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1. The noise
bypass caps, C63,C64 are connected in series across the power
transformer primaries w/the common junction between them
connected to the chassis. This configuration provides optimum
noise reduction when the primaries are connected for 240VAC or
when the receiver is fed through an isolation transformer.
Either way both AC supply lines [or mains] are ungrounded or in
a "balanced" configuration.
If the radio is
connected to normal 120VAC wiring where one side is grounded
[called the "neutral" wire] & the other side is hot [measures
120VAC w/respect to ground] then C63 & C64 form a ground loop &
increase QRM in some cases. The best way to correct this
situation is to, first, replace the AC cord plug w/a polarized
plug so the AC is always connected to the radio the same way.
The conventional way is to connect the hot side to the power
switch, fuse, & one side of C63 & the power transformer primary
& the neutral wire to the other side of the transformer primary
& C64. Leave C63 connected as is & disconnect chassis from C64
& reconnect this lead on C64 to the "hot" side of the
transformer primary.
[If a 3-wire
grounded power cord is used don't connect cord ground wire to
receiver chassis. Connect radio chassis to external ground(s)
via a dedicated ground wire. Ground rods, cold water pipes,
well casings, etc., make good grounds & are usually tied
together externally. Running a ground wire to one of these
items in the building ground system & also using the ground wire
in the AC cord, which is also connected to these grounds forms a
ground loop which invites QRM.]
2. For improved
performance on bands 1 & 2 [LF & BCB] remove loading resistors
from across L1 & L2 secondaries. Any changes in tuning that
result from this can be corrected by the antenna trimmer
control, C2. These resistors are located on the antenna PCB,
p/n 85-123.
3.A. Replace RF
amp., V1, & 1st IF amp., V3, w/6DK6 tubes.
B. Replace
resistors, R15 & R19 w/47 ohm, 1/2 watt resistors. R15 is the
cathode bias resistor for the mixer, V2a, & R19 is the cathode
bias resistor for the 1st IF stage, V3. [These changes are from
the May '68 Popular Electronics magazine.]
4.A. Install a 0B2
voltage regular tube between the Osc. B+ line & ground [V2b
plate voltage]. Pins 1 & 5 are the plate leads & pins 2, 4, & 7
are cathode leads [only one of each need to be used].
B. Replace the
15K, 3W resistor, R49, in the Osc. B+ circuit w/a 15K, 5W type
to handle the additional current from the voltage regulator.
C. Replace the
Osc. B+ filter cap, C61 w/a 0.22-.47uF, 350VDC cap. It's
necessary to lower the capacitance here to prevent the 0B2
regulator from becoming a relaxation oscillator at certain
frequencies [usually at the top end of band E] where osc.
current rise causes the voltage on the 0B2 to fall below the
sustain level.
5. The receiver should be realigned per instructions in the operator's manual w/one exception: set antenna trimmer control, C2, to mid range instead of fully CW as instructed. This helps insure that there's enough adjustment to allow the RF amp to track the other tuning circuits. 6. BFO may not tune low enough in LSB mode. If not, replace C48 w/3-30pF mini trimmer cap. 7. Additional notes. For manuals contact Pete Markevich at http://www.manualman.com/ For any suggestions or comments contact me at wireaddict1@aol.com For tubes, sockets pilot lamp bulbs go to http://www.vacuumtubesinc.com/.
ATTENTION
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