The Philco Beam of Light pickup works on a highly unique principle, one that was unheard of in the 1941/1942 era where it was in production.
Essentially, the pickup has a specially designed light bulb, operating on a high-frequency AC voltage, that shines through a small focusing lens. This light reflects off a pivoting mirror at the front of the pickup, and shines onto a silicon wafer. This generates a voltage which is then amplified.
The mirror pivots on a vertical shaft which is connected through a small arm to the needle. When the needle vibrates in the groove, the vibration is leveraged to the mirror, which rotates and alters the amount of light hitting the silicon wafer. This, in turn, creates a modulated signal to be amplified.
In practice, the system seemed delicate and prone to needing adjustment. But the sound quality in well adjusted units seems to be superior to the traditional phonograph pickups of the era; especially since a tracking weight of 1 1/2 to 1 5/8 oz. can be achieved, extending the life of records.
My unit needed a good cleaning. I was pleased to find that the bulb was still working, having tested it by connecting it to a 1.5 volt ‘C’ battery. However, after a few minutes, the bulb socket began shorting out due to the degraded rubber bushings inside. I dismantled it and used shrink tubing to protect the center portion of the bulb socket.
Once the bulb was up and running, I re-installed it into the head of the tonearm. The needle and arm were detached from the mirror shaft, so I re-soldered that. I also cleaned the silicon wafer and ensured there was good contact with it and the metal frame that holds it. Once it was all back together, I re-focused the light onto the wafer, and tried it out with a record.
Disappointment was the result. The audio was very low, and the noise very high. I tinkered with the focus and alignment of the beam, but I felt fairly confident I had that working correctly. Tinkering further, I grabbed the top of the mirror shaft with my surgical pliers, and twisted. Very suddenly, it began to move freely. The rubber bushings seemed pliable, so I used some silicon lubricant to try to free it up. My next play seemed far better; though the volume was maxed, the quality of the sound was very good.
I have not had a lot of time since December to work on the phonograph, but I am anxious to continue working on this restoration. It will be a real treat to get it working fully again.