[R-390] A Few R390A Issues Zero Adjust

[Bill Hawkins]

The multivibrator tube for calibration is intended to produce
100 KC when it is synched by the 1000 KC crystal. Ten to one
is not easy to do without precise parts. That's probably why
the 390A went to a 200 KC crystal and a simple flip-flop to
divide by two with lots of harmonics

Normally the MV free-runs at something slightly below 100 KC for
worst case environment. The earlier xtal pulse locks it to 100.

If the components drift a lot, it could lock up at any sub-multiple
of the xtal frequency, or jump between them.

There is a trimmer cap across one of the MV timing caps that can
change the free-running frequency by quite a lot. Trimmers are
among the kinds of caps than can fail (actually, all caps can fail).

First, verify that the crystal is producing exactly 1000 KC.

You need a scope to do it right, but I think the cal module can be
pulled without removing the RF deck. Trigger the scope from the
crystal frequency with a 10 microsec/cm sweep. Probe the crystal
frequency and verify 10 cycles in 10 cm. Now probe the 100 KC MV
output (after the buffer amp). If a cycle over or underruns 10 cm,
adjust the trimmer to about halfway between stable at 10 cm and
stable at 11 cm (90 KC).

If the trimmer can't do that, try another tube, measure the resistors,
and finally replace the timing caps with micas for low pF stability.

Bill Hawkins


-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net] On Behalf Of roy.morgan at nist.gov
Sent: Sunday, November 18, 2007 12:30 PM
To: Grant Youngman
Cc: r-390 at mailman.qth.net
Subject: RE: [R-390] A Few R390A Issues Zero Adjust

Quoting Grant Youngman <nq5t at tx.rr.com>:...
> I currently have the R-390 on the bench because of a problem with the 
> calibrator module (still undiagnosed

Grant,

I remember a comment on the list earlier that the R-390/URR calibrator
uses a 1MC crystal, and the mulitvibrator is running if synched
correctly at 100 kc. 
So, the adjustment of the multivibrator is somewhat critical.  If the
calibrator is producing about 90 kc or 110 kc, then this is the
direction to look in.

The 1MC crystal will be less expensive to replace (new) than a 200 kc
one as used in the R-390A/URR.  If it's the crystal, you may well find a
workable substitute for very little.  I don't know what might happen if
you put a garden variety crystal into that oven, but of course you could
make a plug-in adapter to use any crystal from an octal tube base.

Roy
K1LKY

Roy Morgan
13033 Downey Mill Road
Lovettsville VA 20180
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