[R-390] R-390A Use of CX-2919 and CX-1363 adapters

[Roger Ruszkowski]

Larry, While standing in the class room we ask what's the point.Take one side of the balanced input to ground.Place a conductor between the other side of the balanced input and a generator output.Reduce the generator output to a minimum discernible signal level and work on. The input to a vacuum tube is a voltage and it cares not about its current relationship.  More than once over the past 25 years it has been observed that if you are not working in a real and real tight faraday cage all these matchy thingies are for naught.On a maintenance bench with out good room shielding the loss in miss match is below the background RF noise level in the room.Skip the numbers. All those wonder parts have been tested time and time again and are not necessary for the maintenance of a R390 receiver.We know all those old manuals suck. That's why some Fellows took the time to produce a new Y2K maintenance manual version that tried to leave the errors behind.  The lash up is irrelevant to the task of aligning a receiver for best performance. You are poking around doing stage gain analysis so I would recommend an isolation capacitance in your test lead.Several of the adapters in the lids of AN/URM 25 's had a DC blocking cap and were BNC on both ends. One adapter broke out into separate conductors for ground and signal. Nice for injection work. I have never been in an absolute RF quiet tight room and I have been working in shielded rooms for 50 plus years. We made more noise inside than we wanted to let loose while trying to be very stealthy.I have been in some RF tight cabinets and could tell if the door sealed or not by the meter readings. A couple cave bottoms I visited may have been RF quiet areas so we brought our own RF noise makers into the cave with us. Do the math for me. Xc .1 mfd @ 455KHz and .047 mfd @ 455 KHz. Kinks in the test leads offer more resistance than either of these capacitors do to a 455 KHz signal. Either part will block DC voltages in the receivers. Larry, There just are no absolutes and the goal is just the best peak performance available. You have a R390 Receiver and we have expectations for its performance. But we just never get absolute calibrated values. And if we did so what no one else has the same equipment and standards lab to offer a counter comparison. When I suggest 150 uv into the IF deck at 455 and set the DC gain for - 7 volts on the DC load. Their is nothing calibrated in that text book number. The  receiver power output difference of generator modulation on and generator modulation off  must be greater than 25 or the overall receiver will not make 20. A difference of 29, 30, 31 is very common and irrelevant in terms of an absolute number. We know what to expect from a good working receiver in local un calibrated and often un metered bench maintenance environments. We have no idea what these inner stage numbers are as maintainers.HOWEVER, Enquiring minds want to know. This is the age of rebuilt IF and RF decks where most capacitors have been replaced. Oxidized ground point hardware is causing problems. There are several different AGC mods of choice. And after any of that rework some things may not be assembled as required. Thus in need of repair. Larry, we would like to read about the values you do find as it gives us relative real values others can compare to. We also know that tubes can be cherry picked for better performance. Thus tubes dominate in circuit gains. Swap tubes and the whole set of gain numbers go with the tube not the circuit. There are limits but cherry picking makes a difference. This feature of tubes does nothing for your interest in gaining some hands on electronic knowledge. May I suggest that when you get an interesting reading you swap several different tubes into the same setup and compare your results. Just change one tube in the receiver at a time. In the IF deck you want some good spare 5749's and 5814's. Take the time to remediate the little things you do find. All the activates add up and if you were to get three more DB you would be amazed at the doubling of power. So your time in a module for a DB end to end is worth it for many more years. If we cherry pick a stage how much can we get in that stage?   AGC voltages and operation are a topic of interest as the AGC circuit shoves voltage where ever it wants. AGC and MGC comparisons could offer some ideas on what to expect for normal operation. Respectfully, Roger