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

[dog]

Thanks again Alan, this morning I messed with it and found 2 5654s that 
are way better than the others. So I ordered 4 NOS from Antique 
Electronic Supply. They were only $3 each. I've gotten some good tubes 
from them in the past. So now nearly all the MCs come up to spec except 
for a few strange ones. One of them is 24MC on #1 and swapping the #2 
crystal doesn't make a whit of difference. But that MC is also low in 
#2. So it's still not telling me if its the xtal or components. I'll 
hold off until I get these NOS tubes and see what they do before I get 
all involved in the circuit. Those 5654s are also the last tube in the 
IF and I noticed some of them weren't very good either. Maybe in the 
meantime I'll see if I can cool off that 1st osc in #1.

Dave

On 7/27/2019 16:24, Alan Victor wrote:
> 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 
> <mailto: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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