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The potentiometers (Part I, Section 19) should be readjusted for the position of minimum hum
whenever the Radiotron prongs are cleaned.
The prongs should be cleaned by using a piece of fine sandpaper (Figure 6). The use of emery
cloth or steel wool is not recommended. Before re-inserting Radiotrons in their sockets wipe the
prongs and base carefully to make certain that all particles of sand are removed.
In placing Radiotrons in the UX sockets care should be exercised to make certain that the two
large pins and two small pins of the Radiotrons match the socket holes. The UY-227 Radiotron has
five prongs all of the same size and will fit in the socket only one way. If a Radiotron will not fit into
a socket without considerable pressure being applied, look for excessive solder on one or more of the
prongs. Excessive solder on prongs may be removed with a file or knife.
Figure
8
—
Adjusting the contact arm to secure improved contact
with resistor strip.
(10) LOOSE VOLUME CONTROL CONTACT ARM
A loose volume control contact may cause noisy or intermittent operation and should be
remedied. If the contact arm is loose, the remedy is to bend it slightly so that it makes firm contact
against the resistance strip. In order to do this it is necessary to remove the chassis from the cabinet as
described in Part II, Section 1. The volume control is then readily accessible. By removing the two
screws (Figure 7), that hold it to the metal frame it may be completely removed. The small U-shaped
washer is removed from the shaft and the spring contact arm is pulled out to clear the resistor strip.
The spring contact arm may now be bent sufficiently to make a good contact. Figure 8 illustrates the
bending of this contact arm. After adjusting the spring contact arm, replace the mounting screws and
return the chassis to the cabinet and replace screws and control knobs.
(11) ADJUSTMENT FOR SLACK DRUM CONTROL
The main tuning condensers are controlled by a cable and drum arrangement giving a smoothly
acting vernier movement that has no back lash. (See Figure
10.)
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