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

[Jordan Arndt]

P.S....

The third sentence should read: "...B+ leads to the tube socket and the 
capacitor bank fed off the plate...."

73...Jordan...

----- Original Message ----- 
From: "Jordan Arndt" <Outposter30 at shaw.ca>
To: "Larry H" <larry41gm at gmail.com>
Cc: "R-390 Forum" <r-390 at mailman.qth.net>
Sent: Saturday, November 20, 2021 7:04 AM
Subject: Re: [R-390] R-390A 2nd Oscillator alignment...?


> 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
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