[Milsurplus] Intercom amp (AM-26/AIC)

[Mike Hanz]

  On 9/12/2010 11:28 PM, Mike Morrow wrote:
> I don't see any reason to operate vintage equipment in any configuration
> other than original today, when orders of magnitude better performance may be
> had for very little using modern solid-state gear.  There is value to operating
> vintage equipment in original (totally original) configuration for the historical
> perspective today, but when ham enhancements and design "improvements" are added,
> that value is completely lost and the effort becomes pointless.
> Post-war hams (and a few today) seem to have been convinced that they were much
> more intelligent than the original designers of the surplus equipment.

Most of the folks on this list, at least the old timers, have staked out 
well-worn positions about modification or "original use", but I guess 
I'm somewhat flexible about the issue.  Use in the ham shack is a bit 
different than it is in a WWII bomber, and time has made some 
unauthorized changes in what were once Dave's "Mona Lisas" that make it 
necessary to accept some changes if we are to continue to operate them.

My "thing" has always been the systems integration viewpoint, so one of 
my fascinations has been putting together a complete set that has *not 
only* the piece parts of the series nomenclature, but also extensive 
details like UV and/or incandescent cockpit lights to illuminate the 
dials, cabin lights above the position, microphone switch on the deck, 
the interphone system (including jack boxes) that was used with the set, 
and even the associated ash receiver that was attached to or near the 
operating position. :-) The changes forced by Father Time are 
progressive, forcing me (for example) to follow Dave Stinson's lead and 
find dynamotors that are lower and lower in output voltage to reduce the 
stress on old capacitors (I am using AN/AIC-10 dynos on the ARC-5 set 
now - 160v output in the same DM-32 frame, similar to the RAX-1 receiver 
B+).  That in turn reduces the power output of the audio stage of the 
set, but since there is an associated (and appropriate) interphone 
amplifier that is switched into the set's output, it really doesn't 
matter.  Note that doing this doesn't require any modification of either 
an unobtanium receiver in the set, nor even the interphone amplifier.  
It's all about the cabling and switching, which is certainly not 
original.  I may not be as intelligent as the original designers, but I 
know a needed improvement for the operating environment when I see it, 
and don't consider those sorts of excursions to be pointless.

Another example of time's progression is the gradual cementing of carbon 
granules in our beloved T-17 and RS-38 microphones.  Faced with a low 
modulation problem due to this effect in KB4DMF's beautiful SCR-274N set 
that he restored for the Collins memorial event 
(http://www.collinsclubs.com/carc/b-29/B-29_Gallery.html ), I suggested 
a tiny change in the RC-36 interphone system he had included in the 
display, one that required a reversible modification in the wiring of 
the BC-366 jack box.  (See bottom of 
http://aafradio.org/docs/RC-36.html  )  Voila - microphone goes through 
the interphone amplifier before it reaches the modulator, and he had 
gobs of modulation from the old T-17 he was using to test the system.  
Yes, I know...there are ways to fix the microphones, but this is easier 
and transparent to the operator.

I am unrepentant in making small changes like these.  There are pieces 
of this gear, like a BC-366 jack box, that will never become an 
important piece of history with its own softly lit display case - at 
least IMO - and changing a wire or two inside doesn't bother me one iota 
if it helps to make operation of the priceless sets more effective and 
enjoyable.

Oh...and no Mona Lisas were injured or inconvenienced in the making of 
this e-mail. :-)

Best 73,
Mike  KC4TOS