[Collins] 75S3C Bfo control

[Dr. Gerald N. Johnson]

I replaced that pot and switch in my new 75S-3B about 53 years ago. I 
didn't like having to lose the tunable BFO setting when changing modes 
and going back to the crystal BFO so I assembled a pot with a push-pull 
switch. From the schematic the switch is single pole double throw. It 
grounds the cathode circuit of the BFO tube to be used. I don't remember 
the pot details and I had full access to all the Collins specifications 
that year because I worked in the same building as the component 
specifications department. I think I assembled shaft, pot, and switch 
using universal parts in the local TV shop in Central City, likely the 
100K is linear because the schematic shows it loaded with a 51K resistor 
connected wiper to end which makes a linear pot logarithmic. With that 
resistor in place with the pot at the center, there will be 50K to the 
high voltage end and 25K to the low voltage end with a linear pot and so 
with something like 48 volts across the pot there will be about 16 volts 
pot wiper to the low voltage end. That 48 volts is a guess and probably 
wrong. The voltage across the pot will vary with the wiper position. 
With the wiper at the low voltage end, there will be three resistances 
in series, 6810, 17.8K and 100K with 56 volts applied. That means a 
current of 0.449 milliamps, so 44.9 volts across the pot. With the wiper 
at the other end the resistances are 6810, 17.8K, and 100K in parallel 
with 51K. The parallel combination is 33.775K so the current is 0.959 
milliamps and the voltage across the pot is 32.4 volts. Presuming the 
pot value is really 100K but it has a +/- 30% resistance tolerance. So 
with the wiper at the middle of the pot, the pot and 51K make a 50K 
inseries with 25K. With the other resistors the circuit current will be 
0.562 ma and the voltage across the pot will be 42 volts so if the pot 
is linear the voltage wiper to low voltage end will be 14 volts. The low 
voltage end of the pot is where the 51K and 17.8 K resistors connect to 
the pot.

I havean't looked lately but I suspect the shaft should be round 1/4" 
diameter for a set screw knob, length easily measured by removing the 
knob which you will have to do to replace the pot anyway. If set screw 
it will be Bristol Spline for the set screw.

The 1975 military style manual available on line might have details on 
the knob. I'm not willing to go dig out my receiver to take off the knob 
and measure the shaft right now. I have other projects I'm working on today.

There was a time that Surplus Sales of Nebraska would have had that pot 
but they don't today. I looked through several bins but I didn't come up 
with one that looked like Collins with a switch other than the typical 
spst. I did find one with four connections on the switch, but it looks 
took cobbled to have been a Collins part. With having collected parts 
for about 60 years and having moved a few times, parts can be hard to 
find around here.

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

On 5/20/2017 10:10 AM, Doug Hensley wrote:
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> Hi Jerry,
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> I'm searching for a replacement BFO control for a 75S3C where the BFO
> switch no longer works. The BFO works but not the switch. I'm having
> difficulty defining all the parameteres that potentiometer sellers want
> to know: Linear vs Audio vs Logarithmic taper, round shaft versus press
> fit versus half shaft, length of shaft, 3/8" vs 7/16" vs 11/32"
> diameter, switch: spst, spdt, dpst, dpdt, etc etc. Do you happen to
> remember ever handling this control and if so what you can remember in
> terms of describing it? I think it is a 100K but that is all I know.
>
>
> Thank you,
>
>
> Doug W5JV
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