RCA BTA-250M Instructions Manual page 14

approximately 50 ma.
After adjusting the PA plate
circuit, open-the PA and PLATE switches.
Replace the plate cap removed previously from
the 866A tube.
Set the POWER OUTPUT control
to mid-position.
Since the tap setting on L106 is
approximate only, it is necessary to readjust this
tap for maximum loading.
Shut off power and follow an adjustment pro-
cedure of moving the tap on LlO6 slightly in one
direction and then noting the plate current when
power is reapplied.
Repeat this adjustment cycle,
changing the direction if necessary, until maximum
plate current is obtained.
This maximum should
be a peak reading which should drop off on either
side of resonance.
If resonance can not be ob-
tained, the output coupling is excessive, and the
tap setting on LlO5 must be changed.
After obtaining peak plate current, readjust the
PA TUNING control for minimum plate current.
If appreciable detuning is evident, the output
coupling is excessive and must be reduced by read-
justing the tap on L105.
After resonating the output circuit by readjust-
ment of L106, output may be set at the required
level by readjusting the coupling tap on LlO5.
Slight readjustment of Cl 18 may be required to
keep the circuit at resonance.
After the output circuit has been completely ad-
justed, the efficiency of the power amplifier may be
improved somewhat by a slight detuning of Cl 18
in the direction of increasing output.
Final adjust-
ment of the output power may be made by adjust-
ment of the POWER OUTPUT control. Check the
PA cathode current on tubes VI 02 and VI 03 in
positions 5 and 6 of the meter switch.
To adjust the modulator, open the PA and
PLATE switches.
Using a screwdriver, adjust the
modulator bias potentiometers R15 1 and RI 52 to
the maximum clockwise position, which will apply
maximum bias to the modulator tubes.
Close the PLATE breaker and MOD breaker
switch S I 02, thus applying plate voltage to the mod-
ulator stages. Observe the modulator cathode cur-
rent on the panel meter, at positions 8 and 9 of the
METER switch.
Now adjust RI 51 and RI 52 so
that the cathode currents on V 106 and VI 07, re-
spectively, are 30 per cent. The designation I,;,
refers to VI 06 and I,<, to VI 07.
Open the PLATE switch and connect a l,OOO-
cycle audio signal of suitable level to terminals
TBA-2 and TBA-3. At 100 per cent modulation,
meter Ml 04 should indicate approximately 100 per
cent in positions 8 and 9 of S 105.
To adjust for minimum hum, open the PLATE
switch. Rotate RI 55 to the extreme counterclock-
wise position, and set RI 58 and RI 59 at the center
position, i. e., the shaft screwdriver slot must be
vertical. Reapply power and, using a distortion and
noise meter, measure the hum level. In the
sequence listed, adjust R 158, R159, and then RI 5 5
for minimum hum.
When a noise meter is not available. rotate RI 55
to the counterclockwise position. Adjust RI 58
and RI 59 to the center position, using a scale not
exceeding 10 volts maximum on an a-c voltmeter,
for setting these potentiometers at the electrical
center.
OUTPUT CIRCUIT
The output circuit is designed to match any im-
pedance between 20 ohms and 250 ohms resistance.
Inductor L 107 is provided to cancel the capacitive
component of the antenna impedance when the
antenna is connected directly to the transmitter.
The circuit is normally an "L" type low-pass
filter, for antenna resistances greater than 70 ohms.
When the antenna resistance is less than 70 ohms, a
"T" type low-pass filter circuit is used.
In determining the output circuit components,
first note whether the antenna resistance is greater
or less than 70 ohms.
If the antenna resistance
is less than 70 ohms, adjust LlO7 so that its re-
actance equals the capacitive antenna reactance;
or if the antenna resistance is less than 70 ohms,
the reactance of L 107 should equal the antenna
reactance plus 35 ohms. To determine the in-
ductance of LlO7, use the formula:
L =.
159 x,
f
where L is the inductance in microhenries, Xi. is the
required reactance in ohms, and f is the operating
frequency in kilocycles. Then use Figure 6 to find
the number of turns to be shorted.
The adjustment of inductor L106 and the shunt
capacitance may be determined from Figures 4, 5,
and 6.
It should be noted that curves for the
inductance of L I 06 appear on both Figures 4 and 5.
Figure 4 is to be used where the antenna resistance
is greater than 70 ohms: if the antenna resistance
is less than 70 ohms, Figure 5 is to be used.
Following is the procedure for tuning the output
network :
1. Determine the required reactance of L 107.
AS
previously noted, the reactance should be equal
to the capacitive reactance of the antenna when the
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