Re: {Collins} Directional Coupler Diodes



Don, I am sending you a copy of the Q2 05 Signal Magazine that had a great
article By Rod Blocksome of Collins Radio that explains how to calibrate
that wattmeter. It will be a great help. It will come to you direct so the
attachment will survive, but the listserver will strip it off the copy to
the members. For those of you that need it, it can be downloaded for free
from our website.


Bill Carns, N7OTQ (Trustee K0CXX)
Past President, Collins Collectors Association
Founding Board, Collins Radio Heritage Group
Editor, Signal Magazine
Wimberley, TX
512 618 2762  (Cell)
512 847 7010  (Home)



-----Original Message-----
From: Collins [mailto:collins-bounces+wcarns=austin.rr.com@xxxxxxxxxxxxx] On
Behalf Of donroden@xxxxxxxxxx
Sent: Thursday, April 30, 2015 12:48 PM
To: collins@xxxxxxxxxxxxx
Cc: collins@xxxxxxxxxxxxx
Subject: {Collins} Directional Coupler Diodes

I noticed a drop-off in FWD power using my 312B-5 console.

100 watts into the 312B-5 results in a 80 watt reading.
reversing the coupler results in a 100 watt reading.
Both confirmed by Bird 43 and Vectronics watt meters.

The manual shows that the resistors to select 200 / 2000 watts are  
factory chosen during assembly.   9K+- for 2000   900+- for 200 on my  
312B-5.

Both FWD and REF resistors are very close to each other.

There was a difference in forward conduction of the 1N82A diodes between FWD
and REF sides.  ( lower voltage drop on the REF side ) ( expected ).

I purchased a 312B-4 for my 32S-3 and noticed the same thing on it...
A difference between FWD and REF .    Yes, I cleaned and ohmed the switch
and checked the meter movement calibration.

I replaced both 1N82A s  with low conduction / high back resistance 1N5711s.

Great !!!  Now the coupler reads 80 watts both FD and REF.

( confirming that the 200 / 2000 range resistors were once matched to a
batch of 1N82a diodes. )

With that background...........

What is the next logical step ?

I'm considering adding 10K and 1K multi-turn potentiometers and removing the
fixed resistors and calibrating the meters at 100 watts.

Once it's calibrated, it won't ever be seen again until the next owner
decides to open the coupler.

Is there a lower fwd drop diode than the 1N5711 ?
Something closer to the 1N82a ?


Don W4DNR










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