The cathode of the regulator control tube V8 is main- The modernization notes that follow do not necessarily
tained at a constant potential by resistor R33 and give specific adjustments required after completing a
regulator tube V9. Series regulator tubes V6 and V7 single circuit change or modernization since the com-
act as variable resistors whose resistance is con- plete TEST PROCEDURE must be performed after
trolled by the grid bias applied to them by control completing the modernization.
tube V8.
The modernization procedures that follow are given
in the approximate order that the changes were intro-
If the regulated B+ voltage at the cathode of V6 were
duced in production.
to increase, the grid voltage of V8 would be increased
a proportional amount. ?hbe V8 would then draw more
plate current and the voltage drop across R32 would Modernize any instrument by starting with the changes
increase. This would result in a more negative grid given under the serial number group into which the
voltage for V6 whose plate resistance would then in- instrument falls. Complete all changes in the group
as well as those in any subsequent groups.
crease. The increased resistance of V6 would cause
a greater drop across V6 and the output voltage would
drop to compensate for the original increase. - NOTE -
All circuit references apply to the schematic
If the regulated B+ voltage tends to decrease, the re-
diagrams given in Figures 4-9 and 4-10 unless
verse of the abwe action occurs. In either case, the
otherwise noted. The schematic diagrams
voltage at the cathodes of the series regulator tubes
given in Figures 4-7 and 4-8 will facilitate
is maintained at a constant level.
location of components not in instruments with
Ripple in the output voltage is coupled to the grid of Serial Numbers 1350 and above.
V8 by capacitor C18. Variations in the dc voltage are
coupled to the grid of V8 through the voltage divider The modernizing instructions that follow are repeti-
consisting of R34, R35, and R36. The bias for V8, tious in several places. This is necessary to insure
and consequently the regulated dc output voltage, is complete modernization of any instrument and to sim-
determined by the setting of R35. plify the procedure for older instruments. Knowing
? this will eliminate any confusion that might arise when
The heater supply for High Frequency Oscillator tubes
repetition is encountered.
V1, V2, V3, Buffer Amplifier V4, and Voltmeter Amp-
lifier tube V16 is regulated by means of a ballast tube.
The parts required for modernization of a particular
instrument will vary depending upon instrument age.
The ballast tube has a high positive temperature coef-
The parts list following the instructions contains all
ficient. If the voltage applied to the heater circuit
components required to modernize an instrument with
were to increase, current through the circuit and bal-
Serial No. 529 or below and having a type number
last tube V17 would also increase. This increased
other than 4749.
current would cause V17 resistance to increase which
would cause the regulated voltage applied to the tubes
3-9A SERIAL NO. 529 AND BELOW
to decrease and compensate for the initial increase.
The reverse action would take place if the voltage
Instruments with serial numbers below 530 varied
applied to the heater circuit were to decrease.
little from one another. Minor circuit changes were
incorporated during manufacture to simplify produc-
The current through the ballast tube, and thereby the tion, calibration, and testing without changing per-
voltage applied to the tubes, can be set by adjusting
formance specifications.
control R42.
Incorporate the following changes in all instruments.
DO NOT remove any of tubes V1. V2, V3, V4, or V16
Instruments having a type designation of 4749 had
while the instrument is turned on. The current regu-
these changes incorporated during manufacture.
lating properties of the ballast tube will apply a higher
than normal voltage to the heaters of the remaining
Instruments with Serial No. 530 and above do not have
tubes operating from this regulated supply. A burned
type numbers.
out tube may result.
a. Add resistors R84 and R85 (680 ohms, *lo%, 1 watt,
3-9 PROCEDURE FOR COMPLETE MODERNI-
composition) to the screen grid circuits of series
ZATION OF ANY INSTRUMENT
regulator tubes V6 and V7.
Modernization of an instrument should be undertaken
b. Change regulator tube V9 from an OC3 (VR105) to
only when a general overhaul is necessary and in-
a type OA3 (VR75). These two tubes have the same
struments are available for completing the TEST
base connections and it is only necessary to change
PROCEDURE given in this manual.
the values of R33, R34, and R36 as described in the
-
NOTE - next three steps.
An instrument must be completely tested fol- c. Change R33 from 6800 ohms to 10,000 ohms, *lo%,
lowing modernization as the circuit changes 2 watt, composition.
affect output voltmeter calibration, frequency
response, oscillator output, or frequency cali- d. Change R34 from 120,000 ohms to 220,000 ohms,
bration. ilO?%j, 1 watt, composition.
-10- |
ACEC
