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 Post subject: Military Receiver 120 volt Conversion Transformer
PostPosted: Oct Mon 05, 2020 9:13 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 4362
Location: Belleview, FL 34420
Stock military receivers such as the BC-348 or R-648 come with a dynamotor. Receivers made for aircraft are normally 28 volt DC input. Converting to the standard transformer usually only has a 6.3 volt filament winding requiring rewiring the tube filaments and removing the DC filament capacitors. However Fair Radio Sales in Lima Ohio has the perfect transformer for providing 220- 230 volts DC and 25 to 26 volts DC. (BC-348 are better performing on 220 volts vs 250 volts from the dynamotor). Fair Radio part number is 3S92890015. Cost is $13.95. Height is 3-3/4" x 3" wide (trim mounting feet) x 3-3/4" deep and fits where the dynamotor was located plus has enough room for the capacitors and rectifiers. The BC-348 needs thee capacitors for the high voltage not exceeding 1" in diameter X 3" tall (25.4mm x 30mm or 40mm tall are standard). to mount on top of chassis with using a punch for the base and super gluing caps in place. For the 35 volt capacitors, the bottom side of chassis will need four 16 mm diameter by 32 to 40 mm length capacitors. You will likely need four 4700uF in parallel to fit under the base. The R-648 will fit with about any capacitors. Transformer is 80 volts on the high voltage thus a voltage doubler is used. It has plenty of reserve power at 80 volts @ 1.5 amps. It has a 22 volt 3 amp winding that is perfect for a capacitor input power supply using a four amp bridge rectifier. to operate the tube filaments. Most radios use 1 to 1.2 amp for the filaments. The transformer can provide up to 1.5 amps @ 26 volts DC filament or about twenty 6.3 volt .3 amp tubes per simulation software. The transformer has a tapped primary of 100, 110 and 120 volts if you need a boost in output voltages. If the normal 26 volts DC is high, add a 2 to 5 ohm resistor on the DC output depending upon the amount of tubes in the radio.. Schematic is below.


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Last edited by jimbenedict on Oct Sun 11, 2020 5:54 pm, edited 9 times in total.
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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: Oct Wed 07, 2020 8:10 am 
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Joined: Nov Fri 30, 2012 3:35 am
Posts: 618
Location: Phoenix, AZ
That PS design should work fine in that application. But how do you get AC inside the set's case, and how do you switch it? As far as original unmodified BC-348s and -648s with dynamotors, they are becoming scarce. I have originals of both with mounting racks, their dynamotors hum away quietly, the sets work fine, and I value them. If you have a set that's already been significantly modified by someone else (and the mods are beyond reversal), then performing metal surgery to accommodate an AC power supply makes sense. It undoubtedly would be nice to hear the radio play once again, even if its been modified. If we're talking about an original unmodified set, then I'd respectfully suggest trying to design your PS with the transformer, rectifiers and filter caps on a subchassis that mounts in place of the dynamotor without requiring any metal surgery to the receiver's chassis (e.g., reversible mods). Getting AC power inside without cutting or drilling the case may still be a challenge, but it should be doable. The set can be returned to original if desired later on, and its value will be retained ... the best of both worlds. Just my 2 cents' worth.
R/ John


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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: Oct Thu 08, 2020 5:29 am 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 4362
Location: Belleview, FL 34420
UV201 wrote:
That PS design should work fine in that application. But how do you get AC inside the set's case, and how do you switch it? As far as original unmodified BC-348s and -648s with dynamotors, they are becoming scarce. I have originals of both with mounting racks, their dynamotors hum away quietly, the sets work fine, and I value them. If you have a set that's already been significantly modified by someone else (and the mods are beyond reversal), then performing metal surgery to accommodate an AC power supply makes sense. It undoubtedly would be nice to hear the radio play once again, even if its been modified. If we're talking about an original unmodified set, then I'd respectfully suggest trying to design your PS with the transformer, rectifiers and filter caps on a subchassis that mounts in place of the dynamotor without requiring any metal surgery to the receiver's chassis (e.g., reversible mods). Getting AC power inside without cutting or drilling the case may still be a challenge, but it should be doable. The set can be returned to original if desired later on, and its value will be retained ... the best of both worlds. Just my 2 cents' worth.
R/ John

I would not buy a modified BC-348 or R-648 beyond conversion to a 120 volt power supply. The R-648 is modular. The power supply (dynamotor) connector has two pins not used. I use these pins to apply 120 volts to the new transformer primary. As for the power switch, I clip off the side with two leads and insulate. It operates the dial lights. I remove the inductor and cap from the switch 28 volt input and wire direct to the connector for the 120 volts AC. I used the connector ground for AC ground and another unused pin for the 120 volt AC neutral. I keep a spare dynamotor assembly to return to stock design and the 28 volt inductor plus capacitor to reinstall.

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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: May Sat 08, 2021 3:51 pm 
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Joined: Mar Fri 27, 2020 2:20 pm
Posts: 45
Also, having built and rebuilt many HV supplies up to 5kv or more, it is advisable to put equalizing resistors, ie 68-82K across each of the voltage doubler caps. The resistors can also act as bleeders.

I ordered these transformers and looks like they may work ok but it will be a very tight install. Cap size is very important as is parts placement..

I'll let you all know how it works out.

73,
Walter


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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: May Sat 08, 2021 3:54 pm 
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Joined: Mar Fri 27, 2020 2:20 pm
Posts: 45
One more thing, the HV doubler suppy B- does not go to chassis ground so the cap cans will have to be insulated from chassis and from each other.

73,
Walter


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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: May Sat 08, 2021 4:21 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10930
Location: Santa Rosa, CA
Also, I wouldn't use a 250V cap as the final output on a 220V supply. Cutting it too close... 300V at least. Caps are cheap.

Rich


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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: May Sat 08, 2021 4:55 pm 
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Joined: Mar Fri 27, 2020 2:20 pm
Posts: 45
My sentiments exactly!! I always try for ~20% safety headroom if possible. I ordered 300v already!

Thanks for pointing that out.

73,
Walter


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 Post subject: Re: Military Receiver 120 volt Conversion Transformer
PostPosted: Aug Tue 03, 2021 10:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10747
Location: Beaver Falls, PA. USA
Good point on the negative not going to chassis ground; in a stock 348, the negative end goes through a choke (part of the output transformer) to ground, which supplies negative bias to the output tube.

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