[Collins] KWM-2A output

[Dr.Gerald Johnson]

>
>Good points Glen.
>
>And do note that old carbon resistors tend to migrate up in value.  I've 
>found several over the years and a few in my 32S-1 and 75S-3B that were 
more 
>than 30% high.
>
>73
>Bob, K4TAX

Only 30%??? The worst I've seen was about 100% rise in resistance. It was 
the timing resistor in a horizontal multivibrator in a Zenith TV. The oscillator 
was running at half frequency which lowered the high voltage so that only 
half the picture showed on each side of that sync bar down the middle 
of the picture. I had to borrow a scope (didn't have one that worked 
that year) to detect two sync input pulses for each oscillator output. That 
was some time ago, about 1959 and the Zenith TV had a metal picture 
tube. It was older than an antique.

Carbon composition resistors (except those made by IRC which were not 
acceptable to the Collins carbon resistor specification in the 50s and 60s) 
go up with heat, moisture, and age. Sometimes they go back down when 
cooled or dried, but rarely to the same value they had before the 
environmental cycling.

IRC supposedly carbon composition resistors were not really carbon 
composition, but carbon film on a quartz or glass tube with a case 
molded over. The have a few distinguishing features. In later years the 
molded case was greenish in color. The ends were domed, and there's 
generally a mold line visible on both ends and running the lengths of the 
sides. They tend to go down in value when heated and when run a long 
time fairly hot. That seems to be because the heat is concentrated in the 
film that chars the adjacent molded insulator and so makes it into 
carbon. Late in life IRC resistors may be carbon composition, but they 
weren't made that way. IRC also aren't as dependable about burning 
open as true carbon composition resistors when used as fuses and have 
different RF characteristics. I've not taken enough apart to see if the 
resistance element on the insulating tube have a helix or not.

73, Jerry, K0CQ, Technical Advisor to the CRA

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