[time-nuts] OXCO Issues -- Latest

[Don Latham]

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,
no?
can't believe an engineering setup that requires tiny tweaks with
assembly and disassembly. . .
Don

Bob Camp
> Hi
>
> 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.
>
> Bob
>
> 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
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>


-- 
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell


Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com