bolts on the backboard, but only two flasher
circuits. I designed a simple circuit that sequences two
of bright white LEDs to first form a main flash, and a second 'echo'
string that follows the main one. The circuit is shown
below. I started by measuring the flasher voltage on my MM,
was 21.5V. This allowed me to calculate component values and
design the circuit. It works as follows: When the flasher
is activated, the 'main string' (D2 and D4) will light up.
also charges C1 via D1. When the main string is shut off, C1
powers the 'echo string', and R1 completes the circuit. Note
the echo string is held completely off due to the higher forward drop
needed in the D1, D3, D5, and D6 circuit.
The result works extremely well. When tested on my testbench
a pulsed power supply, you can see the momentary bright flash of the
main string, followed by the echo string. Due to the way the
discharges, the brightness of the echo string decays exponentially, and
looks very natural.
The LED flash control circuit. It causes two LED strings to
sequence controlled by a singe flash circuit. Note that the
supply is the top terminal.
By the time I had
artwork and the circuit design, I had figured out how I would build the
backboard itself. I decided that the artwork would be
a foam core frame. The frame is very light and stiff, and
block the light from one flash to another. This leads to a
sharper, more defined series of flashes. Also, the light from
damsel plastic circuit must not be allowed to leak out into the
lightning artwork. The thickness of the foam core also allows
a good place to house all the components into a neat
The black foam core was a scrap piece from my place of employment, but
it should be available at hobby stores.
Frame that holds the printed artwork. It provides structural
support, and light baffling.
The aluminum tray that holds the electronic components.
frame snaps into this one.
The main structure of
replacement is a tray made from white anodized aluminum
I have a large roll of this, and it is used to trim the siding of
houses. Its white color will do a good job of diffusing and
reflecting the light from the LEDs. Using a sheer and a sheet
metal bender at work, the metal
was precisely cut and bent to the form of a tray to hold the foam core
frame. I then added a layer clear tape for electrical
and hot glued the components to the tray. The light pattern
be tweaked by bending the leads of the LEDs. The
overall assembly is quite tidy, and allows me to easily remove the
frame to modify the circuitry.
Back of the aluminum tray. It is a very clean look with only
wiring protruding out.
Using the locations indicated in the artwork, I drilled five holes in
the bottom edge of the aluminum tray so that it can be fastened to the
playfield. I then connected the same kind of connectors
originally used by the backboard for the two flasher and the one lamp
matrix circuit (for the damsel light). This allows one to
and easily swap the old backboard back in.
The finished backboard assembly. Only 3/8" thick.
electrical and mechanical connections are used as the original
An animated view of the assembly. The two left are one flash
circuit, and the two right zones are the other.
As can be seen in the
the two flash circuit branch to the four LED circuits to form a
cascading lightning effect. Note that the player does not see
top inch of the backboard as it is hidden by the panel under the
backbox. As mentioned before, I did not need to cut out the
circle for the ball gate, but that could easily have been done with a
razor. I also decided to not install the original damsel
but to use the one printed with the backbox artwork. I think
looks very close to the original, especially when viewed from the other
end of the playfield, and through the castle tower window.
Time lapse exposure of the upper part of the playfield with the mod
installed. All four lightning bolts can be seen.
The other playfield lights look brighter because they do not flash.
A video of the backboard can be seen above or with this URL.
In the end, I am very
the look of the installed backboard modification. During game
play, the flasher circuit will activate for various reasons, and leads
to a nice light show in the background.
If you enjoyed this mod, you may interested in others I
done of similar complexity: