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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