Tuning the Ultradyne.
A
diary of getting it working properly and how it performs in the Real
World.
14 Dec 2005 -
From a 2005 perspective one can see some of the problems with a design
such as this one as it applies to modern conditions. The most
obvious is the low frequency IF, in my case 60kc. Most modern
sets, in fact most broadcast sets after the 1930s use a 455 kc
IF. With 60 kc there are images will fall 120 kc away from the
desired frequency, versus 910 kc away in the 455 scheme. It takes
extremely sharp front-end selectivity to reject sufficiently signals
that are so close. Moreover, with the 455 scheme there are
essentially no images falling inside the BCB except from stations
transmitting on 1450 kc or higher. In the mid-late 1920s there
were only a relatively few stations operating, mostly with fairly low
power and often with schedules of only a few hours a day.
As mentioned on the main page, a tuned loop antenna can provide much
better selectivity than the internal coil and in the original articles
an external antenna tuner is discussed for applications using an
outdoor wire antenna. The loop has the additional advantage of
rejecting undesired signals by its directivity.
The Wirco antenna coil which I am using leaves a lot to be desired in
its Q. A better coil could be fabricated with larger litz wire
but this really wouldn't eliminate image problems. An external
loop is the best option. Although my version doesn't have the
jack for an external loop one can still be connected via a coupling
link configuration.
The other nemesis that haunts me on most 1920s radios at my location is
a nearby broadcasting station, 5 kilowatts on 1370 kc. The
presence of this strong signal created dozens of undesirable "birdies"
and mixing products. I used a small trap made with a ferrite
toroid and trimmer cap hidden under the antenna block to reject
1370. The radio quietened down considerably after doing so.
This wasn't such a prevalent problem in the 20s with the limited number
of stations but even still there existed all sorts of magic trap-like
gimmicks on the market to achieve this same result. Nowadays in
an urban area with dozens of strong local stations this could indeed be
an insufferable situation. Again, an external loop antenna would
go a long way in minimizing the effects.
Another bugaboo is the harmonics from the local oscillator.
Imagine that if you are tuning a station at 600 kc. The
oscillator is supposed to be running at 660 kc. When doing so it
is also sending out a second harmonic at 1320 kc. Bad enough that
the receiver when set for 600 kc will also tend to receive 720 kc
(other-side injection) but it will also tend to receive 1380 and 1260
simultaneously! If any of these stations are particularly strong
then additional mixing products will be created. More indication
for the need of front end selectivity.
Ok, time to trim the sails....
First, and easiest, is to get the rf input tuning range
established. This will help later in sorting out the images and
oscillator harmonics. With my tuning capacitor of ~475 pf I
had to remove about 3 turns from the Wirco coil to get a tuning range
of about 550 to 1550. Its not particularly sharp at the top end
and I can still receive a station on 1620 with the input tuner set at
minimum. I made a little chart using the signal generator that
can aid me in tuning the desired signals.
25 Dec 2005-
In spite of my best attempts I hadn't been able to get
regeneration. The original Ultradyne calls for a tuned input on
the first IF xfmr. Any attempt to tune my 60kc Victoreen xfmr
would swamp and kill
the oscillator. I suspected the regeneration problem to be
interrelated.
Nothing like a bit of trial and error. I wasn't comfy with the
size and number of turns (24) on the Wirco tickler so I wound a new one
with 40 turns and a slightly larger form. That seemed to do the
trick...at least partly. I had been subbing in some small value
caps across the primary just to see what would happen. It just so
happens that I had left a 150pf tacked in when I tried the new
coil. Removed the cap and no regeneration. So I played
around a bit and it still didn't seem to want to solidly
regenerate. I tried different values down to 47 pf with varying
degrees of success. 100 or less seems to be the magic
number. I left 100 pf in place for the moment and I get a nice
regen edge when rotating the tickler. The antenna tuning cap
interacts with it somewhat but thats to be expected.
This
tickler is slightly larger - 1-5/8" instead of 1-1/2" and has 40
t instead of 24.
So, I think I have licked that problem. The downside is that the
extra capacitor is seen by the oscillator tuning so my previous
trimming of the osc coil has now gone to pot. I had also changed
the osc tuning cap to a one with better SLF characteristics. It
tunes well but is a tad floppy mechanically so I may change it
again. Remains to be seen how much this added cap is going to
bugger up the top end of my tuning range. Those second harmonics
from the osc may come in handy after all!
How much improvement is there with the regeneration working?
WOW! I was tinkering while using a lashup with diode and phones
connected across the output of the 2nd IF xfmr....that means osc, mixer
and 1 rf amp stage and no attempt to match the phones. Without
the regeneration I could just barely hear a semi-local station on 840
kc. When the regeneration kicks in its at comfortable headphone
listening level. That old Wirco antenna coil got decidedly
sharper too! By the way, I measured the Q on it and its not as
shabby as I thought. Its better than most small basketweave
coils. So I think the regen will go a long way in sorting out the
images and make a marked improvement selectivity-wise. I might
also add that there's not a bunch of howling and squawking with this
regen scheme. It acts more like a smooth rf gain control until
you hit the edge where it drops out.
*
* *
* *
21 January 2006
The set has now been rewired with brass rod, 120kc Ultraformers have
been fitted and ready for testing. Initial results were quite
pleasing. The higher IF certainly cuts down on the images.
Two 'bad' things were noticed, however.
1. There seems to be much more pulling of the local oscillator
when the regeneration kicks in. Regen also seems kind of
'rubbery' which aggravates the problem. The rubbery aspect may be
because I had previously increased turns on the tickler but the pulling
aspect when tuning the tickler or antenna condensor is on the verge of
being objectionable.
2. Bandwidth has increased proportionately. Thats great for
nicer audio on a sparsely populated band but I'm not sold on it.
My first thought was that the Ultraformers are overcoupled and
considered going back to the Victoreens and a 60kc IF. That would
be a major chore since the terminals on the two types of IF xfmrs do
not match up for an easy replacement.
So, I tried lowering the IF freq on the existing Ultraformers. I
went down to about 77 kcs and indeed the selectivity sharpened up
nicely. The quantity of images and mixing products came back
too. (I'm doing this in the daytime when my nearby station is
running 5kw and the trap is not removing it as well as at night).
This confirmed to be that the relationship between IF freq and
bandwidth was predictable.
I then tried the IF at 100kc. This seems to be a good
compromise. For comparison, at 120kc a 60 mile distant station at
810kc was about 5 divisions wide on the 0-180 tuning dial. A
much stronger station at 840 could be heard in the background although
not
objectionably strong. At 77kc the same 810kc station on the dial
was barely 2 ticks wide, nothing heard from 840. The decrease in
audio quality was noticeable as well although that doesn't stand out as
an issue. At 100kc the 810 signal was about 3 ticks wide and
sounded pretty good.
I'm going to run with 100kc for a while and get a better feel for it
around the band. I do think these older IF transformers are too
overcoupled at higher frequencies for todays use so there's another
gotcha in playing with old superhets. It would be fairly simple
to fashion an xfmr to modern standards for better performance but that
seems to be going too far when wanting to experience an old 1924
circuit and
its inherent shortcomings.
At this point I think its time to clean up the old loop antenna and rig
it with fresh litz wire. The shop cats have now matured and don't
seem
to have much interest in it anymore!
To be continued....
Back to main Ultradyne page
21 Jan 2006
