The AM Forum

Receiver: Belmont BC-348R

Band	MDS	Blocking DR	Two-tone DR
		(20 kHz)	(20 kHz)
80 meters	-137 dBm	106 dB	87 dB
40 meters	-137 dBm	105 dB	86 dB
20 meters	-134 dBm	101 dB	84 dB

AM audio S/N: 40 dB

AM Audio Frequency Response: in dB, referenced to 1 kHz measurement   

100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz	3 kHz	4 kHz	5 kHz	6 kHz
-6	0	+1	+1	0	0	-1	-3	-8	-12	-17

AM Audio Distortion:

Mod	100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz
30%	28%	14%	3.5%	3.2%	4.0%	4.5%	4.0%
50%	35%	14%	3.2%	3.2%	4.0%	4.5%	5.6%
70%	35%	14%	3.5%	3.5%	4.5%	5.6%	10%
90%	35%	14%	4.5%	5.0%	7.1%	10%	16%
100%	35%	14%	6.3%	7.1%	10%	13%	18%

Notes: Crystal filter used for MDS, blocking and two-tone dynamic range measurements.

AM distortion numbers show the somewhat typical increase at lower audio frequencies - most often caused by the low audio frequencies riding on the AGC line. Also there is the usual increase in audio distortion as the modulation percentage increases. This is due to the detector's inability to cleanly demodulate the higher amplitude levels. 

Info on the measurement setups and test results on other receivers can be found at:

http://www.w1vd.com/BAreceivertest.html (http://www.w1vd.com/BAreceivertest.html)

I love that receiver. My novice buddy got one from his elmer when he was 15 and I used it when ever I visited his shack. Very cool radio. I finally got one at Hosstraders 4 years ago from WA1QHQ Mark, and have been collecting knobs for it.  It was knobless so I put a hodge podge of knobs on it and started using it. I just need the MGC/Off/AGC lever switch to complete the set now. Then I will strip and paint them with crackle and complete the radio.

The crystal filter was cleaned up and resoldered and then I alligned the receiver IF around the peak. This worked slick. The Dynamotor is very quiet and well filtered in this receiver. The one knob gain is clever but you have to ride the gain on CW and in that mode it is strictly MGC. The AGC works OK on AM. It tunes nicely and there is no backlash. The top band is a bit weak and tunes very fast on 20M. It certainly is a usable, sensitive and stable receiver and it does a good job on the modern bands.

An outboard Q multiplier is a bonus and peps it up considerably. The audio is OK to drive a speaker through a transformer but the set was really meant for headphones.

If you want to see one in action, rent the movie Island in The Sky with John Wayne and Slim Pickens - it is a classic for military radio buffs and features the BC-348, 375, RDF and gibson Girl in action.

Mike WU2D
 

Mike

Yes it is quite a receiver.

This one came with a homebrew 117 VAC supply that needed some tidying up and a few misc. problems like a couple resistors that were replaced with incorrect values - off by a factor of 10! In chasing down out of spec voltages and 'cracklies' a large percentage of the .01 uF micamold caps (paper caps that look like square postage stamp micas) were found to have high leakage. So many were bad I elected to replace all of them.

Using the receiver before making DR measurements (uncolored initial impression) it was clear that this was a good receiver. Your comments are right on with the possible exception of the highest band...this unit measured only a couple dB less sensitivity.

One wonders what it was like using a BC-348 in the back of a WWII bomber. From the tests I've run and using the receiver on the air it appears the guys had a pretty good receiver. Bet there was a lot of noise in that environment.

                         

Thanks for Your work on the BC-348. Just finished with a mica mold replacement with "orange drops" an alignment around the filter after the symmetry was correct. I really love how you can hear the "personality" and work put into each station I hear.

Best Regards
Ted KC9LKE

The BC-348 was the first usable receiver I had when I was a novice. My scoutmaster and Elmer, K2EAW, bought a half a dozen of them in a surplus auction and we built 120 vac power supplies for them under his guidance and he loaned them out to all the kids in the troop who were interested in radio. I had mine paired up with a 50C5 one tube transmitter on a piece of pine board - a copy of the transmitter on the cover of an Electronics Illustrated magazine and my first totally home built project.

I recall the 348 as not being all that selective for CW in the 40 meter novice band, but looks like for AM phone it would be pretty good. I used to enjoy tuning around the low band with it and listening to the aircraft beacons. I remember the feel of the dial.


That transmitter was kinda scary. No transformer. Built point to point between brass nails driven into the board. It called for a 400 ohm, 20 watt wirewound resistor in series with the 50 volt filament. I didn't have one of those, but I made up an array of random junquebox resistors that totalled 400 ohms more or less, but I doubt I had 20 watts dissipation and it certainly wasn't evenly distributed. One evening when I plugged it in, one of the resistors popped and acted like a roman candle and started spewing flame and sparks onto the curtains around the window that the desk was in front of. I think that was the first time my mom threw all the ham gear out the second floor bedroom window. Fortunately it had been raining and the ground was soft...

The BC348 took the landing fine. I suspect it had seen worse.  ;D

The receiver of choice by Ashtabula Bill W8VYZ for a long, long time; yeah.

"There ain't a slop bucket in North America can bother me"  ;D

My BC-312 is a very enjoyable radio. I wonder how it compares to the BC-348.  The 312 was for ground operations and certainly very heavy. I aligned mine not too long ago around the crystal freq. It is the most pleasing to my ear of any I have up and running now.

Mine is dated 1944, an "M" model. I have the appropriate 500 ohm speaker.

I've never used a Q multiplier. Wonder what anyone suggests?

I use a BC-312N and ATC (Navy ART-13) with minimum mods to both mostly on 17M AM. The 312 was rather deaf on 17 after recap and alignment and a simple 2N5109 Norton style lossless feedback preamp tucked inside works wonders. With a 1.5 dB NF and only 11dB gain it improves the sensitivity enough with no overload experienced even with a 4el yagi.

The ARRL surely didnt think much of the 348 in a Feburary 1946 QST editorial and claimed it was excessively noisy on the higher range compared to high quality pre WW2 ham receivers. I used a friends once on 20M CW at a 1958 Field Day and that was enough. Went home to my HQ-129X!

Carl
KM1H

Carl,

I built Q multipliers for my command sets and BC-348 TCS etc based on this article. The principle works great on higher IF frequencies like the 1500 kc stuff too.

Mike WU2D