01-24-2014, 05:32 PM
Davidff,
Does it make ANY sound at all? (Even at a very low volume) If it does, check first that the light strikes the shadowbox in the correct position. The lamp is set so the the light is vertical, and quite narrow. If the lamp socket can be moved, slide it GENTLY back toward the the rear. then push it forward GENTLY until you see the filament of the lamp come into focus on the photocell. Push it forward until is just goes OUT of focus. Rotate it GENTLY so the light is exactly vertical. Now you adjust the positioning screw (slowly) until the light is exactly split at the left edge (as you face the machine) of the shadowbox between the shadowbox and the photocell. I ermphasize GENTLY and SLOWLY because the part we are dealing with is VERY OLD, and made of aluminum. We don't know its history, and it may have been abused. Over about 70 years, lotsa of things might have happened to it, and they don't make them any more!
On to the photocell. They DO fail, but... If you have a VTVM, clip the input to the photcell leads. Positive is the rear side. Now take your flashlight and shine it on and off the cell. You SHOULD see some action on the meter. I honestly do not remember how much, but it should 'bounce' up noticeably. If it does, I would first check the plate resistor on the preamp tube. 470,000 Ohms. Also check the screen bypass capacitor is your set uses the 7C7 for the preamp. (.05 mFd) If it is shorted, change the screen resistor as well. Also 470,000 Ohms.
If the preamp stuff checks out you may have a 'bad' photocell. Don't panic just yet. The clip on the front of the assembly can be removed CAREFULLY. This will release the cell. Look it over carefully It is a selenium cell. Hold the leads of your meter to the front and back of the cell, and have a friend flash the light on it. Once again you should see some activity. If not, clean the back of the cell and the contact places on the front with a Q-tip, moistened with Isoprople alcohol. Try for voltage again. If there is, clean the contact points on the assembly, then reassemble the cell to holder, and try it.
Sorry to be so windy. But I like these things just for their uniqueness, and also because the techs I worked with years ago HATED them. If the above soesn't help, let me know. I have a couple more tricks up my sleeve.
Randy
Does it make ANY sound at all? (Even at a very low volume) If it does, check first that the light strikes the shadowbox in the correct position. The lamp is set so the the light is vertical, and quite narrow. If the lamp socket can be moved, slide it GENTLY back toward the the rear. then push it forward GENTLY until you see the filament of the lamp come into focus on the photocell. Push it forward until is just goes OUT of focus. Rotate it GENTLY so the light is exactly vertical. Now you adjust the positioning screw (slowly) until the light is exactly split at the left edge (as you face the machine) of the shadowbox between the shadowbox and the photocell. I ermphasize GENTLY and SLOWLY because the part we are dealing with is VERY OLD, and made of aluminum. We don't know its history, and it may have been abused. Over about 70 years, lotsa of things might have happened to it, and they don't make them any more!
On to the photocell. They DO fail, but... If you have a VTVM, clip the input to the photcell leads. Positive is the rear side. Now take your flashlight and shine it on and off the cell. You SHOULD see some action on the meter. I honestly do not remember how much, but it should 'bounce' up noticeably. If it does, I would first check the plate resistor on the preamp tube. 470,000 Ohms. Also check the screen bypass capacitor is your set uses the 7C7 for the preamp. (.05 mFd) If it is shorted, change the screen resistor as well. Also 470,000 Ohms.
If the preamp stuff checks out you may have a 'bad' photocell. Don't panic just yet. The clip on the front of the assembly can be removed CAREFULLY. This will release the cell. Look it over carefully It is a selenium cell. Hold the leads of your meter to the front and back of the cell, and have a friend flash the light on it. Once again you should see some activity. If not, clean the back of the cell and the contact places on the front with a Q-tip, moistened with Isoprople alcohol. Try for voltage again. If there is, clean the contact points on the assembly, then reassemble the cell to holder, and try it.
Sorry to be so windy. But I like these things just for their uniqueness, and also because the techs I worked with years ago HATED them. If the above soesn't help, let me know. I have a couple more tricks up my sleeve.
Randy
