SX-42 BFO Woes

[W7SOE]

I thought restoring the missing 150V to the plate of the 7A4 oscillator would solve my BFO issues, think again Kimosabe!

The Hallicrafters SX-42 uses a 7A4 as a BFO oscillator, both for the 455 kc and the 10.7 mc IF sections.

During alignment of the BFO I can barely hear the tone as I null it with the BFO coil.  The 10.7 tone is loud and clear.

I have:
cleaned the mode switch which restored the plate voltage

Checked/replace associated resistors and caps.

Replaced the micas in the BFO coil.

Replaced the entire coil/can with one out of a known good part rig.

No change in behavior throughout.

The BFO oscillator appears to feed into the output of the third IF at the detector.  With my scope I can see a nice signal there that varies with the pitch control, both 455 kc and 10.7 signals.

The plate of the plate osc is resistively coupled to the local oscillator.  That signal is hard to see and looks weak.  I don't have a good enough understanding of how it works to know whether it is ok or not.

So even though I can hear a weak tone during alignment the bfo is as good as absent during normal operation.

Would going back and re-peaking the 455 kc IF have any effect?  I doubt it.

Any ideas and/or wisdom would be greatly appreciated.

73

Rich

PS  I am certainly not ruling out anything stoooopid like having a switch in the wrong position etc.....   

[w3jn]

Get at it with a freq counter, or calculate out the signal frequency from the scope display.  If you're seeing a nice strong signal on the scope at 455 KHz as I think you said you are, it's either way off freq or it's not coupled tightly enough to the detector.  The detector is, as I recall, where the BFO signal goes in the SX-42.

[WB6NVH]

This doesn't address your problem, and you may have already taken care of this, but Loktal sockets are somewhat notorious for bad contact issues.  In the past I have wasted a lot of time chasing issues which cleared up with some DeOxit on the tube socket contacts.

The SX-42 was rushed to the stores before the bugs were even close to being worked out.  I used to work in a former service center for Hallicrafters and we still had the service bulletin sets and correspondence back and forth between us and Chicago when I started there.  I have never seen such a volume of material for just one model.  The bulletins on the Rf section were practically asking you to rebuild the entire front end!  I could see the stains from the technician's tears on the pages/

Regarding a "strong signal," that's a bit relative unless it is referenced against the signal level at the detector from the IF stage upstream. I am not at home so I can't look at the manual, but how does the BFO couple?  Gimmick or capacitor coupling?  Presuming it's on frequency you may have to mess with the coupling.

Do make sure the AVC is off and the audio gain all the way up with the RF gain being used to set audio level on CW or when checking the BFO.  I presume you knew that anyway.

[W7SOE]

Get at it with a freq counter, or calculate out the signal frequency from the scope display.  If you're seeing a nice strong signal on the scope at 455 KHz as I think you said you are, it's either way off freq or it's not coupled tightly enough to the detector.  The detector is, as I recall, where the BFO signal goes in the SX-42.

Well duh Rich.   If I am measuring my sig generator with the counter why not move that little clip over to the BFO and read.....

three hundred something kc. 

It never occurred to me to look for another null.  Backing out the adjustment screw (a looong way) I find the proper null and all is well!

Now do I beat myself up over all the work I did?  Nah, if we did that we would have all given this crap up a looong time ago.

Thanks guys for the help and my apologies for my own ineptitude.  Be prepared for more as I hit the RF section!

Geoffrey,
   You are so right about this radio.  It has to be a labor of love.  I am on my second attempt at this one and it is going well only because I found another one that was in ok shape and I can use my last rig for parts.  If I had any sense I would have stopped wasting my time. 
I WILL have a fully functioning SX-42 if it kills me, there is no logic to it.

73

Rich

[w3jn]

Rich, been there, done that, and wasted a lot of time chasing down a fully working (but off freq) BFO 

That's what the forum's here for!