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VFO
RECALIBRATION PROCEDURE:
Central Electronics 100/200V
1.0 Introduction
The Central Electronics 100/200V uses a precision, permeability-tuned
oscillator for output frequency control. The vfo tunes the
range of 5 to 6MHz with 5MHz corresponding to the high frequency
band end and 6MHz corresponding to the low frequency end.
The oscillator circuit is self-compensating for changes
in tube transconductance as caused by line voltage fluctuation
(affecting filament voltage and emission) and normal tube
aging. The vfo tuning mechanism consists of a precision
stainless steel lead screw, supported by a preloaded ball-bearing
actuator.
A mechanical corrector is provided and permits vfo tracking
calibration at 50KHz intervals throughout the 1MHz tuning
range. This built-in calibration feature permits the user
to accurately calibrate the vfo, without an accumulation
of error, throughout the entire tuning range. In practice,
the end-to-end tuning error can be maintained to less than
300Hz.
The transmitter's operating frequency is displayed in 1KHz
intervals on the circular "Kilocycle" dial. A
slide-rule "Megacycle" scale rotates with the
bandswitch so that only the band selected appears in the
vfo window. A two-speed tuning knob permits both rapid and
"finely-tuned" frequency selection.
2.0 VFO Problems
Functionally, the 100/200V vfo is, electrically, an extremely
reliable device. The transconductance equalizer so effective
that tubes failing a transconductance test in a high performance
tube tester routinely provide reliable vfo operation ( just
about any 6U8 that glows will function in this circuit).
Most vfo problems are mechanical, in nature. Some are field
repairable whereas others require services beyond that of
the normal repair service station.
Field repairable items include vfo tracking, dial slippage,
external mechanical binding of parts and normal lubrication.
Excessive vfo drag, as caused by permeability slug misalignment
or defective bearings should be repaired by a specialist
as full vfo recalibration, after the mechanical repair is
concluded, would be necessary.
3.0 VFO Tracking
Vfo recalibration requires an electronic frequency counter,
a precision screwdriver (No. 2 machine screw head) and a
suitable test cable having an RCA plug ternination for interconnection
to the 100/200V VFO Out jack.
Perform the following steps in the order indicated, after
a 30 minute warm-up period:
1.0 Tune the vfo to 5000KHz, which corresponds to the high
end of the vfo scale. The red pointer should be at the last
0.1 MHz integral marking at the high frequency end of the
slide-rule scale. With the kilocycle hairline indicator
set to its correct, vertical position, the kilocycle dial
should read "0". In all likelihood, the observed
vfo output will be some value slightly different from the
desired 5000KHz reading.
2.0 Remove the small plug button between the two vfo tubes
and observe the series of vfo corrector adjustment screws.
The center (second) screw visible in the tuning window corresponds
to the 5000KHz. The left screw corrects for the 4950KHz
setting whereas the third screw visible as the tuning dial
is rotated lower in frequency corresponds to 5050KHz, etc.
3.0 With the vfo set to 5000KHz ("0" on the dial
window), monitor the actual vfo frequency on the counter
and adjust the center corrector screw as needed. It is recommended
that once this initial correction is made that the vfo frequency
be rocked slightly (+/- 5KHz), reset to the "0"
point and the corrector screw readjusted as needed.
4.0 Continue the procedure described in step 3.0 in 50KHz
steps up to the 6000KHz position.
4.0 Overall Frequency Calibration
While the above procedure accurately calibrates the vfo,
transmitted frequency errors can still occur due to drift
in the frequency calibration of the 8MHz single-sideband
generator and the bandswitched heterodyne oscillator crystals.
To calibrate the 8Mhz crystal oscillator, proceed as follows:
1.0 Using an oscilloscope, observe the 8MHz crystal oscillator
level at the jumpered connection, pins B and C, of the RF
phase shift network. Adjust the 8MHz oscillator plate coil
for maximum indicated output level.
2.0 Measure the frequency present at the test point described
in Step 1, above. Adjust the 8MHz oscillator frequency corrector
capacitor (chassis-top adjustment) to exactly 8000KHz.
