[R-390] IF Deck Revisited (Revisited)

[Barry Hauser]

I don't know if this could be it, but you should check the tubes for grid 
emissions on a tube tester, as well as transconductance.  Some testers have 
a grid emissions check setting, but on others it's (I believe) synonomous 
with the gas test.  You can try swapping in new tubes, but if your spares 
came from the same batch, they may be similarly afflicted.

Barry



----- Original Message ----- 
From: <n4buq at knology.net>
To: <R-390 at Mailman.qth.net>
Sent: Friday, September 15, 2006 2:32 PM
Subject: [R-390] IF Deck Revisited (Revisited)


> Since I'm having several issues with the IF deck, I made a geniune effort
> to check resistances and voltages on all tube pins in the IF deck.  This
> time, I recorded all the findings along with their spec'd values so I can
> refer to that list for places to check.  Most things were fine, but there
> are some grids with positive values where they should be negative, etc., 
> so
> I'll begin there.  The first thing will most likely be to try 
> new/different
> tubes in some of those positions as I assume grid emmissions could be a
> problem for symptoms like that.
>
> One problem I'm having is when I switch from MGC to AGC, the carrier-level
> meter jumps up 20dB and stays there.  I found the cathode resistor on 
> V506A
> (R548) to be about 41-ohms instead of 27.  I don't have a replacement, but
> I put a 100-ohm resistor in parallel which should bring it down to about 
> 28
> ohms.  The carrier-level meter jumped up somewhat in MGC, but I can 
> re-zero
> it and when I switch to AGC, it still jumps up 20dB so all I did was 
> change
> the reference point.  Not the main problem...
>
> What I'm wondering is a good way to debug this thing.  Is it possible to
> start at the input and work my way forward (or vice-versa) and try to get
> it working properly stage-by-stage?
>
> Thanks guys,
>
> Barry - N4BUQ (the other, other Barry)
>
> PS: As a side note, now that I have new probe points, I used my HP-410B 
> for
> both resistance and voltage measurements.  I've usually used the DVM for
> resistances because it was a pain to keep switching the pin between the
> probes.  That sure is nice to use a real meter to check resistances!
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