[R-390] Need value of R235 from a RF deck

[Craig C. Heaton]

Roger,
And All,

Barking at the right tree, just haven't found the correct leaf. What you
have described, turning on the BFO, setting the receiver to AGC or MGC, tune
in WWV at 5 10 15 or 20 Mhz, zero WWV against the BFO: and when I switch
from AGC or MGC to cal (back and forth) the calibration jumps around 30 Mhz.


The first inspection found the R235 wrong value according to the Y2K manual,
but I don't think it has any connection with the crystal oscillator. I've
swapped tubes from other sections of the radio, used NOS tubes from the
stash, used another H202 from my Motorola R-390A. No joy yet.

Last night I loosened the retighten the mounting screws for the three tubes
in that section of the RF deck, no joy! Also replaced the silver micas
surrounding V205, no joy there either. 

Thought the air variable cap used to adjust the crystal oscillator might be
bad or damaged, checked it with my Sprague TO-6A, it's spot on, no joy here
either.

Maybe a possible clue! The point where the air variable cap gets the
oscillator circuit to zero beat is near minimum capacitance, plates
unmeshed. So within a few degrees of rotation there are two points that will
zero beat? Minimum capacitance is near 3.5pF

Almost like a passive component has a lead going in the wrong direction or
too close to sometime else? Just guessing here, can't believe it could be
that critical on a point to point wiring radio. Don't care to take both
R-390A's apart to compare circuits.

Craig, 

     

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Flowertime01 at wmconnect.com
Sent: Monday, July 28, 2008 4:30 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Need value of R235 from a RF deck

Craig,

It has occurred to me that you may be barking on the wrong tree.

Try the following,
Turn the BFO on and set the receiver to AGC or MGC and tune WWV at 
5 10 15 or 20 Mhz. Zero WWV against the BFO.

Switch the function to standby and back to AGC and over to CAL and back to 
AGC and MGC.

Does WWV stay zeroed against the BFO when you do this? It should.
You could have one of the oscillators shifting rather than the calibration 
oscillator shifting as you switch functions and Mega Hertz.

Same problems, poor tubes, and or poor caps, and or poor ground points.


I under stand your reluctance to pull the RF deck.

But as long as you have to go in, Be not in a rush to get out. Check and 
change any of the old caps and resistors that may be metering poorly.

Best of luck with this venture.


Roger AI4NI   </HTML>
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