[time-nuts] OXCO Issues -- Latest

Bob Camp lists at rtty.us
Sun May 12 16:30:06 EDT 2013


Several outfits made pot set OCXO's for a *long* time. As far as I know there isn't any sort of premature end of life situation with those designs.


On May 12, 2013, at 3:40 PM, "Al Wolfe" <alw.k9si at gmail.com> wrote:

> Years ago we were taught that it was poor engineering practice to use pots to trim a DC value, especially if any appreciable current was to be drawn from the wiper. (Probably true for any kind of signal on a pot) It seems that current through the wiper would eventually erode away the material under the wiper. This is true especially for set-and-forget applications. If the wiper is used only as a voltage reference and very little current drawn then it is probably OK for most situations.
> The above info is mainly for carbon and ceramic pots. Wire wound ones tend to be somewhat more forgiving but they tend to be step-variable between the wire turns. Often the sweet spot is between the steps and they aren't very useful for fine tuning purposes. Sometimes we would use two pots in series. One was set up as a rheostat and was around 5 percent of the ohmage of the main one. It was used for fine tuning but the problem of DC current through the wiper remains and is exacerbated.
> Probably the best design is to use fixed resistors for most of the voltage divider circuit and the variable element be a small fraction of the total divider resistance.
> Al
>> Well, I decided to make the "Y" incision and pulled the circuit board
>> out of the oven.  (Photos later after I crop them.)
>> Yes, the trimpot is part of the circuit associated with the thermistor.
>> My plan is to check the resistors (surprisingly most are carbon film
>> except those in the thermistor circuit), look at the circuit board under
>> a magnifier, and stick it back in the foam for testing after making any
>> repairs.
>> Thanks again for the continuing comments and advice.
>> Fred
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