[R-390] Okay, don't laugh at me but...

[David Wise]

The 8640B does use a cavity, but there's a varactor diode inside.

Dave Wise

________________________________________
From: r-390-bounces at mailman.qth.net [r-390-bounces at mailman.qth.net] On Behalf Of Barry [n4buq at knology.net]
Sent: Wednesday, March 09, 2011 7:56 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Okay, don't laugh at me but...

Hi Bill,

Okay on the explaination.  I do have a general understanding of FM - I suppose I was just being a bit self-depracating.   It does make sense and is sort of what I thought.  I was just trying to get my head around the idea that "modulating" the 8640B with a triangle signal would "pull" and "push" the frequency that way and I don't understand how that works (given it's using a tuned cavity for the center frequency and not a VCO).

The primary on that last IF peaks at a very odd position (core is quite a bit further out of the can that I think it should be).  Furthermore, tuning is "touchy".  If I don't get the stating centered just so-so, I get some distortion.  Definitely something's amiss.  I would suspect another bad cap but there isn't one across the primary of that transformer.  There are some in the secondary so that still could be part of the problem.

According to the alignment sheet, if using a scope and a sweeper, one should see a double-S curve with the S-es crossing zero at the center frequency of the IF.  That makes sense to me and I suspect that if I could see the pattern, it would not look at all like it should and that's what I'm trying to verify.

Thanks for the reply,

Barry - N4BUQ