Pre-amplifier Boards Looking at side of pickup assembly that faces film. Photo-transistors at right connect to circuit board on the underside of the bracket. The top side circuit board has the second transistor stage for left and right channels. The coupling capacitors that connect the output of the underside board to the input of the top side board feed through holes in the bracket.

View of underside. Plenty of slack on wires from photo-transistors to reduce stress on the fragile wire leads of the photo-transistors. Blue stand-off terminals on left side are termination points for DC voltage and amplifier outputs. The large 1000mfd / 50V capacitor connected to the standoffs is for extra DC filtering. The orange .1mfd / 100V capacitor is for decoupling to prevent high frequency oscillations.

Circuit used for photo-transistor pickup. C1 and C2 are compensation capacitors and very important as they prevent high frequency oscillations that add distortion to the sound. The emitter circuit uses dual resistors with only one resistor bypassed and provides reduced low frequency distortion.

The pick-up / pre-amplifier assembly fits in the same space as the original photocell. Construction of the assembly with pre-amplifiers requires a fair amount of expertise. The assembly as shown above just fits through the old photocell socket hole as installed from the back side of the projector. The hand made circuit boards must be small and parts laid out carefully. Construction like this should only be attempted by someone experienced with electronics.

The PN116 photo-transistor is made by Panasonic. The MPSA18 is a low noise transistor made by Motorola. All the capacitors may have a voltage rating of 35 volts except the 1000mfd capacitor should be rated 50 volts and the .1 mfd capacitor should be rated at least 50 volts.

The 220K resistor located at the output jack is not a load resistor. It serves to allow the output capacitors to "charge" up when the projector is powered up. This reduces the "POP" when connecting the projector output to an amplifier.

Measurements for gain and distortion were made by injecting signals into the base of the photo-transistor and blocking any light from being picked up.

The small bracket that the photo-transistors are glued to is adjustable. The photo-transistors must be all the way back against the main bracket when installed through the old cell socket hole. Once the pick-up assembly is screwed in place, the photo-transistors can be positioned as close as possible to the cover hole.
A light baffle is positioned between the photo-transistors on the cell cover to block light from bleeding across each other from the stereo track.