R-390A Hints and Kinks
_____________________________________________________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 frequency.
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.
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.
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.
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 procedure.
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 ballast.
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.
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 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.