The R-390 and R-390A receivers far exceeded the military's spec, achieving sensitivity in the nanovolt range. Reportedly classified top secret until the late sixties, these receivers were often 'demilitarized' by being crushed by bulldozers. We are most fortunate that some survived.

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INITIAL  POWER  UP

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For equipment coming from basement or garage storage of unknown duration, apply current limited power. Current limiting is simple: wire a light bulb in series with the receiver. A current limiter is made by getting a $5 hardware store extension cord and cutting the wire that goes to the skinny (hot) prong. The two cut ends are spliced with wire nuts to the pigtails of a hardware store rubber lightbulb socket. Now a radio plugged into the extension cord will be in series with whatever wattage bulb is in the socket. If there's a dead short in the radio, the light will hit full brilliance, and current will be limited – a 60 Watt bulb will only allow a half amp, for example, into a dead short.   Preventing current surges avoids blown up filter capacitors and other components, minimizing risk of damage, saving hours of labor.

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With a functional R-390A, a 100 Watt bulb typically allows 80 Volts, which is sufficient to get rectifier filaments going enough to get HV circulating. Both resistors and capacitors are hygroscopic which means they absorb water vapor from the air. After years of sitting in storage, this internal moisture makes them  leaky and off value. Leaks in capacitors generate heat, which will dry them out at proper reduced voltage, or may blow them up hot if hit with full voltage. My experience has been that a day or two at reduced voltage reforms filter caps and the gentle internal heating dries out the other components, stopping leaks and bringing them within tolerance. This simplifies troubleshooting by reducing the number of component replacements.

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INITIAL  CHECKS

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If the bulb is near full brilliance, resistance checks are in order before further powering up. Look for shorted rectifier tubes or diodes, good power transformer windings will show a really low (the R-390As primary is one or two ohms) DC resistance, so shorts are hard to detect. Good diodes should show a high resistance in one direction, and conduct at a bit less than one volt in the other. 

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Once the short is corrected, or if the bulb is dim, then further checks can be made. Rectifier filaments need about 70-80 volts to start producing HV, so this voltage is a good starting point for reforming filter capacitors. Voltage adjustment to the radio can be gotten by swapping different wattage bulbs – more watts, more volts will go out. Use the extra extension cord outlet socket to check input AC voltage.

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Filter caps should be monitored for heat to the touch, and voltage should be reduced if so. Find the DC HV in the radio, possibly by removing a tube (not the rectifier) and hunting in the socket for the maximum DC voltage. Notice that it will slowly climb, and may take an hour or more to stabilize. This is a sign that the filter caps are reforming, and voltage can be increased by going to a higher wattage bulb. Once you’ve gotten to the point of using a 275 Watt floodlight, it's time to plug the radio directly in.  

 

A quick and dirty way to check for excessive power draw can be done by monitoring AC line voltage drop at the end of a cheapie extension cord (#16 wire) when the radio is turned on. Be looking at the voltmeter and be prepared to turn it off in less than a second if it drops more than a couple of volts. The drop will increase as filaments energize the HV. If the drop increases steadily beyond a couple of volts, turn it off immediately. Suspect a bad rectifier or possibly a filter cap. Energizing at reduced voltage may save the cap by reforming.

 

In the R 390A, voltage drops exceeding one volt at the two HV fuses can be used to localize problem areas. Unplugging modules can be used to further isolate any problems. This 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 volts, while the HV fuse  near the power supply (if installed) should have no more than .75 volts across it, and the HV fuse by the audio deck, a half volt, usually much less. The HV should rise and filter caps cool off with time; when they stabilize, full line voltage can be applied. 

 

[The following section is specific to the 26 tube R-390A, but is broadly applicable to other radios.]

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TROUBLESHOOTING  SHORTCUTS  

 

At full line voltage, if the receiver has not made noise, its time to work the switches, removing and replacing tubes (check for cold ones), and wiggle connectors, working backward from the audio stages toward the antenna. Where the clicks in the speaker stop is where the signal is blocked. If necessary, remove tubes and check that they are getting HV (on most small 7 pin tubes it's the  5th or 6th pin going counterclockwise from the gap. Bigger 9 pin tubes often have two HV pins.

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The audio deck is checked by removing tubes, cycling the mode switch between AGC and standby (with the local and line gain wide open), and listening for clicks in the speaker.   

