[ARC5] ARC-5 dummy load

[J Mcvey]

"2 Amps through 5 Ohms is 20 watts."
My point was that the meter maxes out at 2 amps whether it has a 5 ohm  OR the 10 ohm  load.!  So something isn't right...
By the amount of heat generated in the 10 ohm load, it seemed that the that it was very near it's  maximum of 50 watts.  

I will try some of the tests in the magazine article to see if this meter responds the same way.

 

    On Sunday, August 21, 2016 1:37 AM, WA5CAB--- via ARC5 <arc5 at mailman.qth.net> wrote:
 

 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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