[R-390] No Peak on T701

[n4buq at knology.net]

On Sun, 30 Apr 2006 15:07:14 EDT, Flowertime01 at wmconnect.com wrote :

> 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?


Roger,

I attempted to perform the "ground E211 and measure the output on the plate 
of V204" test.  I'm not sure, but I may have 'scope problems.  In order to 
see the signal, I have to select a very small vertical deflection value 
(say 0.1V/div or less).  When I do this, the 'scope "wobulates" up and down 
on the screen on the order of 0.05V or more.  If I look at the output of 
the RF-IF output at F103, it is doing the same thing.

I don't know if the power is dirty, the power supply is acting weird, 
the 'scope is flaking, or what.  I have noticed this on other power 
supplies I've checked, so I'm not sure what to make of it.  At any rate, it 
makes seeing small differences in peak-to-peak on the plate of V204 very 
difficult to measure.

Am I doing something incorrectly here or is this something others have 
seen?  I'm still thinking I could have 'scope problems.  It's a TEK 561A 
and is definitely has its own set of problems, but this seems to be a 
repeatable, almost expected anomoly, at least with this 'scope.

Thanks,

Barry