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MAINTENANCE AND INSPECTION 189
tubes are correct, replace this tube with a new one. Should the plate reading for any tube be zero, check the plate and filament circuits of the particular tube for loose connections. If the connections are all right put in a new tube. If the plate current of all the tubes is zero, check the fuses marked "Battery" and "500 Volts."
F. Power Amplifier Tubes
The plate current of the ten UX-845 tubes should indicate 0.325 amperes on the two meters marked "IP Back Tubes" and "IP Front Tubes." If the plate current becomes abnormal and the rheostats marked "Back Tubes IP Control" and " Front Tubes IP Control" have not been changed (this may happen accidentally), check the bias voltage (see the Meter Reading Table, page 193) and the grid circuit for loose connections. Abnormal readings may also be caused by the fact that the plate of a tube becomes highly heated. If this condition is noticed shut down the motor generator set and replace the defective tube with a new one. If abnormal readings are still obtained use the spare amplifier and consult an R. C. A.Photophone service representative. If the plate current for any one bank of tubes is zero and the tube filaments are lighted, check the fuse marked "1000 Volts." When the plate current for any one bank of tubes is zero check the wiring for this circuit.
G. General
If the amplifier does not function and the meter readings obtained are normal, the photo-cell cable should be checked for broken connections. Try a spare photo-cell. <
If no sound is obtained, shut down the motor generator set, remove the cover from the input section of the amplifier, and inspect the switch to see whether it closes properly and whether proper contact is made. Check the fader relay for operation, and also the contact points.
Inspect the connections on the I photo-cell transformer in the input panel.