 

The IF deck is checked by cycling the LIMITER and BFO on/off switches (should click), and seeing if 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 oscillatin​g or some bandswitch contacts dirty. Typically, dead bands and dead crystals will 'wake up' as crud is scraped off the switch contacts by cycling. If it is live only  above 8 MC, suspect the 3rd mixer or 17 Mc crystal oscillator. Try the CALIBRATOR, turning the BFO ON and using the line level meter if  the  signal is not strong enough to deflect the carrier level meter.

 

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 speaker 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 to connect the PTO output cable to the unbalanced antenna jack on the antenna relay, then insert a foot or so of wire into the unbalanced antenna connector on the rear panel. With the KILOCYCLE CHANGE at 455, another receiver should pick the PTO's oscillations 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 frequency.

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CLEANING  &  LUBRICATING

 

Because the gears are going to get a workout during alignment, it is a good idea to clean and lubricate them first. 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. Q-Tips are also useful for crud removal. After cleaning, a capful of motor oil dipped with a dental pick or other pointed tool is used to put tiny drops of oil in gear teeth, axles, slug rack rollers and slides. Use high temperature aviation oil if available.

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MECHANICAL  ALIGNMENT 

 

Mechanical mis-alignment of the six RF slug racks can be spotted more easily by checking cam positions with the dial at 15999 and 3999 as well as the standard 7999Kc.   (I use 999 instead of the standard +000 since the 1 Kc difference has no noticeable effect, and I’ve encountered receivers in the wild that were mistakenly set at 000.) Each slug rack cam will be about one roller diameter from 'top dead center' when at its top frequency: the MC Change is one Mc below the top frequency in its octave, and the KC CHANGE is 999. If a slug rack is off, the cause may be  a broken or loose gear clamp. 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 (or use a small dab of 5 minute epoxy in lieu of a clamp after washing with lacquer thinner). If the clamp is not broken, set the MC and KC to that slug rack's top frequency, then twist the cam to one roller diameter from top dead center, using your other hand to lift the slug rack to reduce the twisting force needed. Wiggle the cam as you tighten, so as not to overtighten the clamp. 

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   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 used instead. You will need a .096 Bristol spline tool and a small insulated screwdriver.

                                                                

To save time, work first where  signals are strong. With a good signal at any frequency, start with the two IF slug racks in back. The right hand one moves with the KILOCYCLE CHANGE. The slugs are adjusted around 900 KC, and the trimmers around 100 KC, regardless of Mc Change. The trimmer nearest the panel has 200V, so use an insulated screwdriver and 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 have slugs done at 1 or 2 MC, and trimmers around 6 or 7 MC,  at any kilocycle. Beware of another 200 volt 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  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 procedure. 

 

The frequencies are marked on the cans below each of the six 'octave' RF slug racks. Set the antenna trimmer at zero and peak the three slugs in each rack near the low frequency end, and the three trimmers  near the high frequency end for each rack. The 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 has 200V.

 

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. The plug-in crystals can be replaced on the spot by unscrewing the crystal oven cover and swapping out the bad one.

 

The mechanical filter trimmers can be peaked at any frequency. Those on the side of the IF deck can be reached by tipping it up while connected. The stagger tuned IFs, and crystal filter trimmers should be left alone unless way off, as these are time consuming without a sweep generator. BFO neutralization is simple with an oscilloscope. Unplug the two connectors from the back of the IF deck, hook up the 'scope to the IF output, and adjust the trimmer reachable from the left side of the radio for minimum trace width.

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ADVANCED  CONCEPTS

 

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, which reduces excessive 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.

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Four CL80 thermistors in series with the AC line reduces the excess voltage common today by 8 volts, and have high resistance till they warm up over several seconds, eliminating tube filament killing inrush.

 

Ballast tube socket jumpers from the existing filament lines (pin 2 to pin 4 and pin 7 to pin 5 will allow plug and play substitution of a 12BH7 as well as the original 3TF7 ballast. Grounding pin 1 will allow use of an improved solid state regulator available from Aesop Engineering.

 

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 (available from Aesop Engineering).

 

The antenna balance trimmers will reduce noise pickup if set properly. These 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 trimmer closest to the front panel on the appropriate octave slug rack is adjusted for minimum.  

 

AGC action can be improved by adjusting the slug in Z 503, which is often out of tune. Adjust for peak carrier meter reading with any signal or the calibrator.  

 

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. Try a 12V filament transformer wired for 'buck' in the AC line.

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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 in this simplified procedure.

 

Using the calibrator and tuning from the calibrator tones 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. Unplug the radio. Undo the PTO captive screws and remove 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. 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. Plug in the radio. 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 anti-backlash spring.

 

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. 

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