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

[Craig C. Heaton]

Roger,

R235, is on one end of the heater element of H202, in series with a BBOD.
The calibration will be off, if I even turn the circuit off then back on
again. I'll check all the ground points within the next few days. Today,
looking thru the junk box, no 47 ohm resistor was to be had. So a trip into
town was necessary to procure said proper replacement of R235. And we all
know what fun it is to remove the RF deck and get it belly side up! 

I don't think R235 has a darned thing to do with the sudden drift of the
100KC calibration circuit, but while the beast is in the proper position,
might as well fix all ills.

Never gave any thought as to the effect of switching to a different crystal
or set of crystals would have on the calibration, but will take note after
the next battle. 

It's not a big issue, the receiver plays well! But it is a nagging little
itch that must be scratched, I'll fix it.

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: Friday, July 25, 2008 1:48 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Need value of R235 from a RF deck

Craig,

I am not finding R235 on my R390/A TM schematic.

The crystal calibrator isn't quite right! If I use the
trimmer to zero beat WWV life is good until I turn the calibrator off or
switch to a different band. Then the calibration is as much as 20-30 KC's
off. 

When you change bands you are using a different set of crystals from the 
mixer sections. Any one of the second mixer crystals can be off by any
amount.

That's the whole idea of having a crystal to beat against and reset the dial

readout 
with the zero adjust.

But if you were just to switch the MC band up and down and come back to WWV 
without changing the KV knob or the zero adjust, you would expect WWV to
still 
be close to zero beat (close being under a couple 100 hertz).

Common problems these days are the ground points. The little screws have 
caught crud after 50 plus years. You just need to loosen the bolts and nut
up a 
turn or two and retighten the bolt and nut to reset the connection. Any one
of 
these ground lugs on the end of a resistor or coupling cap can form a
variable 
resistance that causes problems.  You likely have one of these in the 
calibration section that is giving you drift. Every time you turn the
calibrator on 
you surge a current through that one crud connection and its instant
resistance 
changes. Just enough to yield a different frequency from the calibration 
oscillator.

Good Luck with this. It is repairable. It just will just take some effort.


Roger AI4NI   </HTML>
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