SCR-300 / BC-1000 Set

May 4

Just wanted to share my newest addition to my collection. This is an SCR-300 set, complete (I didn't bother photographing the BG-150 and long whip antenna, AN-131-A). The harness ST-54, waist belt ST-55 and carry strap ST-50, shown here, are original US items, but the back pad M-391 is, I believe, French or Belgian army. The short whip in these photos is a French AN-130, but I also have an original US AN-130.

The set is in fully working condition running off of a transformer-adaptor and 6 volt battery (stored in the bottom, CS-128 battery case). Unfortunately, I can't personally test the transmitter, but it does receive the local public radio station transmitting on FM on a freq that's almost exactly double the channel freq on this set, so I assume that I'm receiving a 1/2 wave bleed over signal. (Pretty cool however it's happening!)

At any rate, a fairly common set, but still one that I've wanted to add to my collection for some time now, and I'm happy to have it.

1,266 · May 4

That is awesome! I had one of these rigs that I bought at a junkyard in Plainview, but I'm pretty sure it was totally French rebuilt. Your set looks great and bonus that it functions. Nice pick up

May 4

2 hours ago, iron bender said:

That is awesome! I had one of these rigs that I bought at a junkyard in Plainview, but I'm pretty sure it was totally French rebuilt. Your set looks great and bonus that it functions. Nice pick up

Thanks! I'm very happy with this one. It's an original US manufactured set, but it did go through a depot overhaul and received a new data plate and Signal Corps R&R Shop assigned serial number. At some time in its past, it was transferred from the US army to (most likely French) service with a NATO ally, and when it was mustered out it was sold to a Belgian surplus dealer.

I believe that the French and a couple of other NATO armies kept the SCR-300 in service into the late '60s (or even early '70s with reserve units). The French manufactured new SCR-300 through the 1950s and probably into the mid-60s. I believe that they supplied these radios to a number of other NATO countries. There were also British, Japanese and Yugoslavian versions that were manufactured after the war. I suspect, but don't have any evidence yet, that the SCR-300 was also used by South Korea and likely kept in service there into the '60s. I have uploaded a research document for collectors about these sets, and the link to the DropBox file can be found in one of the posts below.

I'd love to hear from other collectors about their experiences and collection sets and equipment.

BTW, I'm still in the market for better webbing and canvas items. If anyone out there in USMF land has any SCR-300 related items they're considering for sale, don't hesitate to let me know.

June 8

I wanted to provide a quick update on this topic.

Ever since I got this radio, I've wanted to be able to use an external speaker so that folks who are looking (like during public displays) at it can hear it, too. The SCR-300, however, was not designed nor manufactured with a jack for an external speaker. It does have an AUX PHONE jack, but the audio signal is too weak (only about 50 milliwatts) and has very high impedance (around 2000 ohms). In order to drive an external speaker, so creative fabrication is needed.

(NOTE: All of this COULD have been done by most of the radio repair / maintenance troops during the war using analog electronic components available at that time. Having said this, I have no actual evidence that this was ever done... The VRC-3, the vehicle mounted SCR-300, was normally wired up to the vehicle's interphone system, most of which did include provisions for amplifier powered external speakers. The concept would have been nothing remarkable if someone wanted to do this.)

So, I decided to fabricate two different sized external speakers for my set, one small, pocket-sized and very portable and one that might replicate something that could have been seen in a command post or HQ. For the small speaker, I decided to use an M-365 Resonator, which is a small speaker normally used with the SCR-625 Mine Detector, and item that would have been familiar to most infantry troops at the time. The large speaker would be an LS-3, since I had located a source for an NOS example at a modest price. The LS-3 was one of the regular issued commo items.

I manage to locate an M-356 Resonator that was effectively gutted. No worries, want I wanted was the case. I removed the R-14 receiver that was in it and replaced it with a 2," 4 ohm, 3 watt speaker. The original cable had been cut and was missing the PL-54 plug, but no worries, I had some original 2-lead cable and used a new Switch Craft 440 plug (a direct replacement for the required PL-55). This was all quite easy.

