[Collins] KWM-2 S-Meter Sensitivity

[Dr. Gerald N. Johnson]

Check the value of R147, in my old schematic listed as 82K. These carbon 
comp resistors tend to drift high with age and heat. Also check R79. 
Same reason. Now realize that the S-meter operation has virtually 
nothing to do with the IF stage the meter is connected to. That the 
stage is doing two jobs, one of IF gain the other as a simple DC 
voltmeter displaying AVC voltage as signal strength. Anything that 
limits the negative going AVC voltage (also the RF gain control is 
applied as a negative bias to the AVC line) will limit the meter going 
up scale. Besides the resistors in the bridge circuit around the screen, 
a tube with low emission, and any AVC bypass capacitors with leakage 
will limit the negative voltage range of the AVC line. Not to neglect 
the resistors around the RF gain control and the -70 volt bias line 
coming from the new power supply. If any of the AVC time constant and 
bypass capacitors are black beauties they ARE leaking and affecting the 
very high impedance AVC circuit. If there is a tube controlled by the 
AVC that has grid emission it counters the AVC voltage limiting its 
negative excursion. A simple test for grid emission might be to ground 
the wiper of the RF gain control and with no signal to look for a 
positive voltage on the AVC line. A fancier test is to test the AVC 
controlled tubes with a grid emission tube tester. A leakage test is not 
sensitive to grid emission in either an emission or tranconductance tube 
tester. In my receiver an IF tube with grid emission pushes the S meter 
negative as the tube heats up. E.g. the zero drifts. Its nearly aways a 
6BA6 IF because they are run at high current for more gain.

Also check the IF/smeter screen bypass capacitor for leakage.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 2/11/2012 1:44 PM, Byron Tatum wrote:
> OK-
>      Thanks for reply Dave. Yes, I did notice that R-158 is chosen in
> production for correct S-Meter response. I researched R-158 some in regards to
> the Collins Service Bulletins and comments on the email reflectors.
>      My KWM-2 is a RE unit that as best as I can tell was made somewhere around
> 1967-68, based on D/C's of crystals and electrolytic caps. The 2 mentioned
> resistors in Service Bulletin at turret E-40 were already the new recommended
> values of 68K and with R-158 at 220 ohms
>      I found the procedure of finding the correct value for R-158, whereby a
> resistance decade box is placed across R-158 to get the S-Meter to read S-9
> approx with 100 uV signal, with the receiver gain adj pot set for AVC threshold.
>      I found that decreasing the value of R-158 (from 220 ohms) worsens my
> problem. Right now I have a 470 ohm resistor in for R-158 and my situation is
> better, but I still cannot peg the S-meter (with RF Gain backed off all the way)
> at the desired lower setting of receiver gain adj pot. With R-158 at 470 ohms I
> can peg the S-meter  (with RF Gain backed off all the way) at a lower setting of
> the receiver gain adj pot, however I still need to be able to reduce the
> receiver gain adj pot some more. I guess I will continue to increase the value
> of R-158.
>      My concern is that I am adjusting the correct part for my problem, from all
> indications it appears R-158 is what I need to manipulate but I just wanted to
> ask others to be certain.
>                                          Thanks, Byron WA5THJ
> ______________________________________________________________
>