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

[Alan Victor]

I understand your frustration. I am looking at the Y2K manual, July 2009. I
don't know if the latest Y2K corresponds, but the schematic is Page 5-42
Fig 5-21. The capacitors C408, 12 pF and C409 150 pF control the feedback
in this oscillator. The oscillator is a Colpitts with the crystal running
above series resonance, that is where the xtal appears INDUCTIVE. Obviously
for all the xtals to to run properly, these 2 caps need to be proper.
Assuming they are, we still do not know if we are dealing with healthy
xtals or a feedback oscillator with marginal components, S402 with band
contacts (the band switch) which would play havoc with the plate load tuned
circuit, T401//all those tuning C's adjusted for each band!

In any case, one suggestion to help put the xtal issue to rest, is to
substitute a discreet inductor with a blocking series cap into the xtal
socket in question. The inductor value is chosen to resonate with ~ 12 pF
at the desired xtal frequency. If you see a significant peaking in the xtal
LO output swing that may point to the xtal problem. Otherwise, no change,
not likely a xtal issue.

Also note, C410 screen bypass cap, 5000pF, better be a good cap. As well
C411 plate tank bypass, another 5000 pF.

Alan

On Fri, Jul 26, 2019 at 3:26 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> I have seen R390’s (A and not A) with damaged crystals in them. The
> “problem radios” had multiple issues.
> Tearing the problem parts open showed that the blanks had cracked. My
> guess is that the radios got treated
> very roughly at some point.
>
> Bob
>
> > On Jul 26, 2019, at 5:59 PM, dog <agfa at hughes.net> wrote:
> >
> > Today I turned on #1 RX and started swapping crystals, but first a tube
> that produced -XVDC in RX2 produced more volts in RX1, so that is
> something. But I started swapping xtals, especially the ones that were
> quite different between the 2 RX's of which there were only a few. There
> was only one position that swapping a crystal made a significant difference
> and that was at 31MC. The 2nd osc went from 4.5 to 7.5V. But notice that
> the same crystal is used at 14MC. It made no difference there so WTF? In
> general I would have to say that swapping crystals is a crap shoot at best
> and not worth it in my RXs. I'm not sure if it's components in the output
> circuit of the xtal osc, the osc circuit itself or what. I'm about to just
> go and shotgun the worst 2nd osc (#2) starting with the tube circuit
> itself. Not sure how hard it is to get into the output circuits with the
> tuned caps. I'll see when I take it off.
> >
> > Also there was a general tendency in RX1 to have lower voltages at 8MC
> or lower freqs and higher outputs at higher freqs, just like RX2. But I did
> have one that was 2V at 24MC and swapping the crystal made no difference.
> All tuned circuits had 2 max points and they were not exactly opposite. I
> wonder if I can see underneath where max and min would be on these var caps
> so I can tell easier from the top what they're doing.
> >
> > I wonder if it was a progression of building receivers that someone came
> up with this mixing scheme or if it was laid out from the design of the
> thing. It reminds me of when I used to work for the PO on the letter
> sorting machines, they could put letters in the 'delivery point' sorting
> machine and in 3 passes have all the letters in order for a delivery point
> on each road and the letters sorted up and down the street so the delivery
> guy didn't even have to sort stuff. He just had a box of letters for each
> street, presorted, and had to go up one side of the street and down the
> other. What magician figured that out?
> >
> > I haven't addressed the 20V that's still on the 1st osc in RX1. I may
> first do some 2nd osc tube voltage/ohms measurements before I get too far.
> Is this exciting or what? Maybe this says something for swapping out all
> the mica, coupling and bypass caps and measuring the R's from the start
> while it's apart.
> >
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