[R-390] No Peak on T701
Barry
n4buq at knology.net
Sun Apr 30 15:07:02 EDT 2006
I'm seeing about 1.8V peak-to-peak, unterminated into my 'scope.
The more I think about this, the more I'm wondering if C709 might be bad.
Wouldn't this cause low output? It's spec'd as a 2% so I'm thinking if it
has changed, even slightly, it might be causing a problem. If I recall,
C327 (the cap across L230/T207) is of the same type and it has been known to
cause low output from the 1st crystal oscillator.
I haven't found a 510pF, 2% cap yet, though. :-( Surplus Sales of
Nebraska has them, but they also have a $10 minimum order and I really don't
need anything else from them right now. Any sources, anyone?
Barry - N4BUQ
----- Original Message -----
From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
To: <shoppa_r390a at trailing-edge.com>; <r-390 at mailman.qth.net>;
<n4buq at knology.net>
Sent: Sunday, April 30, 2006 6:47 AM
Subject: Re: [R-390] No Peak on T701
> "Barry" <n4buq at knology.net> wrote:
> > I've been doing some more tweaking on one of my R390As. T701 (output
> > transformer from the PTO) will not peak. I'm getting slightly less then
2V
> > peak to peak no matter how I adjust the slug. Anyone ever seen this
> > behavior? If so, any ideas what's wrong?
>
> 2V RMS is not awfully low. When I scope the PTO output I see 6-8V Pk-Pk,
> the max in the middle of the range and falling off towards the edges
> (especially the high freq/low counter edge). This is the
> raw unterminated PTO output; putting a 50-ohm terminator on it
> drops the value by a little more than a factor of two.
>
> Let's see, 6V Pk-Pk is a little above 2V RMS. Maybe your PTO output
> is a little bit low but I can't say that it's "bad".
>
> It's been a while since I screwed around with it but IIRC there
> was not a lot of variation to be had by swapping PTO tubes around.
> (I was mostly looking for frequency stability not PTO output
> level as I swapped tubes around.)
>
> I seem to recall past postings on this list that suggested that
> the PTO output network was there to "squash" any resonances and
> make the output roughly flat across the PTO range. So actually
> peaking it up for one particular frequency may be not good (may
> in fact be impossible!)
>
> As another data point, most of my crystal oscillator band outputs
> are in the 8 to 10V Pk-Pk range, with some going up to 12 or 13V
> and a few bands as low as 5 or 6V (I guess the variations are
> due to crystal activity or just the way picking off the harmonics
> works; the units have been peaked up on each band.)
>
> Getting back to my mixer design obsession, I guess this means
> that most of the mixers were designed with LO inputs in the
> range of a few (1-3?) volts RMS in mind and that a factor of two
> variation in level throughout the frequency range is not out of
> design specs.
>
> I'm not claiming that my values are golden reference standards
> either, just what I see on my two 390A's. Is there an official
> output level that the PTO is supposed to meet? And is that
> value into a terminator or not?
>
> Tim.
>
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