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 Post subject: BC-455A BFO
PostPosted: Mar Tue 05, 2019 5:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20750
Location: Warner Robins, GA
I have a BC-455A receiver where the BFO had been removed by me nearly 10 or so years ago as at the time I didn't need BFO and needed the triode for a driver tube since a previous owner converted the receiver to drive a speaker with an AA% type audio transformer.

I do not have any of the original parts.

I would like to add BFO back to the receiver.

I want it to be solid state and I have 12.6Vdc available in the receiver which powers the tube heaters.

I want to do something different here which is to make the BFO variable like it is on most good communications receivers.

Now the front panel someone added where the connector would have been is quite busy with a pilot lamp the previous owner added, the volume control and a signal strength light I added so the BFO controls will have to be on the rear where the dynamotor was.

The ideal thing would be to find the original BFO coil and put the stock circuit back in place.


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 9:43 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 186
Location: Ashland Oregon
I have exactly what you're looking for but unfortunately after checking my BFO freq is 1415kc and the one you need is 2830kc. I'm sure someone will have one, be patient. I installed a BFO pitch control on my rear deck and it feels very natural to reach down the left side to adjust it. Just don't touch it because the B+ voltage is present on the stator of that capacitor. All you need is about 5 pf which is one small rotor and one stator plate. Connect the rotor to chassis ground and the stator to the plate of the CW oscillator. Set this capacitor at mid point and then tune your BFO to specs using that little screw on the right side near the rear.

The hot ticket on your set would be to use a 12SN7. Rewire one triode section as the pre amp you have now, and then use the second triode to reconstruct the BFO oscillator. You will really like the way you can fine tune SSB transmissions with this mod. Best wishes, michael


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 12:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20750
Location: Warner Robins, GA
With mine it would be difficult to do the 12SN7 as I would then lose the detector diode.

I do have the empty octal socket where the original 12A6 went and I can install a 12SQ7 there.

Pretty sure I remember where the BFO coil went, but I don't know that I have room there for it anymore, although maybe I could make room somewhere for it.

That said would there be a small solid state option for the BFO that runs on 12.6Vdc?

So basically I need a sinewave oscillator at 2.830KHz.

Also I would need to install the BFO switch on the rear deck as well.

I also installed a tone control there last night as the .002uF cap I put across the volume control (modded mine for AGC so volume control is after the detector before the first audio tube) didn't reduce the upper frequencies enough for my liking and was not pleasant to listen to when receiving weaker shortwave stations due to the static.

I may try to find a small trimmer cap that can have a knob attached.


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 2:02 pm 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 931
Location: Tokyo
Two circuits ideas. Maybe useful.

First, a regenerative IF stage from QST, about 1953. Easy to do and you don't need a coil. And you can also use it as a peaking Q-multiplier.

The other is from a UK publication, Amateur Radio Techniques, c1970. Interesting ring product detector using 1N34 diodes (today I'd use BAT46 or Schottky 1N5711). Functions as an ordinary AM diode detector until you turn on the BFO.


Attachments:
REGEN IF AMP.png
REGEN IF AMP.png [ 126.15 KiB | Viewed 1408 times ]
RING DET.png
RING DET.png [ 169.25 KiB | Viewed 1408 times ]
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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 2:33 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
In a way I like the second circuit, but how would one derive the -DC used for AGC?


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 9:13 pm 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 186
Location: Ashland Oregon
I realized there was more to my dual triode mod posted earlier. Regarding the lack of the diode element in the 12SN7........no big deal. Simply substitute a solid state diode between the last I.F. transformer and ground. When I did mine, I spent some time researching this topic and ended up taking this advice I saw on this forum. "There is a way to improve the sensitivity somewhat. Replace the diode detector with a 1N5711." I also made this change on a Hallicrafters receiver and it works perfectly through all the bands. I'm sure other diodes will work, but I was happy with this one. I still feel this is the simplest way to obtain what you want.


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 10:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20750
Location: Warner Robins, GA
I used that diode on a hallicrafters S-41W and it worked well.

Cannot remember if I tried it with this receiver or not.

I can try tomorrow.

Given the resistance in the detector circuit is similar to the hallicrafters it may just work.

Now I tried it with an RCA K-80 console and it didn't work good at all.


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Wed 06, 2019 10:51 pm 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 186
Location: Ashland Oregon
This might be of some help. The original BFO can measures 1.42 x 1.42 x 1.600. Dan's Small Parts and Kits sells a kit for a dollar that allows you to add a 1/4" shaft to any trimmer cap. If you ask he might just send it in an envelope.

Interesting results with your diode experiment. I guess it does matter which one you use.


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 Post subject: Re: BC-455A BFO
PostPosted: Mar Thu 07, 2019 12:41 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 931
Location: Tokyo
Deriving the AGC voltage: take it off the primary of the last IFT. 22pF cap, 1N4148 diode to ground with a 470K~1M resistor in parallel. That's your AGC source. Then you just add the usual resistor+cap combinations (eg, 1M + .047, etc) to connect it to the RF and IF amps. The AGC voltage is derived ahead of the detector so you can use it with the BFO on. I have a more elaborate fast attack, slow release circuit which uses 3 diodes, also derives the AGC voltage from the IFT primary. Was intended for the HBR series. I'll dig it up if you're interested. Warning: haven't tried myself.

This is circuit I do use and like very much. Appeared in CQ around 1960? Cathode follower ahead of a halfwave voltage doubler AM detector. Exceptionally clean audio. I insert the BFO at the cathode follower's cathode: it's not a proper product detector but it works pretty well. Component values are not very critical. I use BAT46 diodes instead of 1N119.

Rob


Attachments:
CF + HALFWAVE DET.png
CF + HALFWAVE DET.png [ 68 KiB | Viewed 1368 times ]
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 Post subject: Re: BC-455A BFO
PostPosted: Nov Mon 18, 2019 5:14 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20750
Location: Warner Robins, GA
Hmmmmmmmmm.................

Not sure why I never responded to this topic.

Sorry for the delayed response.

Still looking for a BFO circuit.

Thinking the simplest way is to build a 2.830MHz oscillator.

Yesterday I was trying to build a simple transmitter using as 7.2MHz TCXO and an audio transformer which somewhat worked, but I then picked up a ham transmitting in SSB on 7.2MHz and the audio sounded quite so perhaps the oscillator doesn't even need to be a sinewave which would simplify things greatly.

Far as the BFO goes I may build it external to the receiver and just use a jack to connect it to the receiver as I would then have as much space as needed for the controls and circuitry.

Thinking a 1/8" stereo jack would work as that would also allow me to feed the +12Vdc to the box as well so that I only have one plug.

An alternative is to buy a function generator kit that can tune up to 7.5MHz and loose couple that to the antenna. Have done that before using a service grade RF generator and it works (aside from the unacceptable drift), but I know it isn't proper.


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