[R-390] Re: R-390 Digest, Vol 25, Issue 6

[SHELLY199 at aol.com]

Hi Barry,
 
Pse note this is my first post although I've been reading the mail for 5  yrs.
 
The typical output voltage from a Cosmos PTO fed into the rcvr thru normal  
connections and taken at the jack on the RF deck thru a X10 (10 meg impedance)  
is about 2.6 Vac.
 
I have taken the following readings on 4 ea. Cosmos PTO's as follows:
 
1.   2455   2.49 Vac
      3455   2.59 Vac
 
2.   2455   2.78 Vac
      3455   2.76 Vac
 
3.   2455   2.72 Vac
      3455   2.75 Vac
 
4.   2455   2.56  Vac
      3455   2.48  vac
 
I adjust the output cap C706 for max output at 3455.  This gives  the 
flattest response.
 
The most significant item regarding output volts is the tube.  I have  used 
only tubes which have tested good with highest emission out of a  bunch.  I 
then put the best ones one at a time in the PTO and  select the one which 
delivers the highest output.  This can be a ratio of  2:1 for output voltage.  It's 
surprising that it changes so  drastically from tube to tube.
 
I've been rebuilding these things for 5 years and have test cables,  ovens, 
cal dial setups, and you name it.  I've never seen a defective part  in any PTO 
except R702 which is typically high and once in a while R702.   Although I've 
changed many in a shotgun attempt to achieve super  stability which you can 
expect if everything is just right.  I've  rebuilt the 5ea. .005 bypass cap and 
found zero difference.  I've almost  given up on super stability on one and 
decided to clean up the tube socket a  second time and that was the problem.  
The 10 pf caps in the osc tank  circuit often need changed because they've 
changed value and the endpoint won't  adjust without adjusting the turns on L701 
or changing the caps.  I prefer  changing the caps.  Also the 10 pf caps 
usually don't have the original  temp compensation so we change them on that basis.  
The 370 pf caps seem  pretty good and I've never found one of spec from a 
capacitance  standpoint.    
 
Hope this helped.
 
Rich WD2Q