[ARC5] ARC-5 dummy load

[seanbart at fidnet.com]

I posted this a few months ago but there is an article in the April 1953 issue of Radio Electronics magazine with a chart that corresponds the meter reading to RF current for the BC-442.  This is available online at:
 
[ http://www.americanradiohistory.com/Archive-Radio-Electronics/50s/1953/Radio-Electronics-1953-04.pdf ]( http://www.americanradiohistory.com/Archive-Radio-Electronics/50s/1953/Radio-Electronics-1953-04.pdf )
 
Magazine pages 80 and 82, pdf pages 48 and 49.
 
Sean
KB0OVD
 
 
-----Original Message-----
From: "WA5CAB--- via ARC5" <arc5 at mailman.qth.net>
Sent: Sunday, August 21, 2016 12:36am
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] ARC-5 dummy load


2 Amps through 5 Ohms is 20 watts.

 I haven't looked at a BC-442 or RE-2 in several years but thought that I remembered that the meter in them is not calibrated.  So I went and looked in the parts lists in the SCR-274-N and AN/ARC-5 maintenance manuals.  The original meter has an arbitrary 0-10 scale.  The parts list does not give enough information from which to calculate the actual full scale reading.  The meter full scale sensitivity is 19.5 mVDC.  The thermocouple output (unloaded) is 19.5 mVDC with 0.75 A through the primary side.  However, there is a current transformer whose primary is connected between the T and A push-posts and whose secondary is connected to the thermocouple primary.  The effective turns ratio at 3500 to 4000 KC isn't given.  So from all of the info in the parts lists, you cannot calculate the full scale sensitivity of the assembly.

 Your best bet is to get another 0-2 to 0-5 RFA and connect it in series with the meter and dummy load bank.

 In the parts list, the meter is described as of the "expanded scale" type.  I'm not certain what that means but it may mean that the meter has a suppressed zero.  Meaning that it takes some current flow before the needle ever moves off of zero.

 Someone else mentioned that you could test the meter at 60 cps.  With most thermocouple type RFA's, this is true.  But because of the current transformer in the antenna relay unit, you won't get any reading.  So another direct reading RFA in series is the only reasonable solution.

 FWIW, the reason for the current transformer (the COL-52286 and T-47/ART-13 also have one, for the same reason) is that if the thermocouple primary happens to fail open, the transmitter will not be affected.

 Robert Downs - Houston
 wa5cab dot com (Web Store)
 MVPA 9480

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