[R-390] IF module ---

Flowertime01 at wmconnect.com Flowertime01 at wmconnect.com
Wed Dec 7 18:40:24 EST 2011


I'm chewing whether to add the trimmers or not.

Bob - N0DGN
-------
Bob,

Aggravation exceeds return on investment.

Call Fair Radio and buy a bare module (less the filters) ask then to 
include the stand off and trimmer caps from where they pulled the filters out of 
the module.

Move your filters over to that module. There is about 32 little parts of 
difference between without and with trimmers. The lay out off the module is 
different. The can over the trimmers is a different height. The no trimmer can 
will not let you put trimmers under it. Reworking the bottom end to add the 
trimmers is a lot of rearrangement beyond just the parts.

One version was top only trimmers. Then current flavor was with top and 
bottom trimmers. Pulling the square can off the filters to do the adjustment 
should not cause any of the filters to go into oscillation. 

The goal of the adjustment was not to peak each filter for max. The idea 
was to match all four filters for about the same amplitude. Yea right, we peak 
each filter for max. So much for engineering theory. 

Once you get the filters peaked with the trimmers you find they pretty much 
stay peaked and readjust to the same point each time you align them. We did 
not find that decks without trimmers had lower output or poorer signal to 
noise than the decks with trimmers. Think that the caps are optimum for each 
filter. Set the IF gain and go on with the alignment. Bumping on one of the 
IF cans or swapping some tubes will get more out of the module than trimming 
the filter caps.

Because the trimmers will let you get real far off, we think the fixed caps 
were not well matched and optimized to start with so maybe a lot can be 
gained by adding trimmers. Not a true conclusion. The fixed caps were selected 
and close to peak. So if a cap has not gone bad then the circuit is close to 
peak. 

Put a gimmick across a cap and see how short you need to get it to bring a 
filter above where you were before you added the wire, If you can get the 
gain above where you started. That should give you some idea if trimmers would 
help.

Roger AI4NI
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