To calibrate the heterodyne oscillator crystals, proceed
as follows:
1.0 It is possible to measure the heterodyne oscillator
crystal frequencies from the first and second mixer cathode
test jacks, located immediately adjacent to the stage tubes.
A convenient test-jack extender jig requires a 5-inch length
of 20gauge bus wire. Starting from one end, form a 1/8th
inch diameter circle such that the circle is approximately
3/8th inch from the beginning end. Slide a 3 inch length
of spaghetti tubing over the long end and fashion a second
1/8th inch diameter circle to secure the insulating tubing.
The remaining uninsulated wire length forms the balance
of the extender.
2.0 Insert the jack extender in the first mixer cathode
jack, set the bandswitch to 20 Meters and observe the 27.500MHz
crystal oscillator injection signal. On 100V's, adjust the
crystal trimmer capacitor for 27.500MHz. If the capacitor
has insufficient range of adjustment, carefully adjust the
corresponding oscillator plate coil (plug-in can) for proper
frequency. Note that if this slug is detuned too far to
obtain exact calibration, power output may drop to an unacceptable
level or the Crystal may not oscillate reliably. If this
problem occurs, it may be necessary to replace the crystal,
itself, to achieve proper calibration and output level.
3.0 Repeat the above for 10 Meters and observe the 47.700MHz
crystal oscillator injection.
4.0 Insert the jack extender into the second mixer cathode
jack, set the bandswitch to 80 Meters and observe the 17.500MHz
crystal oscillator injection signal. Adjust the crystal
trimmer as described in step 2.0. Continue this procedure
for 40 Meters (20.500MHz) and 15 Meters (34.500MHz). If
the set is equipped for 160 Meters, 15.500MHz injection
can be monitored at this same test jack location and correct
if necessary.
5.0 Routine VFO Lubrication
In order to reduce friction within the vfo
mechanism, the following internal vfo components should
be coated with a high temperature Teflon grease:
Threads of the vfo lead screw shaft
Top of the corrector post
Bottoms of the 2-56 corrector screws
These parts are accessible by removing the U-shaped bottom
cover of the vfo, below the corrector screw access plug.
Normally, this level of service is recommended at four-year
service intervals
Additionally, one or two drops of light machine oil should
be placed on the Megacycle indicator drive/gear wheels and
pivot points as part of this routine maintenance.
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5.0 Depot Level Maintenance
5.1 Introduction
After prolonged periods of use (measured in
decades), the vfo drive may develop a "lumpy" feel
as the device is tuned throughout its normal operating range.
In extreme cases, the vfo tuning knob may be difficult or
nearly impossible to rotate.
In these cases, the usual cause is a hardening
of lubricant within the pre-loaded, ball-bearing actuator
assembly. Repair requires the removal, cleaning and repacking
of the two ball-bearing assemblies. This activity requires
specialized service equipment and is not a recommended field
procedure.
5.2 VFO Removal
The vfo may be removed for servicing via the
following procedure:
1.0 Remove the 100/200V from its cabinet.
2.0 Lay the unit on its left-hand side.
3.0 Disconnect the vfo output (white coax)
phono plug from the extension receptacle and the vfo power
plug from the rf chassis.
4.0 Remove the vfo tuning knob. This is accomplished
by first locating the aluminum dial hub on the back of the
100KHz circular dial. The dial hub contains two radial holes
(approximately 1/4' deep) that allow access to the recessed
tuning knob set screws. Loosen the two RECESSED set screws
and pull the knob assembly out from the front panel.
5.0 Remove the ball-chain from the sprocket
on the right side of the Megacycle dial drum.
6.0 Loosen, but do not remove, the four vfo
screws which secure the square vfo mounting posts to the front
panel.
7.0 Remove the bottom two vfo square mounting
posts.
8.0 Remove the top two vfo front panel screws
(loosened in step 6.0) and gently remove the vfo assembly
from the top of the 100/200V.
Installation of the vfo is the reverse of
the above described procedure.
