Attack from Mars Strobe

Attack From Mars features a high-intensity strobe located under the saucer. It fires during the saucer destruct sequence with occasional flashes. During strobe multiball, it strobes at about 7 flashes per second. It also strobes occasionally during attract mode.

The system has two boards. One generates the 200V needed to run the strobe. The other carries the strobe tube itself. The two are joined by a high-voltage cable.

AfM Strobe Board

WARNING: THIS BOARD USES HIGH VOLTAGES AND CARRIES A RESIDUAL CHARGE

The strobe controller uses 50VAC from the transformer for its power. It has four main sections.

 The voltage multiplier, which turns 50VAC into 250V DC
 The reservoir capacitors, which hold the flash energy
 The trigger circuit, which applys a 4,000V pulse to the tube to fire it
 The control circuit, which isolates the trigger circuit and allows the WPC system to fire the tube with a low voltage transistor.

What Goes Wrong

The most common problem is that C1 fails. If this happens, the red LED will not glow (note: the glow is very dim on a working machine). C1 is run at about 160V, compared to a specification of 100V. Therefore, it fails within a few years of operation. The simplest fix is to move C2 to C1, and replace C2 with a new 100uF 100V non-polarized capacitor. The original C2 has plenty of life left in it.

Second most common problem is that the vibration causes the trigger transformer to break its fine leads. In this case, the LED will glow but the strobe wil not fire. Replace the trigger transformer. You can sometimes scrounge one from a cheap disposable camera.

Third, the trigger capacitors (orange blocks near the trigger transformer) sometimes crack internally. These capacitors need to be replaced.

Occasionally, previous repair attempts may have left flux on the board, which can cause all kinds of problems with this high-voltage board. It is really important to clean off flux residue.

Finally, the triac and SCR can fail with an overvoltage. Also, the reservoir capacitors can wear out. But the problems are mostly the first three.