[R-390] No Peak on T701

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


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?



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