[Collins] 20V-2 motor driven rheostat

Fri May 7 03:34:59 EDT 2004

Hi Dave,  I have an RCA BTA-1 & it has a motorized rheostat in it.   It's
used to tweak the power to exactly 1KW or the correct cutback power.  The
RCA uses a 400ohm 225W rheostat in series with the PA B+ right before the
modulation reactor.

Looking at the 20V-2 diagram, it uses series/paralleled fixed resistors and
a tapped resistor (R-166,7 & R-194,5,6 in the B+ to tweak the cutback power
level.

73, Bill   N2BC

----- Original Message ----- 
From: "David Knepper" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 06, 2004 5:58 AM
Subject: [Collins] 20V-2 motor driven rheostat

Could someone please tell me what the purpose is for the motor driven
rheostat that is located on the right rear wall of the cabinet.  I have one
here for my 20V-3 but I believe that Collins deleted this unit since my
schematic/manual makes no mention of it.

Specifically how is the unit connected?  I did connect the wiring from the
front panel lever switch to the unit and applied 120 volts AC.  It does work
but I have no idea where the terminals go on the rheostat itself.

I suspect that it was used to vary the screen voltage on the final amplifier
tubes so as to vary the power output.

Any assistance would be appreciated.

Thank you.
Dave, W3ST
Secretary to the Collins Radio Association
Publisher of the Collins Journal
www.collinsra.com

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