The R-390 and R-390A receivers far exceded the military's spec, achieving sensitivity in the
nanovolt range. Supposedly classified top secret until the late sixties, these receivers were
often 'demilitarized' by being crushed by bulldozers. We are most fortunate that some survived.
INITIAL POWER UP
For a receiver coming from basement or garage storage of unknown duration, it may be advisable
to apply reduced power to minimize risk of damage. This allows reforming of filter capacitors
and drying components by internally generated heat without blowing them up. This process also
brings components close enough to spec. that they can be left in. A day or two of heat soak
often saves hours of labor.
A 100 Watt lightbulb in series with this receiver on initial power up limits voltage to about
80 at normal draw, with the light barely glowing, and current to one ampere if there's a
short, with the light at full brilliance, saving the fuse. This is sufficient to get the HV
circulating even in non solid stated powersupplies.
Checking the voltage drop over the fuses is an easy way to detect problems before blowing them
with full line voltage. At 80 volts, the max for the AC fuse should be .15 v, while the HV
fuse near the power supply (if installed) should have no more than 1 1/2 volts across it, and
the HV fuse by the audio deck, 1volt.
When connected to full line voltage, if the receiver has not come to life, its time to work the
switches, check the tubes for warmth, and wiggle connectors, working backward from the audio
stages toward the antenna.
If the audio deck is OK, cycling the mode switch between AGC and standby (with the local and
line gain wide open) will give faint clicks in the speaker. If the IF deck is live, the
LIMITER and BFO on/off switches will make clicks, and the filters will change the tone.
If the RF deck is live, the ANTENNA TRIM and RF GAIN will vary the noise. Try different
bands, as some crystals may not be oscillating or some bandswitch contacts dirty. If only it
is live only above 8 MC, suspect the 3rd mixer or 17 Mc crystal oscillator. Try the
CALIBRATOR, turning the BFO ON if no carrier deflection is evident.
If the RF deck appears dead everywhere, there are two checks for the prime suspect, the
PTO. Try unplugging its mini BNC output connector (the closest one to but not on the
antenna relay). If the volume of noise doesn't change, try swapping the PTO tube with another
6BA6, and if that doesn't work, then check if it is off frequency. An easy way to do this is
to use the unbalanced antenna jack to connect the PTO output to a foot or so of wire. With the
KILOCYCLE CHANGE at 455, another receiver should pick it up as a carrier near 3000 Kc. Try
tuning around, it may be way off . If so, use the clamps on its shaft to get it back on
Mechanical alignment can be checked for all six octave slug racks more rapidly by checking cam
positions with the dial at 15999 and 3999 as well as the standard 7999Kc. Each slug rack cam
will be about one roller diameter from 'top dead center' when tuning the top frequency in its
octave. If one or more slug racks are off, the cause may be a broken or loose gear clamp, or
misalignment. (I had one radio mistakenly set for 7000 instead of 7+000) Try holding one of
the misaligned cams while turning the KILOCYCLE CHANGE. If it slips, try to identify and
inspect the loose clamp. If the clamp is broken, skip alignment for that octave. If the clamp
is not broken, gently try to twist the cam to its top frequency position with the dial set at
that frequency. It is usually necessary to use your other hand to lift some slug racks to
reduce twisting force. Tighten the clamp, no more than necessary, as they can break. Doing the
cleaning and lubricating before further use reduces twisting load and risk of recurrence.
CLEANING & LUBRICATING
Because the gears are going to get a workout during alignment, it is a good idea to clean and
lubricate them before. No need to disassemble. With the receiver on a table so the geartrain
overhangs a generous spread of newspapers, a squirt bottle full of kerosene or other slow
evaporating solvent (paint thinner) can be used to wash gears while turning them and brushing
their teeth with a toothbrush . Slug rack rollers should be turned by hand while washing till
they are free. Split gears are pushed slightly apart, washed, then worked by gently twisting
a screwdriver in their teeth to exercise them. Especially important is the split gear on the
KILOCYCLE CHANGE shaft. After cleaning, a capful of motor oil and a dental pick or other
pointed tool is used to put drops of oil in gear teeth, axles, slug rack rollers and slides.
Use high temperature aviation oil if available.
BASIC ELECTRICAL ALIGNMENT
Electrical alignment can be done using the calibrator, an off the air signal, or even internal
noise. If a signal is too weak to deflect the CARRIER METER, the BFO and LINE METER can be
To save time, work first where signals are strong. With a good signal at any frequency, start
with the right hand slug rack in back, which moves with the KILOCYCLE CHANGE The slugs are
adjusted around 900 KC, and the trimmers around 100 KC. The trimmer nearest the panel has
200V, so don't short it for more than a moment, or you'll cook a resistor. It usually takes
several iterations between slug and trimmer to get them peaked. The left side slug rack in
back is next, with the slugs done at 1 or 2 MC, and trimmers around 6 or 7 MC, beware the
hot trimmer nearest the back. The lone transformers to the right of each rack can be peaked
at any frequency below 8 MC
Any slug or trimmer can that doesn't peak is easily replaced. The procedure is a simple
lifting away of the slug rack, undoing the screw in the bottom of the slug tube in the
defective can, and unplugging by wiggling. A replacement can is inserted by reversing the
This should get decent signals to the six 'octave' RF slug racks, whose frequencies are marked
on the cans below. Set the antenna trimmer at zero and pick strong frequencies near the edges
of each octave, peaking the slugs near the low frequency end and trimmers near the high
frequency end for each rack. The extra trimmer closest to the panel is for antenna balance,
and is not done. When properly aligned, rocking the MEGACYCLE CHANGE knob should show that the
maximum meter reading is in the detent. The middle trimmer in each rack is hot.