In my search for stuff to use to shack-up an external speaker setup for my SCR-300, I also bought an NOS LS-3 from Robert's Military Radios. This piece of kit was a real beaut! To prepare it for my needs, I simply disconnected the built-in 8000 ohm impedance matching transformer, cleaned up the jack and wired the speaker (3.2 ohm) directly the jack. This speaker uses a cable with PL-68 plugs on each end, however the center contact is not wired in, so only the tip (+) and shaft / body (-) are used for the speaker. As you will see, I had a BC-606 interphone control box that I was going to use to house a small battery bowered amplifier, so I used the two PL-68 plugs and piece of the 3-lead cable to fabricate the required cable for the LS-3.

I built a small battery-powered amplifier to take the audio output from the SCR-300, amplify it, and power the speaker. The amp unit I used was an inexpensive Arduino based device that I got from Amazon. It's designed to take a stereo audio output from something like a smart phone, a tablet or laptop computer, amplify it and send it to a mono-speaker. It's powered by a common 9V battery. It took a little creative wiring to get all the connections correct, but nothing too hard. I'm certainly not electronics genius (not even a mildly competent amateur, for that matter), but I managed to figure it out.

I did remove the built-on volume control (potentiometer) and moved it to mount it on the BC-606 box. I also added a single-pole-single-throw toggle switch to turn the power on and off. The only other thing that required a bit of thought was figuring out how to wire up the audio input. The Arduino amp is made with a common 3.5 mm stereo jack to receive the equally common stereo plug. I was concerned about trying to de-solder this jack, so I just soldered pig-tails onto the bottom of the circuit board to connect the audio in cable. For the audio in, I used another piece of 2-lead cable with a PL-55 plug. I wired a C-410 impedance matching transformer inline with this audio cable. The C-410 is the same inline 8000 ohm impedance matching transformer that's commonly found on HS-30 headsets. (I have several that I got with a lot of surplus headset bits and pieces. I think Fair Radio Sales has these, NOS, in stock. They're usually pretty cheap.)

Anyways, the photos show most of the rest of the story. The audio cable (PL-55 with the C-140) plugs into the AUX PHONE jack on the SCR-300. Either of the external speakers (M-356 or LS-3) plugs into the BC-606. The amp and battery are located in the BC-606, and once all the connections are made and the radio has warmed up, the power is switched on, and... voila! Amplified audio from the SCR-300 phone jack!

Unfortunately, the only station that I can receive is our local NPR (which is an oddity in itself since they actually broadcast on an FM frequency that is about double the frequency that I receive it on. Some kind of weird 1/2 wave image IF effect, I suppose.) Comes in with a good bit of static, but now I can listen to it VERY LOUDLY! LOL!

Thanks again, Paul!

Finally, this is the battery powered amp that I used:

Acxico LM386 10w Mini Power Amplifier

$9 for two on Amazon. Pretty good deal. All of the other bits (2" speaker, toggle switch, and 9v batter connector) also quick picks on Amazon. The Switch Craft plug and spring loaded jack covers can be found on eBay from a number of vendors.

BTW, this solution should work with just about any of the period US Signal Corps radio sets that DO NOT HAVE provisions for speakers, that is just Microphone and Phone jacks. Most of these have similar audio output specifications (~50 milliwatt at 2000 ohm impedance) to drive headsets, like the HS-23, and the receivers on handsets, like the TS-15. Sets with external speaker provision should be matched to the correct components specified in their applicable TMs. That is, if it has a speaker jack then there was a speaker that was intended to go with it. If it only has phone jacks, then it was only designed to run / drive a headset.

So, if your radio accepts the TS-15 handset (or equivalent) plugged directly into it, OR it will take the HS-23 headset (or equivalent) OR it will take the HS-30 headset with the C-410 inline impedance transformer, then what I've described above will probably work. However, as I said above, I am not an electronics expert or even someone with basic electronics knowledge. If you're not confident in what you're doing, then seek and find a real expert to assist you. Impedance matching of the speakers you will use and the radio set's audio output is very, VERY! important. If it's not done properly, there is a risk of burning out one or more of the radio set's vacuum tubes because of current feedback or excessive current draw.