Hi All,
My Titan 425 has a drift problem with tuning on 160 meters. I have
posted this question before and got some conflicting answers. All seem
to agree that the problem is caused by capacitors in the tank circuit
that change their capacitance as they heat up, and that the problem can
be cured by replacing the large disk ceramic capacitors with bigger,
better doorknob capacitors. Some have said the problem is in the plate
DC blocking capacitors (the ones that couple the RF from the anodes to
the Pi-L tank, while blocking the DC plate supply) C6 & C7. Others have
said that it is either C15 the capacitor that is paralled with the plate
tuning variable capacitors C14 and C28 when on 160 meters (my schematic
leaves out an important line, and shows C14 and C15 connect to nothing
except ground and the band switch), or the problem is caused by one or
more of the capacitors C16, 17, 18, & 19 which are paralleled with the
variable loading capacitor C21. While it is possible that all of these
capacitors may contribute to the problem, I suspect that there are one
or two that have a larger role in this problem than the rest. I wonder
if anyone can tell me which ones? When I posted a question about this
problem many months ago I got referrals to suppliers of the neccessary
capacitors, which I promptly ignored, because I was sure that I would
find my stock of doorknob capacitors and not need to buy any. Having not
found my doorknob capacitor stock, it seems that I will have to buy some
after all. So I need a repeat of that information too.
Thanks in advance for any help,
Ken N6KB
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