[Drake] TR-7 PTO Drift

[Wa1cvn at aol.com]

Thanks Hank for the info and comments.  I did not think of baking the  board, 
but will try it.  The amount of drift on one of the PTO's is 1.2KHz  for the 
first hour, down in frequency, the second is 700Hz.  I have  replaced the 
later PTO in my TR-7(32XX) with a PTO board from a T-4XC(29xxx)  that was struck 
by lighting and got the drift back to the 20 hertz level.   The board in the 
T4XC was the same board as the TR-7 except the compensation  capacitors were 
mounted under the board and were tubular ceramic, whereas,  the TR-7 board had 
disc ceramic capacitors mounted on the board.   Ceramic disc capacitors are not 
very good for temperature compensation, unless  that short coming is what 
Drake counted on; better sensing. I find mica  more stable in high temperature 
environments.  I have set up a test station  from a RV-7 and am starting testing 
the effects of different capacitors versus  temperature.  Wish I had the lab 
at work to do this is would be much  faster.
 
Schematics are hard to find for the PTO, as you stated the service manual  
does not have any info on the PTO except mechanical alignment.  The  RV-7 manual 
is the only place to find them and there are two designs, only  slightly 
different.  The schematics in the T-4XC manual have the starting  capacitors 
values which are not noted in the RV-7 manuals.
 
Thanks again,
Dave
WA1CVN