[Boatanchors] [Milsurplus] BC-348-J BFO Help Needed (Corrected)

[Joe Connor]

 Thanks for the help. The problem is now solved.
The BFO was off frequency. The military manual that I have is silent on how to adjust the BFO frequency, but there is a way. You have to remove the BFO knob because the front of the case contains a stop mechanism that allows the BFO knob to rotate only a certain distance. With the knob off, you can turn the shaft on the BFO transformer farther. This enabled me to bring it within frequency. 
By the way, the knobs on this receiver are fastened with hex-head screws that require a metric key. I learned this after failing with standard and Bristol keys. 
Again, thanks.
                                   Joe Connor
    On Thursday, January 3, 2019, 2:02:35 PM EST, AKLDGUY <neilb0627 at gmail.com> wrote:  
 
 No. In the Command sets, the grid leak combo (100K and 100 pF) are external to the coil box. The resistor is connected from grid pin on the 12SR7 to the appropriate coil terminal. The capacitor is soldered across the resistor with very short leads (as short as they can be made). The two are suspended in mid-air. I was in no doubt at all that the capacitor was bad. The BFO started reliably as soon as it was replaced and ran reliably afterward.
Neil ZL1ANM

On Friday, January 4, 2019, Joe Connor <joeconnor53 at yahoo.com> wrote:

 Neil, was that a mica cap on the BFO coil itself? Interestingly, the prior owner replaced all the mica caps on mine. I'd better check and make sure he used the correct values.
                               Joe Connor
    On Thursday, January 3, 2019, 12:55:15 AM EST, AKLDGUY <neilb0627 at gmail.com> wrote:  
 
 This is exactly the problem I had with my 3-6 Mc Command receiver BFO. I changed tube, dismantled the coil, replaced the grid leak resistor, and still the BFO would refuse to operate until I touched the grid leak with a screwdriver. The problem proved to be the mica grid leak capacitor (100 pF). I suspect it went open circuit and the screwdriver provided just enough capacitance to bring the BFO to life until next switch off. The BFO played perfectly after replacement with a new 120 pF mica unit that was left over in my junk box from an abandoned project.
Neil ZL1ANM


On Thursday, January 3, 2019, Joe Connor via Milsurplus <milsurplus at mailman.qth.net> wrote:

Guys, I need some help and advice for a BC-348-J I'm working on. The link for the manual is below (AN 08-10-112) and the schematic is on pp. 75-76.
My problem is no BFO. With the BFO switch on, the voltages look pretty good, but no BFO. While I was doing the voltage testing, I found that when my probe touches the grid (pin 2) of the 6SR7 (2nd Det/CW Osc.) with the BFO switch on, I get the BFO loud and clear. With the probe on but the BFO switch off, I don't get the BFO tone. 
Any ideas? From the results above, I assume I can rule out the BFO oscillator, the 6SR7 and the BFO switch as the culprits. What else is there?
This receiver is pretty nice. No mods and I have the shock mount, so I'd really like to get it going full blast.
As always, thank you for your help.
                            Joe Connor


http://bama.edebris.com/ manuals/military/bc348/