This page is a recollection of useful information about the R-390A. Most of it comes from the Boatanchors List (thanks to all anonymous contributors!). I have been collecting it since I became interested in BAs. The R-390A attracted from the beginning my attention...
'Organizational Maintenance Manual, Radio Receiver R-390A/URR'
(Air Force number: TO 31R1-2URR-442)
'Direct Support and General Support Maintenance Repair Parts and Special Tools Lists, Radio Receiver R-390A/URR'
February 1972 (Supersedes TM 11-5820-358-35P, 20 March 1962, including all changes).
'Field and Depot Maintenance Manual, Radio Receiver R-390A/URR'
8 December 1961 (Changes 1 and 2 at least were issued. It, together with the Operator's Manual above, supersedes TM 11-856A, 20 January 1956 including changes 1-6.
'Organizational Maintenance Manual': 38 pages. Tells how to remove and replace sub-chassis. Brief descriptions of trouble shooting.
'Direct Support ... Parts and Special Tools List': 160 pages of lists of parts. Mostly useless except for 24 pages of very good photos showing parts locations.
'Field and Depot Maintenance Manual': 189 pages. Contains theory, detailed trouble shooting, inspection and calibration / alignment instructions, voltage and resistance diagrams, parts location illustrations and, the most important, schematics. This is the recommended Army manual to have.
Chapter 1: General Information
Chapter 2: Operation
Chapter 3: Functional Description
Chapter 4: Scheduled Maintenance
Chapter 5: Troubleshooting
Chapter 6: Corrective Maintenance
Chapter 7: Parts List
Chapter 8: Installation
Chapters 5 and 6 are very detailed.
Now, as far as adjusting the little linearization compensators (of which there appears to be one for each 25 kHz of tuning range), I found I really couldn't get an accurate enough position measurement with my paper template method except on a repeating 360-degree basis -- i.e., a single calibration mark per rotation for each 100 kHz point.
To accurately adjust the linearization compensators at *each* 25 kHz point, one would need to make an adjustment jig with a large (or geared) substitute dial mechanism -- or figure out how to make the adjustments with the PTO installed in the chassis. Right now, I'm figuring to work on the latter approach. It appears it will require some specialized tweaking / viewing tool(s). Roy Morgan and I are collaborating (cross-country) on this dubious endeavor. Any and all advice from other intrepid souls who have "been there and done that" is eagerly solicited.
In any case, at the moment I have probably +/- 200 Hz accuracy anywhere along the 1 MHz range, with +/- 100 Hz accuracy at the 100 kHz cal points. Not bad... but not as good as it could be. This was just a first attempt to figure out how it worked and what might be done.
Electrical/thermal stability of this PTO appears excellent, BTW, especially with the oven running. So far, I would give the Cosmos PTO pretty good marks, both in terms of design and implementation.
1) Hook up freq counter to PTO output.
2) Make note of output in KHz for a 10 turn input (ex 991 KHz for exactly 10 turns).
3) Turn the adjusting screw 1/2 turn or so and make note of the amount and direction.
4) Erase old pencil marks and make new ones (what was a convenient mark will now move to a new spot).
5) GOTO 2 until output is exactly 1000 KHz for exactly 10 turns from the spec. start and stop outputs. For example, the R-390A PTO will output from 3455 to 2455 KHz in exactly 10 clockwise turns.
PS: I leave the ovens OFF all of the time. The PTO oven will just cook the lubricant out of the PTO. Leave it off. Let the rig warm up well before beginning PTO work, and do the work at room temp (24 degrees or so).
A: No need to do it unless you have a bad ballast tube and cannot locate another. In theory, the only alignments which might charge are:
1st XTAL oscillator output
In any case, if you were to replace V401 and V701, you'd want to at least check these aligments.
The diode load screws (two) have a jumper connecting them in normal use, and they are found on the back of the set. This is actually the detector signal, brought out just before the noise limiter, and also before the line and local audio amps. There is no audio filtering here, so what you get is what you get.
One can connect the signal to an outboard audio amp via a capacitor. I used a 22uF connected to the diode load jumper. From here I used the center conductor of a shielded audio cable to feed an amp, the shield being connected to the set ground.
The signal level coming off of the diode load is about right to drive the outboard amp on my set, with the volume control on the amp set at about 25% I get a room full of sound.
In normal operation, I had a slight hum, probably coming from somewhere in the audio deck. Now, hum gone. The amp I am using has nice speakers and an equalizer.
Caveat . .if you alignment is poor on your 390X, then your diode load signal will be low...but your audio would be low anyway then with the regular audio deck.