[R-390] R390 Frequency

[Roger Ruszkowski]

Fellows. 
Jose  EA8DCP ask,
Is there an easy way to connect the frequency meter in a point in which the oscillator signal could be found for all mhz positions?. What  would be the  IF value  to offset ? I know that the challenge in this receiver is to obtain the maximum sensitivity and precision via calibration but that's not my league yet and meanwhile I think that a good solution is the frequency meter.
Jose
EA8DCP
Yes there is an easy way to get a digital readout of the center frequency the receiver is tuned to. The military used 4 each 19 inch racks for 1 R300/A, a nixie tube read out, 8K of ferrite core memory, a hand wired mother board and discrete integrated circuits to make up the processor pre 1968. You could look across the bay and know where the receiver was tuned to. The radio frequency scanner program was loaded from punch tape after a 16 instruction manual memory load to boot the machine after a power interruption. Every mega hertz has a different 2nd oscillator crystal. The VFO runs 3.455 to 2.455. Below 8 the 17 MHz crystal is also in the mix. Patch the signal generator into your antenna lead in. Count the signal generator. Zero the generator against your station. Un patch the signal generator when you have spotted your signal. You read the AN/URM25' s on the high level output and use the micro volt attenuated signal to couple into your receive system.  Yes you will radiate 4 or 5 Uv of CW signal right in the middle of the RF spectrum. So spot your self and uncouple the generator from the antenna and receiver system. Give the generator a dummy load to soak up the energy. This zero beat is not dependent of the IF center frequency of the receiver. If you only need to work a small frequency segment of the spectrum you can count your 2nd mixer once and track the VFO doing the math by hand for your final number. The R390 receivers are frequency meters. Tune the signal to zero beat and read the front panel to within 100 hertz.  60 hertz is a hum. 1948 Leading Earth best of science on the planet. You can zero to one cycle in ten seconds or better watching the line load meter. You can center the signal through the 100 hertz wide crystal filter in the IF deck front end and know flip flop down the way can do no better at naming that frequency than you can. Your R390/A was worlds best machine. Any child can be taught to watch the meter move. Read your maintenance manual. It says inject some thing through the IF deck crystal filter. Tune the injected something through the IF deck to the diode load to peak the diode load voltage value. The input is 455,000 hertz +2 or -2 because of the manufactured crystal filter part. Crystal filters at 455,000 do not oscillate at 455,000 so you do not have a 455,000 power source. The filter is a frequency reference notch (of theoretical infinite depth)  that is filtered against. Daily production was zero to under 4 cycles off the assembly line. You must judge signal fade against filter skirts to discern your measured value. The crystals in the receivers are all within 20 hertz of where we expect them to be. If you need to get more exact than your R390/A will read from the front panel ask Tisha about on air radiation to measure the frequency of an external signal source. Counting the oscillators in the R390 receiver is just not the way to measure frequency today.   Is there an easy way to connect the frequency meter. Get the super glue and single strand lacquer coated copper wire.Pull the 5654 and 5759 oscillator tubes out of the receiver. Neatly wind 20 or 30 turns of wire around the tube envelope and secure with glue.Leave long leads on the sensor coil you just assembled around your oscillator tube. Another solution was to use short 7 pin tube extenders with modifications.A small hole is drilled into the side of the plastic tube extender barrel.So a small diameter coax cable can be inserted through the hole.Tie the coax shield to a grounded tube pin conductor. Use a 100 pf or smaller mica cap to tie the coax center conductor to the oscillator plate tube pin conductor. Re assemble your modified tube extender with a take off coax tap.The 5654 oscillators and 5749 both have the plate on pin 5 and pin 2 is the screen returned to ground. One adapter works in all the right places.  These adapters will cause tubes to extend out of the chassis and for the VFO this needs to be considered in daily operations. If you use the coax and connector from a dead VFO the connector will couple to the rear panel 455KHz out put feed through adapter to a standard BNC connector. One military project punched three new holes in the back panel. Inserted the stock feed through connectors in the back panel. Used a short 1 inch high tube extender with the cap and coax addition. The coax exited high enough on the side of the extender to clear the tube socket shield ring. First osc, second osc, VFO, and BFO on the original jack. Today you ask your micro processor to gate and count each of the signals just doing a running total to display.  Every five seconds each oscillator is counted to the hertz and tallied for the final display value. You can pull the dial counter out. install a display, tape the micro process to the 2nd Osc deck, make up DC from the dial light power for the micro processor and digitize your receiver readout. Back in 1968 a simple ring counter went tick on the second and each operation stepped off in order. Update display, preset counter to 455000, tally on VFO for 1 second, tally on 2nd oscillator for 1 second, if under 8 switch then tally on 1st oscillator for 1 sec and call for time check. This was discrete transistors assembled into logic gate modules. Plug in modules before IC chip devices. Clever programming will do long counts on the crystals and store counts of well sampled inputs. So the VFO is one second to settle on the display. And after 20 seconds has a good averaged count. Sub routines spawn off, get a long count, scale the value and update the memory value. On sudden  frequency change flush and update and re build resolution. The tube extender adapter has been used in real military applications. Performance was acceptable and the receivers all worked with or with out the adapter in place. Our system had its set of adapters and we considered the adapters as pure unobtainium. Not many were made and none were expected to remain when the system was scrapped. The only thing between the thermite on the shelf over the receiver was the paper in the bay expected to be under the thermite when humans left the room vacant. If you make up the adapters they will work to bring the oscillator signal out of the receiver for other uses. Respectfully, Roger