In article , Patrick Turner
wrote:
John Byrns wrote:
In article , Patrick Turner
wrote:
But there is rarely a dip in the tuning character of most AM sets.
That was my point and is the reason sets with a tuning eye virtually
always connect the tuning eye to the audio detector, not to the AGC
rectifier.
It may not be critical where you connect the feed to the magic eye in many old
radios.
Then why is the eye tube always connected to the audio detector and not
the AGC rectifier? Connecting it to the audio detector reduces the
modulation acceptance, so I was surprised when I first noticed that the
eye tube in a set I was looking at was connected to the audio detector,
and not the AGC rectifier where it wouldn't reduce the modulation
acceptance. I thought this was just a design blunder in the particular
radio I was looking at, so I looked up as many schematics as I could find
for eye tube radios with a separate AGC rectifier off the primary of the
IFT, and I was surprised to find that they all were wired the same way.
The only explanation that I can think of for this is that it is done so
the eye tube will provide a sharper tuning indication, without a possible
dip at the center.
But I often derive the AGC voltage separately from
a small cap&diode off the anode of the last IF amp, where more AGC
voltage is
available because the envelope amplitude is greater than at the
secondary of the
IFT.
The IFTs can be aligned with a VTM attatched to the AGC voltage,
and the tuning done to generate the highest AGC.
Good luck with this approach, it is likely to result in misalignment, this
sort of set should be aligned either with a modulated oscillator and audio
VTVM on the output, or by measuring the detector voltage.
It works, and its not neccesary to use a modulated carrier.
A carrier of exactly the 455 kHz can even be cap coupled via 10 pF
applied to the
antenna terminal
with the set tuned as low as it will go, and the IF tuned up starting on
the last
IF and
moving forward, and repeated, to generate the highest AGC.
Its also possible to shunt the AGC across the 0.047 uF cap at the tube grid
connections,
and keep the input signal low, and then tune for the highest AGC voltage
or detector voltage.
But this will cause miss tuning of the last IFT when the set uses a
separate AGC rectifier connected to the primary of the transformer, as a
result of the dip in the primary response.
An interesting experiment on a set that takes the AGC from the IFT primary
is to short the secondary of the IFT with a clip lead, you will observe an
increase in AGC voltage when the secondary is shorted, this is the same
effect that causes the dip in primary voltage when properly tuned.
I have not tried that, but common sense tells me that a shorted IFT sec
will reduce the load seen by the IF amp severely, and reduce the
envelope amplitute
at the IFT pri,
or the tube anode, and thus generate less AGC,
But this is one of those situations where "common sense" is wrong, and was
the first amazing lesson I learned from an early mentor, a lesson I have
never forgotten. Try it, connect the primary of an IF transformer to some
kind of impedance measuring device like an RX meter and see what the
primary impedance looks like with and without the secondary shorted. If
the primary and secondary Qs are the same and if the coupling is critical
the primary resistance will double when the secondary is shorted, if the
coupling is not critical the primary resistance will change by a factor
other than 2 X.
Regards,
John Byrns
Surf my web pages at,
http://users.rcn.com/jbyrns/