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

[Alan Victor]

Forgot,  we need to add in with the 12 pF the control grid to cathode C and
the holder C of the xtal. So that inductor you calculate will be somewhat
less. That's 4 pF from the 6AK5 and estimate the holder C at 6 pF so were
looking at finding an L to resonate where the xtal would be in use as 22 pF
inductive.

On Sat, Jul 27, 2019 at 12:50 AM Alan Victor <amvictor at ncsu.edu> wrote:

> 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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