5.3 VFO Actuator Rebuild Procedure
The vfo actuator housing is a machined aluminum
assembly, mechanically attached to the vfo front plate. This
assembly consists of a tuning shaft, a front-located ball-bearing,
load spring/distance tube and rear-located ball-bearing assembly.
The tuning shaft is tapped to accept the threaded (1/4-20)
vfo lead screw and includes an anti-backlash mechanism. The
input end of the tuning shaft includes a press-fit brass gear
which operates the dial-cord driven vfo Megacycle pointer.
The tuning shaft is 3/8" in diameter,
except that a quarter-inch portion of the shaft, nearest the
threaded end, is approximately 5 thousandths larger in diameter.
This larger portion is designed so that the rear-located ball
bearing is a press-fit, and therefore stationary after positioning.
The front-located bearing easily slides onto the tuning shaft.
A coil spring, located between the two bearings provides an
outward force to keep the bearings separated. A distance tube,
installed between the two bearings ensures a minimum separation
distance is maintained.
The machined aluminum actuator housing is
grooved to accept two locking clips, and so, secure the bearing/shaft
assembly. The housing is somewhat shorter than the extended
distance of the two bearings with the load coil spring uncompressed.
Installation requires that the load spring be slightly compressed,
thereby providing a controlled amount of separation force
which ensures that the shaft bearings are properly located
within the housing. Disassembly of the assembly is as follows:
1.0 Remove the Kilocycle dial from the shaft
by loosening the two retaining set screws.
2.0 Remove the three housing assembly screws from the vfo
front plate and unscrew the entire assembly from the vfo lead
screw. Remove the anti-backlash washers and temporarily place
the vfo aside.
3.0 Secure the housing in a vice (smooth jaws)
with the brass gear upward.
4.0 Gently remove the brass gear. First, using
a large, flat-blade screwdriver, lever the gear up the shaft,
using the housing lip as a fulcrum. Once the gear is approximately
3/16" above the housing lip, use a gear-puller to fully
remove the part from the shaft.
5.0 Remove the internal spring clip. Take
caution as the front bearing will release and move upward
due to action by the compressed load spring.
6.0 Remove the housing assembly from the vice
and extract the shaft, front/rear-located bearings, spring
and distance tube as one sub-assembly.
7.0 Separate the front-located bearing, distance
tube and spring.
8.0 Press off the rear-located bearing. Do
not apply force to the 1/4" portion of the tuning shaft.
This part is press-fit into the shaft tube and will fall into
the shaft if excessive force is applied to that area.
9.0 Disassemble the two bearings, clean out
the old lubricant and replace with a mixture of high-temperature
Teflon grease and synthetic 10W-40 motor oil.
10.0 Discard the original spring and replace
with a 1/2 x 1-1/2 x .041 spring (Handyman SP-9706).
11.0 Reassemble the actuator using the reverse
of the above described procedure. The rear-located bearing
should be pressed onto the shaft until it is nearly flush
(0.010" clearance) with the threaded housing-portion
of the shaft.
Prior to installation of the rebuilt actuator
onto the vfo, perform the following operations:
1.0 Remove all traces of old grease from the lead-screw, corrector
screws and other bearing surfaces.
2.0 Lubricate these parts using the procedure described in
Section 5, Routine VFO Lubrication.
3.0 Verify that the lead screw slides smoothly
into the ceramic vfo coil form. If there is any sign of binding,
it will be necessary to re-center the coil slug within the
ceramic coil form. To do so, proceed as follows:
3.1 Remove the vfo cover.
3.2 Loosen, but do not remove the three screws which attach
the coil form to the vfo chassis plate.
3.3 Gently slide the coil form until the slug
appears centered.
3.4 Tighten the three screws and test for
proper centering.
4.0 Reassemble the anti-backlash washers and
lubricate with high temperature Teflon grease.
5.0 Temporarily reassemble the actuator to
the lead screw and secure, with one screw, to the front plate.
Verify that the vfo tunes the entire 5-6MHz range, with at
least 25 KHz overtravel at each end. If correct, complete
the reassembly process and recalibrate per Section 3.0 VFO
Tracking.
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