[Collins] SB-8 for KWM-2

[Dr.Gerald Johnson]

It appears to me from the schematic and the service bulletin that 
C102 is already connected AT XV15-2 along with R95. Then there was 
a bus wire going from XV15-2 to XV16-1. If you disconnect that bus 
wire you leave C102 on XV15-2 (along with R95 and the connection to 
XV15-3) and you ground that bus wire to the center shield of XV16 to 
ground the cathode of the first audio tube. Then you add the 620K 
and the .01 capacitor to put some delay bias on the AVC rectifier 
which is what it was mostly getting from the cathode circuit of the first 
audio tube.

It appears to me that the mod was made because the audio 
voltage on the cathode off the first audio could get back to the 
AVC detector and maybe change that offset a little. Looks 
inconsequential to me. At best I think the old circuit works, but might 
be improved by the addition off the .01 capacitor to knock off high 
frequencies that the 100 mfd didn't especially when the electrolytic 
got old.

Since the 2.2M grid resistor of the audio stage returns to cathode, not 
to ground, the resistor in the cathode circuit of the audio stage 
doesn't affect the tube bias so removing it doesn't affect the tube 
bias. Using the separate 620K resistor adds a little bit of heat to the 
radio, instead of using the cathode current of the audio tube.  And 
added about 27 cents to the cost of building the radio. 2 cents for 
the resistor, 25 cents for labor.

I don't see how the mod could help a lot, but its easy to accomplish 
and other than the heat in the 620K with maybe 275 volts applied. 
0.12 watt, there's no harm.

The schematic often doesn't show the which end of the jumper wire 
the parts were connected to, that would come from the production 
wiring pictures. A schematic diagram doesn't purport to show that 
wiring detail, it just shows how the electrons will flow at low speed. I 
recall having a half day argument with a draftsman over a 
production drawing. For symmetry in a HF tuning discriminator, I 
wanted a bypass capacitor connected to the middle of a jumper 
wire, but he insisted it didn't make any electrical difference. He didn't 
consider the few nh of inductance difference. In this instance in the 
KWM-2, only the placement of components in the chassis matters, 
wire length isn't a consideration, but its likely the production radios all 
connected C102 to XV15-2 so the mod doesn't need to move it.

Like the service bulletin says, trace out the wiring to see if the mods 
have been performed. Its entirely possible that someone has 
replaced C102 and neglected the original placement and wired it to 
XV16-2. Then it will need to be moved, because it won't do the first 
audio stage any good when shorted out by moving the jumper to 
ground and it won't do the AGC offset filtering any good by not being 
there.

73, Jerry, K0CQ, Technical Advisor to the CRA.

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.