[ARC5] [Milsurplus] SCR-287 (BC-375 & BC-348) WW II Operational Question

Sun Oct 5 00:38:24 EDT 2025

I like this thread.  Here are some additional details regarding using the
CMS on 80M CW.

The transmitter module was removed from the case and used standalone.  I
left it 100% stock.  Receiving was ably handled by a restored BC-348-N.
The manual states that one could use 6L6's at 450V from a line operated
supply or 6V6's at 250V from a battery or hand crank gen.   I used a bench
PS.  First up was the 6L6 and it had keying problems with a crystal.  I
switched to a pair of JAN metal 6V6's and it keyed beautifully with a rock
or as a VFO.  There was a lot of pulling with the VFO and a thermal warmup
period so I elected to go with an FT-243 rock.  Using the lowest Z output
coil tap I was able to get 5W output with a little over 300V applied (max
rating for a metal 6V6).

I will admit that I did make one teensy weensy temporary mod.  I snuck a
thin pair of wires through a factory adjustment hole in the rear.  The
wires were tack soldered to the B+ ON/OFF slide switch on the front panel.
This allowed me to use a foot-switch operated T/R relay for snappy QSO
exchanges.

With my antenna tuned to 50 ohm and measuring 5W, I then swapped the
antenna for the 50 ohm input of my spectrum analyzer.  This is how I
recorded 1.6mW of backwave.  The 200 mi path between my friend and I is a
good one around dawn.  When he copies me 599, the backwave is heard.  It's
not a one time event; it happens month after month. There is a ceramic
neutralization trimmer in the CMS circuit that I left untouched.  Was it
adjusted for 6V6 or 6L6?  I don't know.  I did not readjust it for fear of
breaking that cap.

Note that the telegraph key switches the B+ line.  That is why I chose the
Navy flameproof.  Lots of fun using this little rig.  I welcome comments on
the usage of the CMS set.

Dennis AE6C

On Sat, Oct 4, 2025 at 5:35 PM Hubert Miller <Kargo_cult at msn.com> wrote:

>
>
>
>
> Sent from my Galaxy
>
>
>
> -------- Original message --------
> From: Frank Barnes <fbw4npn at gmail.com>
> Date: 10/4/25 15:03 (GMT-08:00)
> To: Hubert Miller <Kargo_cult at msn.com>
> Cc: Milsurplus at mailman.qth.net, ARC5 <arc5 at mailman.qth.net>
> Subject: Re: [ARC5] [Milsurplus] SCR-287 (BC-375 & BC-348) WW II
> Operational Question
>
> I attach the output of my shop LM-8 (similar to the BC-221) to a random
> wire antenna about 20 feet long, strung along the floor joists in the
> ceiling of my indoors basement shop.  My understanding is that the output
> of these units is something like 10-15 milliwatts.   I receive the signal
> on my shop's Yaesu 840 and if the LM-8 is set to 7.04 Mhz (base frequency
> is 3.52 Mhz), I get receivable harmonics all the way into the 30 Mhz range.
> Some time ago, I keyed the LM-8 antenna while testing another receiver
> tuned to 7.04, and to my surprise, a station came back to me from well out
> of town.  Very surprising. Wonder what would happen if I coupled the LM-8
> to my twinlead-fed 144' Delta "loop"? - but too many harmonics!  Time to
> build an LC circuit to restrict the output to the desired frequency?  Might
> be fun.  QRPP  The only problem will be how to use my electronic keyer to
> send the code by breaking the antenna connection - I can't use my old J-38
> hand key much any more, due to a nerve injury, but I can loaf along with
> the keyer.
>
> On Sat, Oct 4, 2025 at 4:01 AM Hubert Miller <Kargo_cult at msn.com> wrote:
>
>> Your post is interesting but doesn't address my question. I imagine the
>> RF route for osc to antenna in the CMS is via grid-plate capacitance in the
>> 6L6 final
>> and thence to the output circuit. With the LM or BC-221 as transmitter,
>> as Bill related, i tend to think this would only work with a high Z
>> antenna, like 1/2 wave.
>>
>> I also have the CMS, never used it tho. The black plastic sheet for
>> covering the key, to keep hands off, got the sticky disease, so i discarded
>> it. I am thinking
>> i will plug in 1 volt receive tubes in the xmtr; maybe improve the
>> stability that way.
>> I wonder how the regen works with no actual gain control. That circuit
>> design is very old.
>> I suspect the CMS was intended for Philippines. It is a U.S. Navy radio.
>> The Navy also owned the MBM, which seems not to have been the insurgents'
>> answer
>> to prayer either. I further suspect the CMS was just too hobbyist - like
>> for a radio issued to people with only a basic operator level training. My
>> speculations
>> only. The CMS did stay in the Navy catalog for some years after the war,
>> apparently; witness that AC supply for it, which is clearly of a newer
>> generation of
>> electronics.
>> -Hue Miller
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>
>
> --
> Frank Barnes
> www.W4NPN.org <http://www.w4npn.net/>
> Chapel Hill, NC
> Grid Square FM05
> Cell 919.260.7955
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