[Collins] KWM-2 tube socket voltage and resistance charts

[Dr. Gerald N. Johnson]

I welcome those charts. I wish there had been more care taken in the 
Collins manuals. When I rewrote much of the text for the October 1964 
edition of the 75S-3B/C I left it to the tech writing department to see 
that the V/R charts, parts list, and schematic were up to date. Turned 
out when I went through all editions I could get to to compare 6AQ5 vs 
6BF5 audio output stages there were serious differences between the 
three. That the power transformer voltage updates over the history of 
the receivers was not always matched in the voltage chart or the 
voltages on the schematic. The voltage/R chart for the early 32S-1 shows 
serious differences between the two 6146 sockets that really reduces my 
trust of the charts. Some differences are obviously typos. As if they 
were NOT proofed by knowledgeable eyes.

It would be a great service if someone would take several copies of 
working radios and actually measure voltages and resistances. The carbon 
comp resistors will have drifted generally higher in value adding 
confusion to the measurements, lowering voltages (especially screen) 
while raising resistance measurements. It would be prime quality if the 
drift of the resistors were individually confirmed as much as possible.

Resistance values from plates and screens depend on ohmmeter polarity 
and the quality of the B+ filter capacitors when the negative probe of 
the ohmmeter is grounded. Indeed the resistance seen through the 
electrolytics can vary according to applied voltage. If reversed the 
circuit can include the transformer secondary through the rectifier (if 
the power supply is separate and connected). It would be a useful test 
for later trouble shooting to either short the power supply for the 
plate and screen resistance measurements or to make those measurements 
from tube plate and screen back to the rectifier cathode or some + 
supply voltage test point. When not powered those lines supplied only 
when transmitting aren't going to be connected to the power connector. 
Likewise determining some of these resistances by circuit analysis gets 
messy because of circuit complexity. Yet these resistances can seriously 
affect radio performance, especially increased value screen resistors 
overheated by leaky screen bypass capacitors.

I've not looked under the chassis of a KWM-2 but I can imagine it being 
tightly packed. I suspect many a turret was wired before it was bolted 
in place. It may take a hooked test prod to reach those buried most deeply.

I think the original voltage readings were made with an HP410B with 100 
meg or so input R on DC volts so using a Heathkit with only 11 meg input 
Z or a Simpson 260 at 20K ohms per volt will make some voltages read 
low. The precision of the 5 or 10% resistors probably didn't justify 
measuring resistances with a laboratory digital bridge, and the analog 
meter was adequate. The known aging and thermal drift in the carbon comp 
resistors means the precision is less today. But a digital resistance 
meter may be a benefit in making a new chart today.

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

On 3/10/2012 10:34 PM, Glen Zook wrote:
>
>
> I have been "pulling my hair out" working on a particular KWM-2A.  The radio is in excellent physical condition but has some very serious electrical problems.  Of course, it is necessary to measure the resistances and voltages at all of the tube sockets.  However, it is a REAL "pain in the posterior" to go to the resistance / voltage chart in the manual, then to the diagram showing the tube socket locations, and finally digging through all of the layers of wiring that is present in the KWM-2- series.  After making the measurement on a certain tube socket pin, then going back through the charts to get the next value and then relocating the proper tube socket through the layers of wires gets very time consuming to say the least.  Anyone who has worked on a KWM-2- series radio know of which I speak.
>
> Finally, this evening, I decided to make some single page charts showing the tube socket locations and the resistance and voltage measurements at each socket.  Basically, I scanned the under chassis locator diagram and then eliminated all of the components except the tube sockets and a few other components which are very prominent and help locate any particular tube socket immediately.
>
> Someone else may have done this.  However, I can't find any documentation.  So, I took the information from the voltage / resistance chart from a fairly early KWM-2 manual and a fairly late KWM-2 manual and made labels for each tube socket.  Those labels were then put on the revised locator diagram with lines drawn to the tube socket to which the information belongs.  There are a total of 4 charts:  1 for early resistance measurements, 1 for early voltage measurements, 1 for late resistance measurements, and 1 for late voltage measurements.  In certain circuit locations there are significant differences in the value of the voltage and in the value of the resistance.
>
> Of course, any particular KWM-2 unit may differ from a certain value for several reasons including factory production changes and in which Service Bulletin was installed.  But, either the early version or the late version chart should be enough to get someone started in repairing their KWM-2 transceiver.
>
> The charts are posted on my website at the following URL:
>
> http://k9sth.com/uploads/tube_voltage_resistance_charts.pdf
>
>
> The charts can be downloaded and printed to be used at the repair bench.
>
> Glen, K9STH
>
>
> Website: http://k9sth.com
>