[Collins] 312B-4/5 wattmeter coupler query

[Dr. Gerald N. Johnson]

On Fri, 2008-08-01 at 09:42 -0500, mmaseda at gte.net wrote:
> Sounds like it might be the 1000 watt version coupler.

Might be, but the high power range resistors are in the range of values
used for both the 312B-4 and 302C-3, manuals that I downloaded today to
check. The resistors for the low range are a little low in value. Maybe
some project had a 200 microamp meter with a little more resistance. Or
it truly was set up for a 200 microamp meter at 100 and 1000 watts full
scale in some application. The diode variation is too great to be sure
just from the resistor values.

Earlier I said I though those resistors might vary 20%, actually there
is almost a 2:1 resistance range selected from for calibration. For the
312B-4 coupler the high range resistors are to be selected from the
range 5100 ohms to 10K. The low range from 787 to 1780 ohms. The coupler
assembly is CPN 543-8118-004. In the 302C-3 manual that coupler
assembly has the same part number and the resistors listed range from
5360 to 11.5 K for the high range and 953 through 2490 ohms for the low
range. And that parts list messes up on the 10 ohm resistors labeling
them 3190 ohm in the parts list. The 312B4 manual gets the description
correct and both show the same part number. Unfortunately neither book
gives the meter sensitivity or resistance and the two use a different
part number meter. The schematic in the yellow book shows a 290 microamp
meter but I've been assured by those owning such that the meter is
actually 200 microamps. The draftsman slipped his lettering template
too far.

The only way to be sure is to set up a known wattmeter, this wattmeter,
a transmitter (and maybe PA), and a dummy load and compare readings. A
wattmeter with a built in power meter (probably a voltmeter calibrated
in watts at 50 ohms) would work as well.
> 
> >From: Yong Surk Lee <yonglee at yonsei.ac.kr>
> >Date: 2008/07/31 Thu PM 11:51:45 EDT
> >To: Collins at mailman.qth.net
> >Subject: [Collins] 312B-4/5 wattmeter coupler query
> 
> >Hello;
> >    I own a new coupler(rf pick up for wattmeter) which I am planning to use in a 312B-4/5.
> >This coupler looks exactly the same as the one in 312B-4/5 but internal component values are different.
> > This one has 
> >R3=  6810 
> >R4 = 7320
> >R5 = 619
> >R6 = 681 ohms which are different from the old one in the 312B-4/5.
> >Can this coupler be used with 312B-4/5? 
> >Or was this coupler made for different equipment?
> >

Now I ask the question, what is the coupler assembly part number? If 
its 543-8118-004, then it is the same coupler. Just that it required different
calibrating resistors to adjust for diode variations.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer