[time-nuts] OXCO Issues -- Latest
djl at montana.com
Sun May 12 18:16:01 EDT 2013
If there's a pot set that's inside the case, indeed inside the "foam"
then there has to be a procedure for setting it before final assembly,
can't believe an engineering setup that requires tiny tweaks with
assembly and disassembly. . .
> 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
> 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
>> 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.
>>> 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
>>> My plan is to check the resistors (surprisingly most are carbon film
>>> except those in the thermistor circuit), look at the circuit board
>>> a magnifier, and stick it back in the foam for testing after making
>>> Thanks again for the continuing comments and advice.
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