[R-390] RF deck alignment

[Tom M.]

In a related matter, a sure sign that your RF stages need alignment is that if you move the MHZ change dial a tad and the signal gets better before it changes bands, you need to realign.
 
Tom


________________________________
From: Charles P. Steinmetz <charles_steinmetz at lavabit.com>
To: 390 list <r-390 at mailman.qth.net> 
Sent: Monday, August 5, 2013 7:01 AM
Subject: Re: [R-390] RF deck alignment


Tisha wrote:

>has anyone ever gone through the process of aligning the RF deck stages and
>bands for linearity?
>
>      *  *  *    what happens when a slug has been replaced,
>repaired or the spring was accidentally stretched? That one slug will have
>a different peak than the other in the string when the band selector switch
>is lined up that way.

The RF tuned circuits are substantially wider than the widest IF 
filter, so there is considerable room for RF misalignment without any 
performance penalty.

To adjust linearity (tracking), you would have to adjust cam profiles 
-- not practical, and not necessary given the large margin for RF 
misalignment.  If a spring has been stretched, you want to get the 
slug back to its original height in the coil form.  If a slug has 
been replaced or repaired, you may want it at a different height than 
original if the mass, length, and permeability differ significantly 
from the original slug.  A two- or three-point check should be 
sufficient to set this properly.

>The only way I could think to do it would be to inject a known RF signal
>level and to use a RF microvoltmeter (like a Boonton 92) at test points to
>walk through the stages.

The RFs are all peaked, so you should be able to just monitor the 
level at the IF.  This will also account for the offsets of the 
crystal oscillators (although, again, those should be much less than 
the large margin for RF misalignment).

>Am I wrong-headed to think that the more selective the RF stages are the
>better the desired response would be at the IF?

Because the RF tuned circuits are substantially wider than the widest 
IF filter, the IF response will be determined solely by the IF 
filters unless something is broken.  The RF filters are there to 
limit the out-of-band signals the front end has to deal with, not to 
affect the ultimate response.  There would not even be any practical 
benefit in terms of out-of-band rejection or in-band insertion 
loss.  Indeed, if one made the RF as narrow as the IF, it would 
almost certainly be detrimental because of the inevitable mistracking.

Best regards,

Charles




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