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

[Larry H]

John, You're right about the oldham coupler, it should have about 1/32"
space between the center piece and the end piece.  Also, the mounting
brackets on the vfo should be adjusted so that the vfo shaft and the kc
knob shaft line up as best as possible.

Regards, Larry

On Mon, Apr 29, 2019 at 3:15 PM <jgedde at optonline.net> wrote:

> 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
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>