[ARC5] MC-385-D

[Dennis Monticelli]

Robert,

Well I got around to measuring my MC-385-C and was initially confused
for a few minutes until I realized the little bugger didn't use an
isolated transformer but rather an autotransformer.  The turns ratio
is 3.33 which equates to 11.1 impedance transformation.  Assuming a
600 ohm headset for a load, 6660 ohms would be presented to the
driving source.  This adaptor would probably do a decent enough job
from 4000 to 8000 ohms or handling two paralled headsets fed from 4000
ohms.  Maybe that was the intention....to be general purpose.  My unit
(made by Rola) simply states it's for "high to low impedance."  BTW,
DC resistance is 80 ohm on the low Z side and 267 ohms on the high Z
side.  Measuring from Tip to Tip gives 187 ohms thus confirming that a
common wire size was used for the windings as one would expect for an
autotransformer.

Dennis AE6C

On 11/26/12, WA5CAB at cs.com <WA5CAB at cs.com> wrote:
> OK.  Of the 8 or 10 that I apparently have, at least 5 are NOSB.
>
> In a message dated 11/26/2012 23:57:54 PM Central Standard Time,
> dennis.monticelli at gmail.com writes:
>> Mine looks nearly new so it was probably still packaged up at the
>> close of WW2 and then sold as surplus (perhaps via Fair Radio).  So it
>> is probably 4000.  If I get the time I will run a simple test on the
>> bench and answer the question once and for all.
>>
>> Dennis
>>
>> On 11/26/12, WA5CAB at cs.com <WA5CAB at cs.com> wrote:
>> >You're welcome.  The only other logical possibility is that it is
>> 8000:600
>> >since all of the common late Pre-War and early War headsets and
>> loudspeakers
>> >
>> >were either 7000 or 8000 ohms.  If I get around to checking one, I'll
>> post
>> >it.
>> >
>> >Unfortunately, I tracked the thing back through SIG 5 to the part number
>> >for the actual transformer and no where does the catalog list impedance
>> or
>> >turns ratio numbers except that in one place it says that the intended
>> load
>> >is
>> >600 ohms.
>> >
>> >In a message dated 11/26/2012 20:42:25 PM Central Standard Time,
>> >dennis.monticelli at gmail.com writes:
>> >>Thanks for the response, Robert.  I will assume it is 4000 ohms.
>> >>
>> >>Dennis AE6C
>> >>
>> >>On 11/26/12, WA5CAB at cs.com <WA5CAB at cs.com> wrote:
>> >>>I doubt it.  First, incrementing the suffix letter didn't always mean
>> >>> a
>> >>>revision.  It could mean a different contractor (and internal parts
>> >>>might
>> >>>not
>> >>>mechanically interchange) or sometimes (though not often) as was
>> >>> common
>> >>with
>> >>>
>> >>>the Navy's suffix numbers, it could mean just another order.  Second,
>> >>> I
>> >>>don't know why they would have needed a 2000 ohm input for.  All of
>> >>> the
>> >>>receivers involved were 4000 ohm output.
>> >>>
>> >>>SIG 5 says that the phone plug was tapered to a slightly smaller
>> >>>diameter
>> >>>compared to the ones on the -A and -B models.  Doesn't say why.
>> >>>
>> >>>In a message dated 11/26/2012 16:23:56 PM Central Standard Time,
>> >>>dennis.monticelli at gmail.com writes:
>> >>>>
>> >>>>I have an MC-385-C and had many years ago marked it as 2000 to 600
>> >>>>ohms.  Is that correct for the "C" model?
>> >>>>
>> >>>>Dennis AE6C
>> >>>>
>> >>>>On 11/26/12, Geoff <geoffrey at jeremy.mv.com> wrote:
>> >>>>>Thanks to all who replied with its specs.
>> >>>>>
>> >>>>>
>> >>>>>Sent: Monday, November 26, 2012 8:28 AM
>> >>>>>Subject: [ARC5] MC-385-D and a BC-221-D Question
>> >>>>>
>> >>>>>
>> >>>>>>According to the "bible" (1973 Fair Radio catalog), the MC-385-D
>> >>adapts
>> >>>>>>4000 ohm to 600 ohm and is a High to Low adapter.
>> >>>>>>
>> >>>>>>This makes perfect sense given the radio output impedances used in
>> >>>>>>WWII,
>> >>>>>>enabling you to plug a 600 ohm headset into your BC-348 without
>> >>>>>>changing
>> >>>>>>the tap on the audio output transformer.
>> >>>>>>
>> >>>>>>I have one, NIB.  And given that Fair was selling them for 50 cents
>> >>>>>>used
>> >>>>>>and 75 cents new, I suspect that a great many were taken apart to
>> >>>>>>enable
>> >>>>>>the phone plugs and jacks to be employed for various nefarious
>> >>>>>>purposes.
>> >>>>>>
>> >>>>>>By the way, I have a BC-221-D, made by Bendix, 1941 contract date,
>> >>>>mounted
>> >>>>>>
>> >>>>>>in a BC-221-AK case.  There is a large ceramic core coil with many
>> >>many
>> >>>>>>turns in 4 windings hanging below the detector tube socket.  It's
>> >>>>>>come
>> >>>>>>disconnected at one end.  And I am not sure it is original, either.
>> >>>>>>Anyone have clue where this thing goes or if it's even supposed to
>> be
>> >>>>>>there? The set looks pretty original, otherwise.
>> >>>>>>
>> >>>>>>Thanks,
>> >>>>>>Wayne
>> >>>>
>
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