[R-390] R: PTO learnings and sensitivity un-mod

[fdigio1 at gmail.com]

> As far as endpoint drift, this seems to be related to mechanical stress on
the Oldham coupling.  As far as I can speculate, the two ends and the center
plate shouldn't be tightly sandwiched together.

They should have a clearance of 1/32".
Regards
Francesco Di Giovanni - IN3XZP

-----Messaggio originale-----
Da: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> Per conto
di jgedde at optonline.net
Inviato: martedì 30 aprile 2019 00:16
A: 'R-390 Forum' <r-390 at mailman.qth.net>
Oggetto: [R-390] PTO learnings and sensitivity un-mod

I've been tracking down a sensitivity issue with my R390A.  I found a few
bad 2.2K resistors which I changed, but sensitivity still wasn't up to the
gain stage charts in the TM.  Slowly I came to the conclusion that my PTO
was feeble (apart from having endpoint drift).  This is where it gets good.
The PTO was indeed weak and putting in a new tube just made it worse.  So, I
got under the hood and found the 220k screen resistor came in at 290k.  It
was the subject of a change from 56k to 220k at some point.  Changed that
out, but got a far from flat amplitude vs. frequency.  So, I began to wonder
what that 220k change was all about...  After some reading I decided to
experiment.  Put in a resistor substitution box and found output amplitude
got better and flatter vs frequency with a decrease in value.  I made the
decision to revert back to the 56k pre-mod value.  Wow!  What a
difference!!!!  PTO output went from 750 mV to 3.3Vp!  It is virtually flat
from 2.4 to 3.5 MHz.  The  receiver is WAYYYY more hot too!  I haven't gone
back and checked stage gains, but this is very promising.  Carrier meter
picked up 12-15 db!  Very pleased.  The 56k un-mod also made the PTO much
more insensitive to tube changes...

As far as endpoint drift, this seems to be related to mechanical stress on
the Oldham coupling.  As far as I can speculate, the two ends and the center
plate shouldn't be tightly sandwiched together.

I learned all about how the Cosmos PTO is linearized and yes I had to take
off a turn of the endpoint coil, but now I have a drift free (so far) PTO
linear from end to end within <300 Hz.  40 little tiny screws and a few
hours and I'm a happy camper.

It's a good R390A day!

John

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