Finally, each trimmer in the crystal deck is peaked. Dead frequencies typically indicate a bad
crystal, bad trimmer, or bad padding capacitor, which should be noted for future repair. Often
a bad crystal can be replaced on the spot.
The mechanical filter trimmers can be peaked at any frequency. Those on the sideof the IF
deck can be reached by tipping it up while connected. The stagger tuned IFs, BFO
neutralization, and crystal filter trimmers should be left alone unless way off, as these are
time consuming without a sweep generator.
Tuning feel and ease can be improved drastically by three simple tasks. The front panel
KILOCYCLE CHANGE bushing is often out of alignment, which can be fixed by removal of the knob
and repositioning and lubing the bushing while paying attention to shaft torque. With the
receiver on its back during cleaning, the large split gear on the KILOCYCLE CHANGE shaft
can be disengaged by loosening its clamp and sliding it, allowing one half to disengage so
as to move by itself, reducing spring tension. Care needs to be taken to avoid changing its
position if both halves disengage. When re-meshing and sliding back into position on the
shaft, observe clearance between the disc and the DIAL LOCK. Rubbing between them is often
the cause of excessive drag. A simple front panel check is to remove the dial lock knob and
loosen the dial lock nut. If it wiggles when the KILOCYCLE CHANGE knob moves, it's dragging.
A pair of 1 Amp, 1000V diodes soldered to the plate and cathode pins of the rectifier sockets
replace the failure prone tubes, reduces heat, and allows running on lower voltage.
Four CL80 thermistors in series with the AC line reduces the excess voltage common today, and
applies it over several seconds, eliminating tube filament killing inrush.
Ballast tube socket jumpers from the existing filament lines to pins 3 and 4 will allow plug
and play substitution of a tube such as a 12BH7 without affecting the use of the original 3TF7
Sensitivity and noise rejection are improved by a 200 ohm balanced antenna connection.
This can be done with ten bifilar turns of telephone wire on a half inch toroid. The 50 ohm
coax shield is connected to the toroid's "center tap" while the toroid coil ends go to the
balanced inputs, with the coax center to either one.
The antenna balance trimmers can be set if a strong signal is available near the top of each of
the radio's six octaves. A 100 ohm resistor is connected to each side of the balanced
antenna connector and the other ends connected together. A strong signal is applied to the
junction, and the appropriate octave trimmer adjusted for minimum.
Reducing line voltage is a good way to find and replace weak tubes. Measured sensitivity was
unaffected by line voltage, but overall gain dropped about 10 dB for every 10 volts reduction.
That same reduction cut power and heat generation almost 20%, and filament voltage 8%,
offering a way to substantially increase tube life.
PTO ENDPOINT SETTING
Getting the PTO endpoints within one KC makes frequency spotting far easier, and enhances
sensitivity. No fancy paper dials or test equipment is involved.
Using the calibrator and tuning from the calibrator tone at 000 to +000, note how many KC
spread the dial covered. Remove the PTO spring, then move the KILOCYCLE CHANGE so that the
coupling disc rib on the PTO side is horizontal. Undo the PTO captive screws and the backside
mount. Remove the coupling disc, tip the PTO coupling end up. Be careful not to move either
the PTO or KILOCYCLE CHANGE discs, and beware of the live wires behind (or unplug the radio).
Undo the screw-in plug hiding partially behind the PTO transformer. Let the screw-in plug
drop out of the way (it won't go far) and get a small screwdriver that will reach the adjuster
behind. Turn this adjuster to reduce the KC spread, about one turn will do 10 KC. Clockwise
will bring the spread down (if the dial covered more than 1000 KC).
Replace the disc on the upturned PTO and gently lower and reunite it with the disc on the
KILOCYCLE CHANGE shaft. With one hand on the PTO to keep it in place, spin the KILOCYCLE
CHANGE and recheck the spread, which should be substantially reduced. After about three or
four cycles of tipping up, adjusting, recoupling, and checking, you should be within one KC.
You may want to put a drop of oil in the shaft bearing, and de-oxit on a Q-Tip for the shaft
grounding strap while the PTO is tipped up. On reassembly, remember the screw in plug and
Application of the above procedures has recently resulted in CW sensitivity of 80 nanovolts
(.08 microvolts) for 10 dB S/S+N in a 55 year old R 390A.