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 Post subject: BC-348Q Filaments
PostPosted: Sep Fri 17, 2021 11:08 pm 
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Joined: Sep Wed 23, 2020 5:40 pm
Posts: 18
Hi All.

I'm working my way up the learning curve on boatanchors and military radios. Have not called 911 for anything so far and I intend to keep it that way.
I do have a BC-348Q that I need on the air. It had the dynamotor - not operational - so I built a power supply for it.

I'm at the point I have to change the filament wiring to handle the 6.3V AC from my Hammond HX261M6 transformer.

I am following the surplus conversion document published in 1948. It has the filament modifications that need to be made.
I am ok with the modifications for the 1st IF, 2nd IF, 3rd IF and Output. Where I am having trouble is the line they want between the OSC and 1st RF.

Highlighted in attached.
Is that required and where can I make that connection?

Is there a better way to handle this?


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6.png
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 Post subject: Re: BC-348Q Filaments
PostPosted: Sep Fri 17, 2021 11:39 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11343
Location: Long Island NY
The BC-348 receivers were meant to operate off the 28-VDC bus in larger planes. So with eight, six volt tubes they put two groups of four in series. R501A is a dropping resistor to get from 28 volts down to 24 and R501B is a balancing resistor used because the output tube heater draws more current than the other tubes' heaters. To convert to 6.3 volt operation the resistors are bypassed and the series circuits are converted to parallel.

So with the resistors bypassed as shown, the center horizontal line becomes your 6.3 volt AC "hot" line and the chassis of the radio is ground. What they are telling you to do is find the heater wire that goes from the first RF tube, either pin 2 or pin 7, to the second RF tube on pin 2 or pin 7. Ground one end of it. Then find the wire that goes from either pin 2 or pin 7 of the first RF tube to pin 2 or pin 7 of the oscillator tube. One end of that should be connected to the 6.3 volt "hot" line at any convenient point.

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 Post subject: Re: BC-348Q Filaments
PostPosted: Sep Sat 18, 2021 12:33 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 4256
Location: Austin, Texas
I got one of those that had a beautiful conversion to AC power. All the 6.3VAC filament connections were done with a twisted pair of wires and looked like they came that way from the factory. The radio operation was terrible with oscillation and overloading on all bands. I removed one of the twisted pair wires and grounded one side of the filament to the chassis on each stage. That got rid of the feedback through the filament circuit and the radio worked great.

Jay


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 Post subject: Re: BC-348Q Filaments
PostPosted: Sep Sat 18, 2021 1:44 am 
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Location: Victoria, Australia
These read like hearer tubes, if so I go with the idea of parallel. It should be noted that a lot of manufacturers staggered the wire so that if heaters were 2 & 7? Pin 2 was perhaps grounded and on the next one pin 7 and so on. Watch the wire gauge. I have sets here where somebody forgot (factory) that as you get closer to the transformer the current in the wire increases.

Marcc


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 Post subject: Re: BC-348Q Filaments
PostPosted: Sep Sat 18, 2021 9:06 pm 
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Joined: Sep Wed 23, 2020 5:40 pm
Posts: 18
Now I can see what I need to accomplish.

Thanks Chris08 for the clarification on that.


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 Post subject: Re: BC-348Q Filaments
PostPosted: Sep Sun 26, 2021 11:43 pm 
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Joined: Sep Wed 23, 2020 5:40 pm
Posts: 18
I must be missing something totally obvious.

I have the filaments wired to match the schematic for 6.3V Ac. Or at least I verified that as best I can.

I expected a good continuity signal from my MM from pin #3 or 4 of the Jones plug. Or from lugs 1-5 that were connected to the now missing dynamotor.

I get bumpkis.

I was going to pull the 6.3V AC in and test the heaters. I cant do that unless I have good test results from the MM that everything is properly connected. IMHO.

What am I doing wrong?


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 Post subject: Re: BC-348Q Filaments
PostPosted: Dec Sun 19, 2021 7:32 pm 
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Joined: Sep Wed 23, 2020 5:40 pm
Posts: 18
I'm still working on it. I am researching this voltage problem but suspect a bad tube.
Without a tube tester its a bit of a challenge. So feel free to chime in if you have any suggestions.

Starting with the good news
- I have 6.3V AC on all the filaments now.
- I have B+ 225V DC @ 117V AC from the power supply

The bad news is
- I have continuity on the filaments but no tube heater glow on my 6F7/vt-70.
- DC voltage at the metal caps is as follows


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 Post subject: Re: BC-348Q Filaments
PostPosted: Dec Tue 21, 2021 6:06 am 
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Posts: 8061
Location: Liberty, Missouri 64068
I well realize this is of little to no help, but I just used a separate 24V transformer for the filaments when I converted mine about 50 years ago. Never had any problems with hum.

Image

There are also the original MODIFICATION SHEETS from Hallicrafters, that were supplied with their 115VAC replacement power supply, to the USAAF after the war. Click the pics to enlarge them for reading/duping.

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