[ARC5] BC-453 drift test

[Tom Lee]

Thanks for the detailed description of what you did. I am a little 
surprised that the only effect of the original caps was to slow down the 
settling, but that the total drift was about the same before and after. 
I'm trying to come up with a quantitative model of the drift sources. My 
expectation is that LC drift would dominate, but there would certainly 
be secondary contributions from the vacuum tube (and its bias). Without 
knowing anything about the inductor's TC, I would guess that the tank 
would contribute somewhere between 50 and 100 ppm/C drift downward. If I 
take the middle of that range, then the tank's temperature rise would 
have to be about 20 degrees C to fit your observations.

Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)

On 1/10/2018 6:23 PM, J Mcvey wrote:
> No, most of those tank caps are mica. Generally they are usually OK. 
> They don't leak but when they are  bad, they will either be 
> intermittent ( silver migration), or shorted.
> The standard operating procedure is to change all of the paper and 
> electrolytic caps in the "cans" . I cut them open with a pipe cutter, 
> gut the contents and replace with either mylar or ceramics. The 
> electrolytic cans get 105 degree types inside. Then I solder the can 
> back together. looks like it never happened except for the thin band 
> of solder.
> Hot air soldering makes the band almost disappear.
> Those bypass caps that I replaced are ALWAYS leaky and will mess with 
> the tube biases, etc
>
>
> On Wednesday, January 10, 2018 8:59 PM, Tom Lee 
> <tomlee at ee.stanford.edu> wrote:
>
>
> Yes, WSPR is definitely more challenging!
>
> The capacitors you changed -- I assume that a couple were part of the 
> LO tank (say, C-8 and C-10 in the schematics I have). Is that right, 
> or did changing others reduce the drift? I'm curious...
>
> Tom
> -- 
> Prof. Thomas H. Lee
> Allen Bldg., CIS-205
> 420 Via Palou Mall
> Stanford University
> Stanford, CA 94305-4070
> http://www-smirc.stanford.edu <http://www-smirc.stanford.edu/>
> 650-725-3383 (public fax; no confidential information, please)
> On 1/10/2018 5:24 PM, J Mcvey wrote:
>> The test was done at a more or less arbitrary frequency. The LO 
>> started at 540Khz and ended up at 539.300Khz give or take a few hz.
>> So, yes that would be about 0.13%.  However 700Hz and 1Hz/second  is 
>> a big deal with WSPR! Once it settles down, I can decode WSPR, though.
>> The radio stabilizes faster now that the caps were all replaced. It 
>> used to take twice as long to get there. Going to give a try again 
>> tonight.
>>
>>
>>
>>
>> On Wednesday, January 10, 2018 7:48 PM, Tom Lee 
>> <tomlee at ee.stanford.edu> <mailto:tomlee at ee.stanford.edu> wrote:
>>
>>
>> Without knowing the LO frequency it's hard to give a precise answer, 
>> but that warmup drift represents roughly a couple tenths of a percent 
>> at midband. That sounds reasonable, as does the warmed-up drift. The 
>> dropping in frequency during warmup is consistent with the inductor 
>> and capacitor expanding as it heats up (L and C both increase under a 
>> linear expansion), so it makes sense.
>> -- 
>> Prof. Thomas H. Lee
>> Allen Bldg., CIS-205
>> 420 Via Palou Mall
>> Stanford University
>> Stanford, CA 94305-4070
>> http://www-smirc.stanford.edu <http://www-smirc.stanford.edu/>
>> 650-725-3383 (public fax; no confidential information, please)
>> On 1/10/2018 3:45 PM, J Mcvey via ARC5 wrote:
>>> I recapped this radio and tested the LO drift afterwards.
>>> Coupling the plate via 7pf cap and a x10 scope probe to a frequency 
>>> counter, the results were as follows:
>>> The LO dropped in frequency at a rate of about 1 HZ / second upon 
>>> power up.
>>> The drop rate slowed until it more or less stabilized an hour later 
>>> 700 Hz lower than the start up
>>> measurement.
>>> Once at the bottom of the descent, it would sit and go up and down a 
>>> couple of Hz in a very slow undulation.
>>>
>>> I guess this a normal warmup scenario? What say you?
>>>
>>>
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>
>
>

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