[R-390] No Peak on T701
Barry
n4buq at knology.net
Sun Apr 30 15:29:52 EDT 2006
----- Original Message -----
From: <Flowertime01 at wmconnect.com>
To: <R-390 at mailman.qth.net>
Sent: Sunday, April 30, 2006 2:07 PM
Subject: Re: [R-390] No Peak on T701
> Barry - N4BUQ Reports that he has done some resistance checking on V701
and
> nothing is all that far off. Since the slug tunes the plate tank, I'm
wondering
> if the problem might be with C706. It's supposed to be adjustable, but
how?
> Reading the Y2K manual, it states C706 is a variable glass dielectric cap.
Not
> sure what that is, but with the PTO out of the radio and the cover off of
> Z702, I see a clear cylinder in which the slug moves. Is that C706? ---
YES ---
>
> Barry - N4BUQ Reports that he has been doing some more tweaking on a
R390A's
> T701 transformer (output from the PTO) that will not peak. Getting
slightly
> less then 2V peak to peak no matter how he adjust the slug.
>
> By the way, FC7 (changing R702 from 56k to 220k and thereby reducing the
> output of the PTO voltage) has not been done on this PTO.
>
> He ask, Anyone ever seen this behavior? If so, any ideas what's wrong?"
>
> -----------------
> Barry,
>
> R702 is the screen gird resistor, Changing the value from 56k To 220K will
> just reduce the screen grid current. Maybe a bit less noise in the
oscillator
> output. Like a little longer tube life. Not a lot of change in the
oscillator
> output.
>
> Tim Shopp recommended that you check resistor values and DC voltages
inside
> the PTO chassis. We know the 2.2K's are always off by a few hundred
percent.
> The one in the plate circuit R703 could be drooping B+ and thus giving you
low
> output.
>
> Figure 49 page 75 in TM 11-5820-358-35 8 December 1961 Shows Z702 with
both
> the cap and slug adjustment on the top of the can. If you are working on a
> Cosmos PTO or some other clone of the Collins Variable Frequency
Oscillator then
> all bets are off.
> ---------------------------------
>
> Hmmm. It appears the adjustment I *thought* I was making to the
transformer
> was not the transformer at all, but C706! Although the schematics are
drawn
> with an arrow through T701 (which I always took to mean an adjustable
(usually
> slug-tuned) transformer), it appears the transformer in Z702 is fixed (the
> windings are on top of each other with no slug tuning).
>
> Now, I'm still wondering what C706 is tuned for. Maximum output? If so,
> I'm not seeing any change with it.
> Thanks again, guys.
>
> Barry - N4BUQ
> -------------------------
> Oops, Barry, you should have a C and a T to tune. Several photos and
drawing
> in the 1961 Tm show both items. My R390/A is bolted in a case and I can
not
> get a look its internals. Some models of the PTO may have put the C
outside the
> Z702 can on the "subassembly chassis".
>
> The Cosmos PTO's may have a fixed transformer with no adjustment. Some
other
> models may also have no transformer adjustments.
>
> Is it time for every one to look at their PTO and report what we find for
> models and setup?
>
> --------------------------------
> Both C706 and T701 of Can Z702 are variable. C706 will be a thread shaft
that
> gets longer and shorter when adjusted. T701 will have a nut on the coil
> thread shaft like T401. What you want is a flat output from 3.455 to 2.455
into the
> load of the V204. Any other test is just an educational opportunity and
has
> no relation to the actual operation of an R390/A receiver. The cathode DC
> current of V204 flows through the secondary of T701. The mixer output of
V701 is
> imposed on the DC cathode current of V204.
>
> For inspection purposes only, place a tube extender under V204, ground
E211,
> and observe the peak to peak output of the oscillator signal on the plate
of
> V204. As you run the VFO through ten turns watch the peak to peak level on
the
> plate of V204. The output should be flat from end to end. Oscillator peak
to
> peak output likes to change with a change in frequency. The tuned circuits
are
> to oppose the change and thus provide a "flatter" output across the tuning
> range. DO NOT expect the oscillator output to peak at any point. DO expect
a flat
> output across the full range.
>
> The 1961 TM offers no suggestions for tuning C706 or T701. The Y2K Manual
is
> based on this early publication and likewise offers on help with C706 and
> T701. We see that in the RF section we adjust the slugs on the bottom
frequency
> and the cap on the top frequency. We see that in the RF section we make
these
> adjustments at the quarter band points. So 2.455 plus 250 = 2.705. Thus
3.455
> minus 250 = 3.205.
>
> Use the procedure above with a tube extender. Watch the B+ on the plate of
> V204. Ground the test point E211 just to keep the V204 mixer output
content down
> to a minimum number of mixed output signals. Adjust C706 for maximum
output
> with the KC knob at 705 on any MC band. Dial the KC knob through the ten
turns
> and watch the output on the plate of V204. Adjust T701 for best equal
output
> across the KC. Repeat the process until you get the output as flat as you
can.
>
> If you find your receiver output is lower at 1.750 than at 1.250 then the
VFO
> Z702 may need adjusting. Or if the band has more output on the other end.
Bad
> PTO adjustment will look likes lower output or less sensitivity on one end
of
> every KC than at the other end. Or you could have a peak where the middle
is
> peaked or sags.
>
> If you have a VFO you think has a low peak to peak output, by all means
start
> checking those resistors.
>
> Most of us do not have a signal generator that we can run over 8 Meg (just
to
> dodge the first mixer) with a flat output across a 1 meg band to check the
> output of the VFO through the mixer. The number of variables just make the
> effort not productive. I am sure some one will be happy to tell me they
can and did
> it. Most of us just do not care.
>
> In summary, check the resistors, scope your VFO output once and then leave
> C706 and T701 in Z702 on your receiver alone for the rest of your life.
>
> Bill Hawkins added,
>
> Also recall past postings on this list that suggested the PTO output
network
> was there to "squash" any resonance 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!)
>
> Ah, les'see, the output PTO coil will not peak. Doesn't seem like it would
be
> a good idea for it to peak.
>
> The PTO generates 2.5 to 3.5 megacycles, and they've all got to be mixed
with
> the masticated RF from the RF deck. A sharp peak at 3.0 MC would seem like
a
> very bad idea. If you want a reasonably level signal for 3 +/- 0.5 MC, you
> want a really low Q circuit.
>
> Bill Hawkins asked, "What have I missed?"
>
> Bill's right, the VFO output needs to have the same output level across
the
> output range of 2.455 - 3.455.
>
> Tim Shopp, tells us 2V RMS is not awfully low. When he scoped the PTO
output
> he sees 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".
>
> Tim Shopp's following is also relevant knowledge.
>
> 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?
>
I don't know the brand name of this PTO. Someone removed the label before I
got it. It definitely has no T adjustment, though. The primary and
secondary are wound together on a "resistor-like" form. The glass cap is
definitely there and I can see the slug in it moving as I turn the
adjustment nut (made a special tool to do this).
I'll have to try the tests you describe.
As you may have noticed my earlier reply, I'm wondering about the cap across
the output. Ever seen one of those go bad?
Thanks, Roger.
Barry - N4BUQ
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