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Author Topic: Will HF/RF Preamp Benefit a WWII BC-348?  (Read 2856 times)
WZ6F
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Posts: 98




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« on: December 28, 2012, 08:08:41 PM »

Hi,
I just had my BC-348 repaired and it is working although hard to pick up SSB signals on it. I am told this is a characteristic of the radio (i.e. do not expect too much sensitivity). Wondering if a hf/rf preamp will help? If so, any specific recommendations? The radio is intended to work with a long wire versus a 50 ohm antenna.

Thanks for looking,

Alan
wz6f
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G3RZP
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Posts: 4391




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« Reply #1 on: December 29, 2012, 01:17:26 AM »

Depends a bit on the model. Generally, yes, a preamp will help, but you then get 3 stages before the mixer which won't help the already not brilliant dynamic range.  If it's a BC348J, N or Q, it could be worthwhile trying to substitue a 6SG7/VT211 for the second RF stage. Sadly, you can't do that for the first RF stage without changing the wiring slightly.
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WZ6F
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Posts: 98




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« Reply #2 on: December 29, 2012, 08:43:30 AM »

G3RZP,

Thanks for the response! Would your suggestion potentially work on an "O" model (if you know)? If so, would you mind giving a couple sentences more detail (since I am not brilliant on the electronics piece - do much of the other in radio? Also (if you know), is it worthwhile to replace the tubes with solid state versions (if they can be found)?

Thanks,

Alan
wz6f
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AA4HA
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« Reply #3 on: December 29, 2012, 09:50:32 AM »

This is a great resource for the BC-348 series;

http://www.jamminpower.com/main/bc348.html

On model numbers there are two variants. Basically as James Moorer points out the "J,N and Q" series receivers were different in that they combined the mixer/oscillator and did not have a voltage regulator (see under BC-348 Tidbits for more explanation).

Another great reference can be found at;

http://www.hpfriedrichs.com/rr-bc-348.htm

Lots of the BC-348's were hacked up in the '50's and '60's to eliminate the 24VDC dynamotors or to add knobs and switches. Sometimes you will find one in pretty good shape, most other times they look like someone went crazy with a Weller soldering gun and acid core solder. I have three of the receivers (fortunately all "Q" versions), to me they are fun to work on. Even going back and fixing the messed up mods is not a problem and I have retrofitted one with an original dynamotor and rewired the tube filaments (they are 6.3VAC filaments that are wired in series-parallel with ballast resistors).

For a radio that was intended to be operated from a bomber aircraft during WWII it is a pretty amazing piece of work. It is also historically relevant and featured prominently in the B-29 "Enola Gay". They were frequently paired up with the ART-13 transmitter (other photos shown on the following web site);

http://aafradio.org/NASM/Enola_Avionics_Descriptions_-_Radio_Op.htm
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
G3RZP
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Posts: 4391




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« Reply #4 on: December 29, 2012, 09:53:24 AM »

Alan,

The BC348-O uses a double ended RF tube, the 6K7, with the grid on the top cap. The J, N and Q models used the single ended 6SK7. The 1st RF amp in those has the suppressor grid grounded, and the 6SG7, although twice the gain, has suppressor and cathode tied together, and so won't work in the first RF amp position, unless the lead from pin 3 to ground is clipped. Changing from a double ended tube to a single ended tube in those positions is no job for someone without a fair bit of experience.

Download the manual (free) from the Vintage and Military Amateur Radio Society (VMARS)web page at http://www.vmarsmanuals.co.uk/archive/files_index.htm.

Search for it by hitting ctrl F and entering 'BC348'

You should get a 15dB signal plus noise to noise ratio for a 9 microvolt signal, modulated I suspect at 30% with the function switch in MVC and the crystal filter out.

If there is an internal AC power supply, check that it does NOT have the negative side grounded directly, otherwise you can kill the output transformer.

Or so many have said, and I have no reason to doubt that. Not that I've ever had a BC348, though.
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WZ6F
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Posts: 98




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« Reply #5 on: December 29, 2012, 02:50:27 PM »

AA4HA,

Thanks for the good background! I will consider putting a good picture of a B-29 behind it. I love the history and your information is very pertinent..

Thanks Again,

Alan

G3RZP,

Great information. I now have the manual and will look at testing the signal to noise ("or signal plus noise to noise ratio"Smiley) and not having the negative side grounded directly.

I was also told that some of the tubes are a bit weak. I am wondering if there is an impact to that 15dB signal to noise ratio. It will be a good test.

Thank you!

Alan 
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KE3WD
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« Reply #6 on: December 29, 2012, 06:06:40 PM »

If the weak tubes are anywhere along the signal path, certainly changing to stronger tubes should make a difference. 
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AA4HA
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Posts: 1378




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« Reply #7 on: December 29, 2012, 07:05:32 PM »

There are also some potential issues with the 915 KHz IF frequency and the crystal filter. On one of the sites I posted there was details on high insertion loss when the filter was selected. The solution was to disassemble the filter and flush it out, then whatever the new crystal filter frequency was the next step was to align the IF stages to that new frequency.

I would imagine that those >60 year old capacitors and resistors in the stages probably should be checked and replaced if any are out of tolerance or bad. It sounds like if you go through and "gold plate" the alignment you can do much better than the basic RF sensitivity values. In those manuals on James Moorer's web site you will find a pretty detailed alignment procedure.

(btw; "gold plating" does not mean to literally apply gold to anything. It is to go back and forth through the alignment procedure repeatedly to achieve the peak performance.)

If you still do not get the performance you desire out of the receiver, then I would consider things like tube swaps or heaven forbid, a pre-amp.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
G3RZP
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Posts: 4391




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« Reply #8 on: December 30, 2012, 12:08:19 AM »

Back in the 1960's - post 1965, but I can't be more precise - there was an article in 73 Magazine on reducing the loss in the BC348 crystal filter, which was claimed to be very effective.
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WZ6F
Member

Posts: 98




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« Reply #9 on: December 31, 2012, 04:54:10 AM »

Thanks for all of the replies! I will go for the low hanging fruit first: tubes (dare I say try the preamp just for curiosity Smiley) while I investigate the other suggestions. The links have been great in this regard. Also, the reference to 73 magazine has been useful. The 73 articles that turned up are (based on suggestion from G3RZP):
- A Second Chance "Crystal Filter" for the BC-348" http://archive.org/stream/73-magazine-1966-06/06_June_1966#page/n47/mode/1up
- Modifying the BC-348 http://archive.org/stream/73-magazine-1964-06/06_June_1964#page/n89/mode/1up
- Band Spreading the BC-348 http://archive.org/stream/73-magazine-1964-06/06_June_1964#page/n89/mode/1up
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