[R-390] Update to 3 Mc issue with R390A

[2002tii]

Ken wrote:

 > I find this very interesting...I have an R-390(non-A) that exhibits
 > the same behaviour in the crystal calibrator module...
 >
 > Turn it on and it's good for about 30 seconds or so then
 > dies a slow death. I had initially thought it may have been a cap
 > or resistor that's gone out of spec. The idea of crud getting in
 > there and causing mischief hadn't occured to me.

It's certainly possible, but the most common cause of flaky operation 
of the calibration oscillator is drift of the multivibrator.  The 390 
and 390A both synchronize an astable (free-running) multivibrator to 
a crystal reference (1 MHz in the case of the 390, 200 kHz in the 
case of the 390A).  If the multivibrator's free-running frequency 
drifts too far, the "kick" from the crystal oscillator is mis-timed 
and fights rather than assists the multivibrator, which damps the 
oscillation.  In both the 390 and 390A, the multivibrator should be 
able to lock throughout the (small) pulling range of the crystal oscillator.

In both radios, if there is no output from the crystal oscillator, 
the calibrator will generally be running but far off frequency.

On the 390, check for 1 MHz output (couple a 10x probe through 10-20 
pF to the plate of V901).  If that is OK, try adjusting C912.  If 
that doesn't work, suspect drift in the components around the 
multivibrator, V902, or V902 itself.

On the 390A, check for 200 kHz output (couple a 10x probe through 
10-20 pF to the plate of V205A).  If that is OK, suspect drift in the 
components around the multivibrator, V206 (there is no trimmer on the 
multivibrator free-running frequency on a 390A), or V206 itself.

Best regards,

Don


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