[ARC5] ARC-5 dummy load

[WA5CAB at cs.com]

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

In a message dated 08/20/2016 18:45:32 PM Central Daylight Time, 
arc5 at mailman.qth.net writes: 
> Found a bag 100 ohm 5 watt wire wound resistors the other day. I didn't  
> what to do with them, so I tried making a lo-z dummy load for the
> ARC-5 /SCR274 equipment. 
> 
> 10 of these in parallel is 10 ohms, 50 watts. You say "hey those are 
> inductive WW resistors!" . Yes but the inductance is greatly reduced in
> parallel.  In fact at 3.5 mhz it was 5+j17 which is more than swamped by 
> the series 100 pf transmit cap (-j454).
> 
> 
> The TX units are supposed  load into 5 to 12 ohms with high capacitance, 
> so it's in the sweet spot.
> The resistors got warmer than I thought they would, although the meter 
> said it was pushing 2 amps. That's only 40 watts.
>  There is some serious reactive voltage at the cap. I had the cap barrel 
> laying too close to metal and I saw some blue corona discharge.
> 
> 
> 
> I'm a bit suspicious of this BC442 meter which never seems to budge above 
> 2 amps. I get the same max current with a 5 ohm load!!
> Are they known to be inaccurate?
> 
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