[R-390] Testing the 2nd osc, swapping xtals

[Alan Victor]

I understand that Dave. Once you get into one of these projects, it hard to
shake it off!
The fundamental oscillator circuit is pretty straight forward. However, all
the mechanics particularly the tuning capacitor band through the switch can
cause irritation! Assuming its solid and not so temperature sensitive, I
would also concentrate on the oscillator components. Even something as
simple as a poor intermittent ground return of one of those 5000 pF caps,
is going to create an issue. If you have a scope and a FET probe to prevent
loading, boy that would go a long way to pointing to what is going on.  The
idea of taking the xtal out and replacing it with its inductive equivalent
is pretty straightforward. I would do it on only ONE select xtal freq. The
one that appears to be most troublesome. I wish I knew the Collins spec on
those xtals. It could be they are simply correlated to 32 pF which is an
industry standard. The 22 pF I mentioned earlier is pretty close. The 32 pF
correlation C simply translates to an inductor whose reactance is
equivalent to 32 pF at the xtal frequency. Anyway, its worth a check.
Again, you will have to block the inductor say with one of those 4700 pF
C's. As it hangs on the tube grid. You would simply plug the two leads of
the series L-C into the xtal socket.

Alan W4AMV

On Sat, Jul 27, 2019 at 6:19 AM dog <agfa at hughes.net> wrote:

> Alan,
>
> Thanks for taking a look at the circuit for me. Right now the discrete
> component exchange of the xtals sounds like a project, except maybe in
> the case of the few where I have really low voltage. It would be a
> project to do them all but maybe substituting one or 2 may be viable. I
> have a good supply of newer dipped mica caps and may be able to replace
> the feedback caps for the osc and also the 5000pf bypass caps and maybe
> any other thing that seems obvious. I bought a lot of 4700pf 1KV quite a
> while ago thinking of replacing a lot of them but never got around to
> it, there's a lot of them, especially in the IF. I did notice yesterday
> after the rx warmed up good, the voltages wandered around a bit. About
> an hour into it, the osc voltage dropped down significantly (maybe 5 to
> 4 V) and then as it warmed up it slowly increased again, but not to the
> peak I had at about an hour into it. Not sure if that's the tube or
> components, but it also may be worth investigating. I've got about 5
> tubes I can substitute, plus the 2 in the 1st osc position for both rx.
> Right now both of them seem to be operating about the same 2nd osc
> voltage wise but of course there are distinct variations per MC. I'll
> also attempt to de-ox the bandswitch after a bit of cleaning, but it
> doesn't seem noisy or erratic at all. Right now both RXs seem to vary
> from a low of less than 2V to a max of 8. I wonder what they were when new.
>
> I don't know why I'm doing this in the middle of mowing season, I should
> have been working on this either in last week's heat or last winter.
> Evasion I suppose.
>
> Dave N3DT
>
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