[R-390] No Peak on T701

[Tim Shoppa]

"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.