BFO dead on Hallicrafters SX-71 - BoatAnchors Forum at eHam.net

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eHam.net Forum : BoatAnchors : BFO dead on Hallicrafters SX-71

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BFO dead on Hallicrafters SX-71	Reply
by K3STX on October 16, 2009	Mail this to a friend!
It was working until a week ago, and now when I turn on the BFO to tune CW I STILL hear the THUMP THUMP of the carrier, not the nice whistle of CW. It is not the switch itself, as when I switch on the BFO my volume drops ALOT, but alas still the THUMP THUMP. I switched tubes with a good one, no joy. I checked voltages on teh 6SC7 BFO tube, all is OK except G2 reads about -0.2V instead of the "spec" -0.5V (probably within error of my meter). This is the same reading I got when the BFO was WORKING. Any ideas where I should start? The rig has been re-capped, in fact I am using original electrolytics (all seems fine on AM and fine on CW till last week). paul

RE: BFO dead on Hallicrafters SX-71	Reply
by W6OU on October 16, 2009	Mail this to a friend!
Does G1 (pin 4) on the BFO tube go to approximately -4 VDC when the BFO is switched ON? If so, the BFO is oscillating as it should. In that case the CW PITCH control may need realignment.

RE: BFO dead on Hallicrafters SX-71	Reply
by K3STX on October 17, 2009	Mail this to a friend!
Yes, indeed G1 does go to -4V DC when the BFO is switched on. I WAS turning the CW pitch control around with my fingers while repairing the knob, but now that I "play" with that some more I still can't receive CW. I don't want to start replacing caps wholesale without knowing what I am doing (and it also seems to have been re-capped in the past 20 years or so,mostly ceramic caps and orange drops in there now instead of paper caps). paul

RE: BFO dead on Hallicrafters SX-71	Reply
by W5RKL on October 17, 2009	Mail this to a friend!
If the receiver is set up properly to receive CW but there is no beat note heard when the BFO is turned on, I would suspect that the BFO coil, L3, is not adjusted properly resulting in a BFO signal that is too far outside of the IF frequency which would lead to no beat note heard. The BFO signal should be within a Khz or so either side of the IF frequency resulting in a beat note heard. It is also possible that C66, inter-stage coupling capacitor is open which would prevent the BFO signal from being applied to pin 4 of V6, hence no beat note heard. You can check for the presence of the BFO signal by connecting a frequency counter to pin 4 of V6, 3rd IF amplifier. Turn the BFO ON and read the BFO frequency on the frequency counter. Adjusting the BFO knob should result in the frequency at pin 4 of V6 to change. If you have the SX-71 manual, turn to the "Alignment Procedure" page and perform the alignment steps. If you do not have the SX-71 manual, you can download it at the following website: http://bama.edebris.com/manuals/hallicra/sx71/ and/or http://bama.edebris.com/manuals/hallicra/sx71sams/ 73's Mike W5RKL

RE: BFO dead on Hallicrafters SX-71	Reply
by K3STX on October 17, 2009	Mail this to a friend!
Mike, No frequency counter here, and my only signal generator for alignment is an old tube job, I'm not sure it is all that accurate anyway (I assume I can test it by injecting RF into my TS-850S). Anyway, I took off the CW pitch knob and with a screwdriver started "re-spinning" the shaft of the control (like I originally did with the knob off while repairing it). Now after 3 or 4 turns I hear my CW stations again! I guess it was completely out of whack from me playing around!! Is this L3? Anyway, I think it is fine now. I better leave it alone and maybe align it once I KNOW my signal generator is accurate. Not to get that damned oscillator on my DX-20 from going all the time (it oscillates even when it is not keyed, for some reason I have HV on the cathodes all the time). paul

RE: BFO dead on Hallicrafters SX-71	Reply
by W5RKL on October 18, 2009	Mail this to a friend!
Yes, L3 appears to be the BFO coil. However, one can be confused by comparing the original Hallicrafter's manual against the SAM's manual. The Hallicrafter's manual indicates the BFO coil as "L3". The SAMS manual indicates the BFO coil as "L24". After reviewing both manuals, I believe the original Hallicrafter's manual has the BFO coil correctly identified on the schematic and parts lists as "L3". 73's Mike W5RKL

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