[R-390] Looking for details on an old mod

[Gary Weddle]

Greetings again,
   No matter how hard I try, I cannot find it all in a timely fashion. This 
morning after posting my query last night, I re-searched the Y2K manual pdf for 
"relay" and found a different entry in the document which seems to have the 
diagram I am looking for. I apologize for the interruption and to anyone who is 
trying to spend time helping me find this information. However, I will still 
appreciate historical and helpful comments should anyone feel moved to do so. 

   Again, thanks.

<G>



----- Forwarded Message ----
From: Gary Weddle <w1ghw at yahoo.com>
To: r-390 at mailman.qth.net
Sent: Sun, December 19, 2010 10:22:55 PM
Subject: Looking for details on an old mod


Greetings to the list,
   I experienced difficulty with the BFO circuit on my R390A some time ago. It 
would not inject any signal into the audio (I could not read sideband stations 
or CW signals) and the receiver audio would reduce in intensity. I recently 
began to troubleshoot the problem and discovered that my unit had been modified 
extensively on the IF sub-chassis. I traced the circuit as best as I could, but 
feel there yet might be more I have missed. What follows is a description of the 
mod as I see it; what I am asking for is a reference to the article that covers 
the details of this particular mod. During the preliminary search of the 
archived resources, I found references to articles written in Ham Radio Magazine 
in the mid 70's concerning the BFO and  modifications. Ham Radio Magazine is 
hard to find anymore.
   I describe the changes I observed: 
     1. The BFO circuit, V505, has had a 10K resistor inserted into the plate 
circuit (common cathode configuration) and the signal on the cathode picked off 
the top of the resistor is passed through a 5 pf 1000V cap to pin 7 of the 
detector tube, V504B, in place of the signal injected at the same point from the 
plate circuit as in the original schematic.
     2. The original plate signal is now passed through a 0.02uF cap to the N.O. 
relay tap of a Potter & Brumfield relay, PW5LS, which is also lifted through a 
33K resistor to pin 6 of the RT510 (which has been solid-stated with two diodes 
and a ballast resistor). I assume that this is a supply line.
     3. The N.C. tap of the same relay is connected to pin 1 of T503 and goes 
nowhere else. (In the original diagram pin 1  of T503 was passed through a 12 mH 
choke and out to the diode load terminal at the back of the rig.)
     4. The center tap of the relay is connected through a choke (perhaps the 12 
mH choke that used to be attached to T503) to the diode load line out to the 
back of the rig.
     5. The relay coil is activated by the BFO switch on the front panel.
   It is clear that when the BFO switch is off, the relay is not energized and 
the circuit is functioning as the original design schematic indicates. Engaging 
the relay places +/- 455 kHz on pin 7 of the detector and also applies the same 
signal (180 deg out of phase) from the plate to the diode load line, but the 
original 455 kHz IF signal seems to be blocked from proceeding out that line. 
(At this point, I begin to doubt my ability to trace the circuit properly.)
   I am also trying to make a determination as to the value of this mod.  When 
engaged, the audio is reduced to an inaudible level. It is unclear to me what 
was being accomplished by the change. The modifier did a superb job of cutting a 
hole in the chassis and installing a tube socket for the relay. The soldering 
job looks good, but I will go over each joint depending upon whether I undo the 
mod or not.
   Again, I am looking for details on the complete mod, the theory behind why it 
was done and how it works.
   Thank you.

<G>