[time-nuts] Oscillator temperature compensation

Richard (Rick) Karlquist richard at karlquist.com
Fri Jun 21 04:26:02 UTC 2013


1.  Is the 200 Hz drift measured at 660 kHz or at the "final frequency",
perhaps 1.8 MHz?  Even in the latter case, we are talking about 100 ppm.

2.  Was the rig stable before and something changed?  You didn't say.

3.  Have you verified that the drift is not simply due to the crystal
itself?  A 660 kHz crystal might not be AT cut, due to the low 
frequency.  It is easier to believe a crystal drifted 100 ppm than
an oscillator accidentally pulled it 100 ppm.l

Rick N6RK

On 6/20/2013 9:09 PM, Joseph Gray wrote:
> This is not strictly Time Nuts, but it is about a crystal oscillator that
> needs some temperature compensation, or something. Please ignore this
> posting if you like. In fact, so as not to clutter up the list, it would be
> best to email me directly about this.
>
> I have an old AM transmitter that has three 6AL11 compactrons. The crystal
> is a fundamental, cut for 660 KHz. I don't have a schematic for this thing,
> but I believe that one half of one 6AL11 is used for the oscillator.
>
> The problem is, the frequency decreases as the rig warms up. It will
> eventually stabilize, but the final frequency is over 200 Hz low. Not as
> good as it should be. I think the original specification was well under
> half of that.
>
> I have replaced the electrolytic caps. The others are mostly silver/mica
> with a few ceramics. I checked all of the resistors and only found one that
> was out of tolerance (I replaced it).Three NOS tubes were installed. There
> are no tunable components in the oscillator section. Only the output
> section has anything tunable.
>
> I know that there are many Amateurs on the list and I'm sure many know more
> about old tube rigs than I do. Does anyone have a suggestion as to what the
> trouble might be?
>
> Joe Gray
> W5JG
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