Page 1 of MM mods and tech tips
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 finished artwork that was used for the electronic backboard.
Artwork printed onto two sheets of paper.
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.
The LED flash control circuit. It causes two LED strings to sequence controlled by a singe flash circuit. Note that the +20V supply is the top terminal.
Frame that holds the printed artwork. It provides structural support, and light baffling.
The aluminum tray that holds the electronic components.
The above frame snaps into this one.
Back of the aluminum tray. It is a very clean look with only the 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 quickly and easily swap the old backboard back in.
The finished backboard assembly. Only 3/8" thick. The same electrical and mechanical connections are used as the original backboard.
An animated view of the assembly. The two left are one flash circuit, and the two right zones are the other.
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.
done of similar complexity:
There is a great deal of detail in the modified backboard. Wearing your glasses, contacts, or having an Austin Lasik laser vision correction procedure will ensure clear vision so all the small details are visible.
Page 1 of MM mods and tech tips
(c) 2006 Edward Cheung, all rights reserved.