KWM-2 VFO Zener mod #11B
This mod is described in the instruction book for the KWM2 and 2A dated 9th edition, 15 January 1978. The back of this manual has a list of SB's and pages of "Illustrations" of Schematic Changes and Equipment Differences. The Zener mod is #11B. It is was meant for equipment which is used on battery operation of 12 and 24 volts...but of course is necessary for all of us in these days of "being on frequency".
The original mod is as follows:
"Deleted 33-K resistor R131. Added 0.1-uF capacitor C278, 0.02-uF capacitor
C279; 2-mH coil L42; and 1N5383B zener diode CR12. This change improves
frequency stability for +12, +24, and +28-V dc remote or vehicular
operation."
Read the physical description by Dutch below. I think I used an NTE 5161A
zener diode.
----- Original Message -----
From: "Dutch WB7DYW" <>
To: "Collins@qth. net" <>; "Collins List Server"
<>
Sent: Friday, August 10, 2001 5:36 AM
Subject: {Collins} KWM-2 TX/RX off frequency mod.
Well Collins Gang, I just finished installing the Zener
diode mod in my old KWM-2 to get it to transmit & receive on the same frequency and what a difference WOW. It now has virtually no drift and is a great improvement, as it now not only stays on frequency but will RX & TX on the same frequency.
The mod is simple it requires a 22K/2W, 470 ohm 1/2 watt
resistor and a 150 volt zener diode. The following is thanks to John K0LFA.
PTO B+ is set by R73, 15K/2W, & R13, 33K/2W, from pin 2 of
the external power jack to ground. Look on turret E60 to locate.
Remove both. Replace R73 with a 22K/2W. Good place to use metal
oxide.
Replace R131 with a 150 volt 1 watt zener, NTE 5100A.
From the junction of the diode / resistor install a 470 ohm / 1/2
watt in series to the PTO.
The PTO drain is about 3 ma. The drain through the zener is 3 ma.
During warm up the zener goes up to 6 ma for a few seconds until the PTO starts drawing current.
The 470-ohm isolates the PTO from the impedance shifts of the zener
From rx to tx. The 275-volt line actually charges about 30 volts. When this happens, the PTO will still shift unless isolated from the
zener by the 470-ohm resistor.