ARC-5 / SCR-274N receivers

[K9STH]

I finally got around to getting all of my SCR-274N receivers actually working. Although many people refer to the series as "ARC-5", there are actually subtle differences between the ARC-5 and SCR-274N equipment including the nomenclature. Since all 3-models have "BC" in the model number they are actually SCR-274N series.

The BC-454, 3.0 MHz to 6.0 MHz, has been around for decades. It was converted for general use (AC power supply, volume control, BFO switch, headphone/speaker jack added) a long time ago. However, it had not been used for a long time and a number of the bypass capacitors had become leaky. Therefore, all of those capacitors and the 2-electrolytic capacitors in the receiver were replaced. The antenna post had long been replaced with a UHF SO-239 connector.

The BC-455, 6.0 MHz to 9.1 MHz, was given to me 2 or 3-years ago. It had been subjected to one of the "transformerless" modifications that were popular during the 1950s. Basically, the heaters ("filaments") had been wired in series, a 45 volt heater rectifier tube added, an electrolytic capacitor added, etc. Unfortunately, one side of the AC line was attached to the chassis making such a conversion pretty dangerous to use. Also, somewhere along the line, the audio output transformer had been removed. Of course, all of the bypass capacitors and original electrolytic capacitors were suspect and therefore replaced. The heaters were wired in parallel, an audio output from the remains of an old "All American Five" receiver added, and the tube socket which had been installed on the back deck (where the original dynamotor was installed) was rewired as the socket to which an external power supply is attached. The antenna post was missing so an SO-239 was added. There was already a BFO switch, volume control, and headphone/speaker jack mounted on the removable plate on the front of the receiver. The volume control was in bad shape and also was way too high in value, therefore, a new potentiometer was installed.

I got the BC-453, 190 kHz to 550 kHz, in a trade deal several months ago. All of the capacitors, including bypass and electrolytic, were replaced and a BFO switch, volume control, and headphone/speaker jack were added. The heaters were changed from series-parallel to just parallel. Unfortunately, the receiver was dead! I could force an i.f. signal (85 kHz) through it but absolutely nothing through the "front end" or even the mixer stage. It was discovered that the tunable oscillator was not working! In fact, there was nothing on the plate of the oscillator tube! Pulling the "front end" coil assembly showed that there was B+ going into the oscillator coil. However, when the coil was "plugged in", there was nothing on the plate of the oscillator tube! Now the voltage to the plate of the oscillator tube is limited by a series resistor so the high voltage of the receiver was still fine! After going through the entire oscillator circuit it was discovered that the rotor plates on the oscillator section variable capacitor were shorting to the stator plates. The spacing between the plates is extremely small and it was difficult to tell where the short was occurring. Finally, I moved the rotor assembly a small fraction of a millimeter and the short disappeared and the receiver started working.

All 3 of these receivers had been previously modified and there was no real chance of getting them back to original condition. However, all 3 of them are again working fine. As to what I am going to do with them, that is a good question! Right now the only SCR-274N transmitter that I have is a BC-458 which, unfortunately, has to be slightly modified to work in the 40-meter band. Years ago, I had a BC-459 transmitter, which covers the 40-meter band. However, that transmitter was used primarily as a VFO for the 6-meter band.

My next project will probably be a BC-779 receiver which is a military variant of the Hammarlund SP-200-LX Super Pro series receivers. It isn't supposed to be in my hands for another week, or so.

Glen, K9STH

arc-5 receivers-1.jpg

[N0SYA]

Keep up the good work! When you get my sx28 done let me know!

[AF6LJ]

That's great, I love that old stuff

[N2EY]

NICE!

One thing you can do is to build a crystal-controlled converter for 160, 80 and 40 to go in front of the BC-453. Makes a really sweet glowbug RX.

