Small External Speaker ID?

Radio Telephone and Telegraph Transmitting and Receiving Equipment
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SGM (ret)
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Small External Speaker ID?

Post by SGM (ret) » Wed May 10, 2023 2:46 pm

Does anyone recognize and know the correct nomenclature and model of this small speaker unit? Is this a proper US Signal Corps item?

If it's a genuine piece of US Signal Corps equipment, what about the applicable TM or technical specs?

Finally, I gotta ask, does anyone know of or have one for sale? (LOL! Yea, I know, but I still gotta at least ask THE question...)
scr_300_bc_1000_1334325.jpg
I did try to search the website that hosted the photo, but no luck on any details or other info... :cry:


N6FEG
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Re: Small External Speaker ID?

Post by N6FEG » Wed May 10, 2023 3:32 pm

Believe it was used with the SCR 625 mine detector

SGM (ret)
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Re: Small External Speaker ID?

Post by SGM (ret) » Wed May 10, 2023 5:21 pm

Thanks, N6FEG!

I got a similar answer on another forum, too. A quick bit of digging and it turns out that it's the M-356-C Resonator, and it is, indeed, a component of the SCR-625 Mine Detector. That led me to the TM for the mine detector which had a bit more info.

Turns out the thing is essentially just a clamshell holder with an R-14 Receiver held in it. It could also be substituted with the HS-30 Headset.

Gotta dig through my "excess to needs stash" now to see if I have an R-14. (I recall that I had a couple of them some years back as I was putting together an M-1938 Tank Helmet.) I'm hoping to cobble together an external speaker for an SCR-300 radio set that will put out a bit more volume than just the handset and HS-30.

I really appreciate the info, though. Thanks, again!

Radtech
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Re: Small External Speaker ID?

Post by Radtech » Wed May 10, 2023 5:48 pm

It is a resonator for SCR625 mine detector.

SGM (ret)
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Re: Small External Speaker ID?

Post by SGM (ret) » Wed May 10, 2023 5:49 pm

Thank you, Radtech!

N6FEG
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Re: Small External Speaker ID?

Post by N6FEG » Wed May 10, 2023 6:10 pm

You're welcome SGM.
Before I had a bad fall from my bike 4 weeks ago I came across that spkr in box of headphones and the like. Unfortunately it's up in the stufftorium loft and I can't get up there with a fractured pelvis to get it...have a AN/29 ant I have to retrieve for another gent in the group here.
Paul

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Re: Small External Speaker ID?

Post by SGM (ret) » Wed May 10, 2023 6:16 pm

Thanks, again, Paul!

Hope you recover as quickly and as easily as possible. I can only imagine how painful your hip must be feeling!

"Stufftorium"! Love it! (Gonna steal that one, too.) When next you feel up to rummaging around in the stufftorium, keep me in mind if you come across that little speaker box. I'd be very interested in it.

Get well soon!

Mike

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Re: Small External Speaker ID?

Post by N6FEG » Wed May 10, 2023 6:32 pm

Thanks Mike..appreciate it.
I'll let you know when I get up there and find it.
It's going to be for awhile..Goin crazy not being able to work on anything...take care
Paul

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Re: Small External Speaker ID?

Post by N6FEG » Sun May 28, 2023 10:23 am

Mike
I'm able to get around and got the speaker if you still want it.
It'll need the cord replaced as well as the rubber grommet that protects the cord going into the metal enclosure.
The small canvas strap on the speaker that attaches it to the epulet (thankyou Bill) is in excellent shape.
If you still want it PM me with you mailing address...you can have it for the cost of postage.
Paul

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Re: Small External Speaker ID?

Post by SGM (ret) » Sun May 28, 2023 10:24 am

Thanks, Paul!

I appreciate the offer... PM sent!

pyropanzer
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Re: Small External Speaker ID?

Post by pyropanzer » Sun May 28, 2023 1:25 pm

The LS-7 speaker works with the SCR300 as well. That's what I've have used.

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Re: Small External Speaker ID?

Post by SGM (ret) » Sun May 28, 2023 2:11 pm

That's very useful info Pyropanzer.

Did you have to make any sort of allowance for the speaker impedance when using the LS-7?

pyropanzer
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Re: Small External Speaker ID?

Post by pyropanzer » Mon May 29, 2023 11:16 am

I never have. Works ok.

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Re: Small External Speaker ID?

Post by SGM (ret) » Mon May 29, 2023 2:12 pm

Thanks again, PyroPanzer.

I may have to give this a try. I do have an LS-3 which has a built-in 8K ohm impedance transformer which is the same as the C-410 that is wired in-line with the HS-30 headset (when used with the SCR-300).

However, the HS-30 receivers (speakers) are 256 ohm vice the 3.2 ohm speaker used in the LS-3. This has left me really scratching my head trying to figure out what is compatible with what.

I have seen a photo where the headset used was the type with twin R-14 receivers (possibly an HS-23) instead of the usual HS-30. The R-14 receiver was a 2K ohm speaker and is the same unit used in the M-356 resonator.

The audio transformer output for the SCR-300 doesn't seem to provide enough power to really "drive" speakers of any significant size. It would seem that some sort of intermediate amplifier would be needed between the radio set and the speaker to provide much in the way of volume.

My study thus far suggests that the problem is when the load impedance (the speaker) is significantly lower than the source impedance (the radio's audio output). If the R-14 can be used directly with the set, then the set's output impedance must be quite high (2K ohm?). If the HS-30's 256 ohm receivers are matched with a 8K ohm impedance transformer, then it would seem that the LS-3's 3.2 ohm speaker even with its 8K ohm transformer is still way low to safely match directly with the radio set. I really just don't now if this is correct or even close, though...

So, this is as far as my limited knowledge takes me right now.

When trying to research the question about how to match the impedance between the SCR-300 and an external speaker, I'm confounded by a lack of information about the impedance and the power of the radio set's audio output. I believe that the audio signal is provided by Transformer T3B (if I read the schematic correctly) but none of the TM parts lists provide any specs for it. It is Signal Corp Stock Number 2Z9639, but I'm stymied when trying to find out more.

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Small External Speaker: Update with Battery Powered AMP

Post by SGM (ret) » Thu Jun 08, 2023 12:03 pm

I wanted to provide a quick update on this topic.

I did manage to locate an M-356 Resonator. (Thanks, Paul, N6FEG!) 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!

AMP Almost Done 006.jpg
Here's an overall view of the speaker-amp system connected to my SCR-300.

AMP Almost Done 005.jpg
Here are the three components of the external speaker "system." Each of the speakers, the M-356 and LS-3, use different plugs, so the BC-606 has jacks for each (PL-68 and PL-54). In this picture, the LS-3 is plugged in to the amp.

AMP Almost Done 004.jpg
I have a nicer, more "military looking" knob on the way for the volume control. I did add the spring loaded jack covers just because I like the way they look. These are not hard to find NOS in both black and OD. They're still made by Switch Craft, I think.


AMP Almost Done 003.jpg
I was tempted to try to move the red LED "power on" light to the case of the BC-606, but decided not to. The amp components are all soldered on the circuit board very closely. I was worried that I'd screw it up if I tried. (Would have been cool, though...)

AMP Almost Done 002.jpg
You can see just how small the little amp is. The volume control was originally soldered onto the circuit board, but here it's been removed and connected with wire leads.

AMP Almost Done 001.jpg
The battery and small amp are contained in small ziploc baggies for right now to serve as insulation and to keep them from rattling around.


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:

https://www.amazon.com/dp/B081YQRDWM?ps ... ct_details

$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.


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