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

[n4buq at knology.net]

Hi Rich,

Are those peak-to-peak readings or RMS?  I assume since you are talking 
about 'scope readings, they are peak-to-peak, but not sure.  If so, that's 
about what I've been getting.  I wonder if this is because the COSMOS uses 
a fixed output transformer?  As I mentioned, my Motorola has a 
significantly higher output.

I have a sleeve of NOS 5749 tubes.  I may just try those and see what 
results.

Hey, post more often!  This is very helpful, especially since you obviously 
have "been there, done that"!

Regards,

Barry - N4BUQ


> 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
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