[TenTec] Titan tuning drift on 160 meters

[Ken Brown]

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