[R-390] 2nd osc again

[Alan Victor]

On the R390A I had 3 low frequency xtals age to the extent that the series
resonate frequency shifted nearly 15 kHz! Measurement of the xtal on a VNA
at the "stamped" frequency indicated the equivalent series R of the xtal
was in excess of several hundred ohms. The LO would never run on its
intended frequency.

Alan

On Mon, Jul 22, 2019 at 7:13 PM Larry H <larry41gm at gmail.com> wrote:

> Nice report Dave.  Glad the 2nd osc came up.  Yes, the variance in readings
> from 1mc band to another band is normal, although ideally it would not be
> that much.  The variance is usually caused by a less active crystal or a
> trimmer not peaking correctly.  I'm not sure if the lower frequency
> crystals get weaker over time or not.  T401 should be adjusted so that all
> the trimmers will peak correctly.  Remember, when T401 is changed all
> trimmers need to be adjusted.  Good work Dave.
>
> Regards, Larry
>
> On Mon, Jul 22, 2019 at 6:29 PM dog <agfa at hughes.net> wrote:
>
> > I went through 7 6AK5 tubes today in the 2nd osc. They varied from -2V
> > to maybe -4V. Then I tuned T401, things came up a bit. Then I found the
> > ant trim didn't peak at 0 any more. Back to the RF alignment. Found a
> > few that were off a tiny bit and one that was way off, one of the slugs.
> > I ended up going back and forth on the 8MC and 15MC until they didn't
> > interact. Then the individual frequencies were off on the 2nd osc.
> > Readjusted them. I think I went through the T401, RF alignment and the
> > 2nd osc xtals about 3 times. The funny thing is most of the 6AK5 tubes
> > seem to work fine in the 1st osc, about 10V worth. But the 2nd osc I get
> > anywhere from less than 3V at 8MC to nearly 10V at 31MC. The PTO is
> > slightly low on the low end, but I'm not messing with that, I haven't
> > tried swapping tubes on the PTO yet. Does this sound about normal or do
> > some of the 2nd osc crystals get weak as time goes on, especially the
> > low freq ones? There doesn't seem to be any balancing of the T401 to
> > adjust for the xtals low to high. The MC bandswitch seems to be stable
> > between bands now. This is on the Dittamore RX, #2. But all the modules
> > are EAC. At least now I'm getting noise peak on the antenna trim which I
> > didn't before.
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