[R-390] VFO tube circuit changes increase receiver performance

[David Wise]

The 220K PTO screen resistor was a MWO (Modification Work Order) to be applied only to installations that were radiating too much LO and interfering with other equipment.

When my BFO faded out, I found that the tube was fine, as were the surrounding R's and C's.  I concluded that the coil had become just lossy enough to damp out oscillation.  The coil puts a heavy load on the cathode.  I increased the available power, by adding a transistor, wired as an emitter follower.  A "silicon prosthesis", if you will.  Now it is strong.  Schematic sketch on request.

Regards,
Dave Wise
SWL in Oregon
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Roger Ruszkowski <flowertime01 at wmconnect.com>
Sent: Wednesday, May 01, 2019 11:56 AM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] VFO tube circuit changes increase receiver performance

Fellow's We have a short Cosmos PTO story from I believe John. 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 The BFO PTO is also a suspect we wish produced more output. The Cosmos flavor VFO is a candidate for change as reported above.The TM schematic diagrams only has one VFO oscillator circuit in them.All PTO shall match the diagrams as built and delivered.That's the rules.OMG the part values were picked from the stock parts list and not the performance list from design testing At least the Cosmos PTO's can receive replacement parts that help to meet the users needs. Many owners have expressed a desire for a stronger BFO output. The circuit for the two PTO oscillators share the same tube. Thus some BFO output gain could be achieved with some selective resistor changes.Most of us have been of the mind set not to go hacking around just because we can.But some thought full hands on experiments show improvements can be made to the production stock quite easily.Over the years some rebuilders have quietly been tucking some better value parts into their rebuildsA resistor here and resistor there with a very good cleaning cherry picked tube set and alignment produces a smarter than average bearThe report from the field as related above lays down a methodology to explore circuit changes. Respectfully Roger AI4NI

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