[R-390] R-390A 2nd Oscillator alignment...?

[Jordan Arndt]

Hi Larry and the group...

Some background and some clarification may be in order. Sorry if this a bit of a long post...

When I first pulled the RF deck with the 2nd osc. module attached, I found that R406 and R407 had been badly cooked and both were disintegrating and the board underneath them badly discolored.

As a result, I searched thoroughly to see if anything else appeared to have been overheated, including the coil and connections within T401, and all the B+ leads to the tube socket and the crystal bank fed off the plate of the 5654 oscillator tube.

There were no signs of insulation melt or discoloration, or wires having been overheated, so while replacing Rs 406 and 407, I checked the ceramic bypass caps C411, C412 and C413 for leakage and found none at all.

Since both resistors were cooked, it appeared that the B+ "short" had to be after R406 in the plate circuit, which leaves only the primary of T401, the plate of the tube, and the capacitor bank connected to the plate circuit.

There was no tube present, so I couldn't check that for a short, so I inserted a vector socket into the tube socket, disconnected J410, and connected my TO-6 from pin 5, the plate pin of the tube socket, to ground.

Again, I could not detect any leakage while cycling through each of the 31 positions of switch S402 even with 600Vdc applied from the TO-6, so I re-installed the RF and Osc. assembly and began work on the RF deck circuitry which is not quite complete yet.

What I then noticed while setting T401 and the caps was that I could set all the caps for good strong peaks, but on the 2 lowest bands the peak would randomly shift and lower the sensitivity on those bands by 10-20 dB, so I adjusted T401 slightly and reset all the caps, but the bottom 2 bands would still "drift" off the peak, which I when I started searching for a proper method of setting T401.

So far, this is by far the most finicky 2nd Osc. I've ever worked on, but I think I've solved the problem by using a very slightly modified version of the method described in the R-390 non-A manual for setting T401.

That manual says to adjust T401 in the 31MC position so that only 1 peak can be found by rotating the cap, and then adjust T401 slightly away from that position.

What I ended up doing was setting T401 until a single peak could be found while adjusting the 31MC cap, and then adjusting T401 roughly 1/2 turn farther into the coil where 2 peaks appeared within a few degress of rotation of the cap.

I then went through the remaining bands and set the caps for peak noise on the Line Level meter with the AGC off, and the issue with the bottom 2 bands has been resolved.

I can't figure out what caused the burned resistors, but I do recall finding a 3/8" machine screw rolling around under the oscillator deck when I first pulled the rig apart to clean the chassis. Whether it caused a short, or the tube itself developed a short, I can't say, but it appears the damage was limited to those 2 resistors...

73...Jordan VE6ZT


  ----- Original Message ----- 
  From: Larry H 
  To: Jordan Arndt 
  Cc: R-390 Forum 
  Sent: Saturday, November 20, 2021 12:08 AM
  Subject: Re: [R-390] R-390A 2nd Oscillator alignment...?


  Hi Jordan,  None of the tech refs do this subject justice, especially the 1956 Army version (it's totally missing).  This is very noticeable because of the frequent discussion on our forum.


  Although I haven't tried it, I like the R-390/URR tech refs process, but it too does not discuss the very common situation where some components like the crystals and SM caps may have drifted value and this requires a little more finesse.  What you will most likely end up with today is some of the trimmers are at or close to the end of adjustment, some in one direction and others in the other.  When the situation is at hand where a trimmer is at end of adjustment, the T401 needs to be adjusted slightly so that all the trimmers can be adjusted where they are not at the end of travel.  If this is not possible, then replacement of a crystal that has drifted too far off frequency needs to be replaced or the trimmer or its parallel SM cap needs to be repaired or replaced.


  As far as instability goes, I've only seen that when the trimmer involved needs to be cleaned or an SM cap is going bad or some connection is bad.


  Regards, Larry