[AMRadio] Re: GB> 75A-2 project


sbjohnston at aol.com sbjohnston at aol.com
Wed Jan 23 00:38:16 EST 2008


I did try this on the bench but the loss in gain seemed pretty severe 
for the modest bandwidth improvement I could get.  Maybe with a more 
methodical approach I could make it work.

Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
------------------------------------------------
Radio is your best entertainment value.
------------------------------------------------


-----Original Message-----
From: Jim Wiley <jwiley at alaska.net>
To: amradio at mailman.qth.net; glowbugs at piobaire.mines.uidaho.edu
Sent: Tue, 22 Jan 2008 10:49 pm
Subject: Re: GB> 75A-2 project

It might be possible to "stagger tune" the IF stages to increase
bandwidth.  Use of a sweep generator (or a manually swept signal
generator) will let you know how you are doing when trying this.  As a
general rule, you can trade gain and selectivity for increased
bandwidth.  You might be able to get as much as a doubling of the IF
bandwidth by this method.



I am not familiar with the 75A-2, but if it includes more than one IF
transformer between the IF amplifier stages, it would be a excellent
candidate for this modification.  I have a 51J-3, but the two might not
use the same design inside the IF section.  With luck, the 75A-2's IF
system might have some excess gain that you could sacrifice to get more
bandwidth.  Might be worth a try, at least.


The advantage here is that this is a non-intrusive modification - in
other words, easily reversible, requiring no component changes, extra
parts, etc.


Lacking a sweep or signal generator, you could experimentally try
turning the IF transformer slugs or capacitors a bit each way (keep
track of the original positions!!) to see if it gives you what you
want.  If the trimmer capacitors or slugs are the  multi-turn type, 
then
they can probably be detuned a full turn or more without losing too 
much
gain.  Try small steps, such as a 1/4 or 1/2 turn at first.   Half-turn
trimmer caps are more difficult, and you will probably need to use a
series of pencil marks on the can to try and get fractional turn
indicators.



There are fancier ways to do this, such as by using double tuned
circuits and/or  over-coupling, but then you are into the realm of 
parts
changes and not so easy to reverse modifications.


YMMV



- Jim, KL7CC





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*********** http://www.mines.uidaho.edu/~glowbugs/ **************

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Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
------------------------------------------------
Radio is your best entertainment value.
------------------------------------------------


-----Original Message-----
From: Jim Wiley <jwiley at alaska.net>
To: amradio at mailman.qth.net; glowbugs at piobaire.mines.uidaho.edu
Sent: Tue, 22 Jan 2008 10:49 pm
Subject: Re: GB> 75A-2 project










It might be possible to "stagger tune" the IF stages to increase
bandwidth.  Use of a sweep generator (or a manually swept signal
generator) will let you know how you are doing when trying this.  As a
general rule, you can trade gain and selectivity for increased
bandwidth.  You might be able to get as much as a doubling of the IF
bandwidth by this method.



I am not familiar with the 75A-2, but if it includes more than one IF
transformer between the IF amplifier stages, it would be a excellent
candidate for this modification.  I have a 51J-3, but the two might not
use the same design inside the IF section.  With luck, the 75A-2's IF
system might have some excess gain that you could sacrifice to get more
bandwidth.  Might be worth a try, at least.


The advantage here is that this is a non-intrusive modification - in
other words, easily reversible, requiring no component changes, extra
parts, etc.


Lacking a sweep or signal generator, you could experimentally try
turning the IF transformer slugs or capacitors a bit each way (keep
track of the original positions!!) to see if it gives you what you
want.  If the trimmer capacitors or slugs are the  multi-turn type, 
then
they can probably be detuned a full turn or more without losing too 
much
gain.  Try small steps, such as a 1/4 or 1/2 turn at first.   Half-turn
trimmer caps are more difficult, and you will probably need to use a
series of pencil marks on the can to try and get fractional turn
indicators.



There are fancier ways to do this, such as by using double tuned
circuits and/or  over-coupling, but then you are into the realm of 
parts
changes and not so easy to reverse modifications.


YMMV



- Jim, KL7CC





************ The Glowbugs E-mail List Website is: ***************

*********** http://www.mines.uidaho.edu/~glowbugs/ **************

*** Send subscribe and unsubscribe requests to this address: ****

************** majordomo at piobaire.mines.uidaho.edu **************







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