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My new toy: BC-348
http://www.wavebourn.com/forum/viewtopic.php?f=7&t=119
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Author:  wavebourn [ Fri Oct 15, 2004 2:29 pm ]
Post subject:  My new toy: BC-348

Here is an aircraft communications receiver, made at the beginning of 1940'th.
Probably this particular set had been modified for ground operations, since the dynamotor is stamped 12V.

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Author:  wavebourn [ Fri Oct 15, 2004 2:34 pm ]
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Label is missed, but probably it is a model E, it uses tubes with grid caps.

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Author:  wavebourn [ Fri Oct 15, 2004 2:35 pm ]
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Thanks to a robust metal box, it looks clean inside.
However, I did not get a chance to inspect it, since I am still waiting for shipment.

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All photos above are courtesy of John Marshall who sold me this beast, he is a professional photographer. Thank you John!

Author:  wavebourn [ Fri Oct 22, 2004 12:39 am ]
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Crystal filter switch had been broken.

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It was on/off switch shorting a crystal resonator. A bridge filter gave narrow selectiviry good for CW (Morse code receiption). However, for AM audio it is useless.

Author:  wavebourn [ Fri Oct 22, 2004 12:45 am ]
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This rotary switch by Grayhill is exactly what do I need. However, it will switch nothing inside of the IF cover, but I would like to leave controls on the same positions on the control panel of the receiver. Now, instead of Filter on/Off the same position will have a switch for narrow/normal/wide bandwidth.

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Author:  wavebourn [ Fri Oct 22, 2004 12:57 am ]
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How it looks now.

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I soldered a cable to the switch that will go to the second IF amplifier, to switch remotely Electro - Mechanical Filters. Probably, it will be done by miniature electro-magnetic relays.

I plan to remove one IF tube 6F7 -- it is one of the 1'st 1930'th combined tubes, and replace the last IF tube with a pentagrid converter, to convert the 1'st IF 915 KHz to the second IF 500 KHz. The second IF will be demodulated by a double ballanced demodulastor switchable to AM or SSB demodulation. Also, the second IF will go to a BNC connector for further demodulations, for example for DSP by some computer card, I do not know yet what kind of ideas I will have later. Probably, I will install in the same receiver a narrow band FM demodulator if some radio stations will appear on short waves broadcastinf NB FM. I remember a rumor about some Belorussian FM station broadcasting around 7 Mega Hertz.

Author:  wavebourn [ Fri Oct 22, 2004 2:41 pm ]
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Today I've found pictures on Ebay and desided don't modify it unreversable... It is a pitty to see that...

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Dynamotor and capacitors under chassis had been replaced with transformer, rectitier tube and electrolytic cap. Most of IF cans and tubes were replaced. No doubt, IF were feed to another QR5 receiver as a second IF (I wanted to insert for such purposes one more PCB with transistors and ICs)

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Some holes are empty, some were drilled that damaged a nice panel. Convenient double RF/AF gain control and MVC/AVC switch were replaced on pair of separate controls.


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6SK7 tubes were replaced by probably 6BA6 miniature equivalent tubes.

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Also, a huge hole was made for a meter, plus extra holes were drilled for additional potentiometers...

Author:  wavebourn [ Fri Oct 22, 2004 3:02 pm ]
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Conclusion: to install a BNC socket instead of brocken crystal filter switch. The socket will be used to take the IF to feed to a 2'nd IF converter with filters and demodulators (External!), it will be reversable, If I need to restore the original.

Author:  wavebourn [ Fri Oct 22, 2004 4:58 pm ]
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After careful consideration I decided to restore the receiver as close to original as possible. I took a switch from a floor lamp, it was made for Hi Pot and UL listed, so should have small capacitances.

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6F7 tube will stay on the place. However, I'll make an IF output on the rear main socket. IF band switch will be installed on an appropriate external device.

Author:  wavebourn [ Mon Jan 10, 2005 12:47 pm ]
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Last news: the toy works fine. However, I needed to replace a BFO switch, it was open internally. Probably, corrosion because of moisture. I did not have such 2-position switch, but I had a 3-position one. The switch originally feeds BFO with B+ in one position and adds a capacitor in AGC to slow it down. Now, it works the same way in two positions, plus in the middle position it disconnects both B+ and a capacitor, so the middle position is AM one with fast AGC.
Also, I aligned 915 KHz IF using signal generator. Also, heterodyne trimmers and RF trimmers were adjusted using signal generator and digital frequency counter.

As the result, sensitivity of the receiver is great. I heard hams from both coasts, SSB operators from Europe, and AM broadcast stations from around the world. Image rejection is great, thanks to higher than usually IF and three RF selectiviry stages carefully shielded in silvered compartments.

However, adjucent channel selectivity is not good. For experiment I took the IF signal, and connected it to the input of Fisher hi-fi tuner tuned to 915 KHz AM. However, such "second IF converter" has extra amplification that results in high volume of noises between stations (blasting in between stations), but resulting selectivity is really great!

Now, the receiver is totally original like in 1943 except:
1) Previous owner rewired filament powering from 28 to 12 volts. Old wires are in place, so it will be easy to convert it back if necessary.
2) 28 volts dynamotor had been replcaed by 12 volts one by a previous owner.
3) One shorted mica cap in B+ network was replaced by a modern ceramic one,
4) Replaced 2-position BFO switch by 3-position modern one.
5) I plan to hide J-309 FET in the IF can, as a source follower to take IF for further conversion in the external processor. The socket will be installed on an additional bracket fixed by bolts in already existing holes in the chassis.

Author:  wavebourn [ Wed Jan 12, 2005 3:08 pm ]
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Here is a power supply I made for it yesterday (just a prototype yet)

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B+ voltage depends on a bias voltage. Bias voltage increases while the capacitor C3 is charged by a load. When tubes are cold they draw small current. When they start conducting, they charge C3 faster. While bias voltage increases it increases B+ voltage until stabilizes. Both voltages depend on D3 zener, B+ depends also on R7 position.

D1, D2, R2, R4 - "fool proof" protection.

Author:  wavebourn [ Wed Jan 12, 2005 6:44 pm ]
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Here is a filament voltage regulator. Speed of growth of output voltage depends on the C2 capacitor. It helps fo warm up cathodes slowly so filaments will live much longer. Thanks to Shottky diodes in rectifier and FET loss of voltage on open diodes and transistors is less than if to use ordinary diodes and bipolar transistor, so 12.6V AC from a filament transformer may be easily converted into the slowly increased 12.6V DC for less hum and longer tube life.

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Author:  wavebourn [ Sun Jan 16, 2005 8:41 pm ]
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Today I suddenly found in the net a picture of Russian receiver US-9...

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Investigation reveils, it was a BC-348 modified for Soviet tubes, and with remote control added.

Author:  wavebourn [ Wed Jan 19, 2005 9:44 pm ]
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Here is the power supply, just prototyping...

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Author:  wavebourn [ Wed Jan 19, 2005 9:45 pm ]
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Here it is, I am listening to the conversation on 3,890 MHz

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