Electronic backboard
Another area that has been modified on the MM is the backboard.  This is commonly done by replacing the plywood panel with a transparent acrylic one with a copy of the original artwork attached to it.  The assembly is then backlit to give it an effect similar to the playfield.  For example, this unit from Tom Altenbach is about $150 shipped.  I decided to build my own due to cost and I wanted 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 and Photoshop work.  It removes rather easily.  Once the two connectors are demated, and the screws at the bottom of the playfield are removed, the backboard can be wiggled out.  Only the damsel plastic (which is suspended inside the castle tower window) provides a little bit of interference, but if you are gentle, it will come out just fine.

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 then 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 with perfect white (RGB=255,255,255) for the brightest flash.  I was disatisfied with the way the blue-black transition scanned, so I added a solid blue background with a vertical black gradient.  I also 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 plastic.  The result is below.  Note that the dots on the edge of the lightning bolts due to the silkscreen process have been preserved.

The big black circle in the artwork 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 (bottom edge).

I printed the artwork on several types of media to find the best result.  Clear transparent plastic produced too many bright spots from the LEDs and did not diffuse the light enough.  Color photo paper was too thick, and blocked too much light.  Translucent craft paper had poor color reproduction.  Finally, I found that regular white paper produced the best image.  The lightning bolts were bright and evenly lit in backlight.

There are four lightning bolts on the backboard, but only two flasher circuits.  I designed a simple circuit that sequences two strings 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, which was 21.5V.  This allowed me to calculate component values and design the circuit.  It works as follows: When the flasher circuit is activated, the 'main string' (D2 and D4) will light up.  This also charges C1 via D1.  When the main string is shut off, C1 powers the 'echo string', and R1 completes the circuit.  Note that 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 with 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 cap discharges, the brightness of the echo string decays exponentially, and looks very natural.

By the time I had finished the artwork and the circuit design, I had figured out how I would build the backboard itself.  I decided that the artwork would be fastened to a foam core frame.  The frame is very light and stiff, and will block the light from one flash to another.  This leads to a sharper, more defined series of flashes.  Also, the light from the damsel plastic circuit must not be allowed to leak out into the lightning artwork.  The thickness of the foam core also allows me a good place to house all the components into a neat sandwich.  The black foam core was a scrap piece from my place of employment, but it should be available at hobby stores.

The main structure of the backboard replacement is a tray made from white anodized aluminum sheeting.  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 insulation, and hot glued the components to the tray.  The light pattern can 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.

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 quickly and easily swap the old backboard back in.

As can be seen in the above image, the two flash circuit branch to the four LED circuits to form a cascading lightning effect.  Note that the player does not see the 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 black circle for the ball gate, but that could easily have been done with a razor.  I also decided to not install the original damsel plastic, but to use the one printed with the backbox artwork.  I think it looks very close to the original, especially when viewed from the other end of the playfield, and through the castle tower window.

In the end, I am very pleased with 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.