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

[Bob kb8tq]

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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