73 de Jim, n2EY

[KO6WB]

I had a BC-455 as my novice receiver for 40 meters. Needless to say, it got replaced really fast. Talk about having to really work to keep in contact. That was bad but compounded by the fact that I really sweat sending CW it was way more work then I wanted. Used to have the BC-453 in my earlier days prior to getting into amateur radio. Used it as an IF and from everything I've learned since I did it wrong. It was still fun to experiement with and at that time the ARC-5 series was still cheap and abundant although the Q5er's were in demand and a little harder to find.
Have fun Glen and good work.
73
Gary

[WA7KKP]

Glad to see they're still desirable . . . in spite of the rather broad IF bandwidth of the HF models. The 453 of course can be used with any radio with a 455 kHz IF (including that BC-779) for a bit better selectivity. I kept my BC453 for that specific reason. Now if it would do FM . . . ???

Gary WA7KKP

[KM1H]

One thing you can do is to build a crystal-controlled converter for 160, 80 and 40 to go in front of the BC-453. Makes a really sweet glowbug RX.

But there isnt any glow in a BC-453. I have 3 of them in use with various radios here and had one in the late 50's with a HQ-129X. That plus a Heath QF-1 made for an excellent CW radio. After trying a 2B in 63-64 and then got the 75A4 I still use, the 129 became a BCB DX and GC SWL radio until I sold it before moving to the Chicago area in 1970. Ive always missed that radio as it was a very late 1953 production and had a factory installed calibrator, Ive only seen one other since.

The 453 of course can be used with any radio with a 455 kHz IF (including that BC-779) for a bit better selectivity

That IF is 465kc. I have a SP-100, BC-1004, and SP-400X; the first two are 465 and the 400 is 455kc. Ive often thought of changing the 1004 to 455 by retuning the IF's, changing the filter crystal and then adding a couple of R-390A mechanical filters to it to sharpen the skirts. Ive also heard of several using the 453 with the USN RAS receivers which had a 175kc IF and no crystal filter (otherwise it was a HRO-Jr); images must have been fun but I bet it was a great CW radio.

I had a BC-455 as my novice receiver for 40 meters. Needless to say, it got replaced really fast

I started with a HB regen in 54 and learned CW and spent a few Novice months with it. I then UPgraded to a BC-454 and 455 and eventually was told about bandspreading by pulling capacitor plates. It didnt help selectivity but sure made tuning easier. The next upgrade was a HQ-129X after a short distaster with a SX-25 which decades later I figured it probably had a bunch of leaky paper caps since it was deaf above 40M and not even very good there.

Carl

[K9STH]

The Surplus Schematic Handbook, published by Cowan Publishing (CQ Magazine during that time period), does list the i.f. as 455 kHz for the BC-779. However, TM-11-866, the military manual on the BC-779, BC-774, BC-1004, and a couple of others of the old Hammarlund Super-Pro receivers, does list the i.f. frequency as 465 kHz.

The manual doesn't give an absolute bandwidth for each position of the "bandwidth switch". However, there are 5-positions for the crystal filter plus and "off" position. The manual does say that the 2-narrowest positions are suitable only for CW and the other 3-positions are suitable for "phone" operation.

Glen, K9STH

[K9STH]

WB:

The BC-455 does have a narrower bandwidth than the Hallicrafters S-107 ("re-boxed" S-53A) that I used for my main receiver when I was a Novice Class back in 1959!

I would have thought I was in Heaven if I had a BC-455 back then! And, the S-107 wasn't, by any means, the worst receiver available in those days.

Glen, K9STH

[KB1WSY]

I'm currently restoring my own BC-453 and building a crystal controlled converter for 80m and 40m based on the W6TNS circuit in QST (January 1956). Some pics, prior to rewiring the filaments and replacing the capacitors:

BC453_front_after_cleaning.jpg BC453_overall.jpg BC453_below_chassis_01.jpg BC453_tuning_01.jpg

That tuning assembly is a thing of beauty. Glen, congrats on getting yours up and running. It is encouraging me to get mine working, too!

73 de Martin, KB1WSY