[R-390] More PTO stuff

[n4buq at knology.net]

Miles,

Thanks for the warnings.  I was thinking of attempting to replace only the 
15pF coupling capacitor.  That one may not be as temperature sensitive as 
the others in the "tank".  Finding temperature compensating caps that are 
identical to the originals may be almost impossible to find.  I see where 
the capacitance tolerances are 1% or 2% on the non- bypass capacitors in 
the PTO, especially the ones in the can.  I don't see a temperature 
compensation designation so I'm not sure what I'd get to replace them.  
NPO, perhaps?

I did some further checking last night.  With the PTO oscillating, the 
control grid is seeing about 1.4VAC (as checked with my ME26D/U).  When I 
connect the signal generator's 2VAC output (measures about 1.8V RMS on the 
ME26) to the control grid, the PTO output goes to nearly 8V peak-to-peak 
(as seen on the 'scope).  It appears a very small increase in grid signal 
causes a vast difference in output voltage.

HOWEVER, I'm not sure what effect connecting the signal generator directly 
to the grid does to the bias point.  This may account for the increase in 
signal more than anything.

Arghh?

Barry - N4BUQ

On Thu, 11 May 2006 08:50:32 -0400, Miles Anderson <k2cby at optonline.net> 
wrote :

> Barry,
> 
> You are borrowing a lot of trouble!
> 
> (1) The values of the capacitors inside the PTO can are absolutely 
critical
> in setting the linearity of the PTO tuning. If you don't get the total
> capacitance exactly  right, you may be able to align both ends, but the
> errors will be concentrated in the middle. Thus, at the "500" point you 
are
> either going to have a huge amount of positive error or negative error
> (depending on whether you have too much total capacitance or too little) 
and
> this is going to be beyond the ability of the screws in the Cosmos pto to
> compensate. It's 100 times more of a problem with a Collins "stack
> compensator" type of PTO.
> 
> (2) All, repeat all, the capacitors inside the PTO can are temperature
> compensating. If you don't get the right delta C/delta T the PTO is going 
to
> drift on warm-up.
> 
> (3) Lead lengths and component dress are critical. They affect the total
> capacitance. Also, they affect the total series inductance, and the slug
> tuned compensating coil is tiny. Too much lead length, too much inductance
> and you are going to have to unwind turns on the little compensating coil.
> 
> Bottom line:  Change components inside the PTO can only when the tuning is
> non-linear. I really don't think any of these components are going to 
affect
> the output so long as the PTO is actually oscillating, which yours is.
> 
> Miles, K2CBY
> 
> 
> 
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
> 
> 
>