[R-390] RE: Mechanical filter repair

[Roy Morgan]

At 10:44 PM 8/19/2004 -0500, Dennis Pharr wrote:
>... I've been looking at several alternatives in the ceramic 
>Filter/resonator line. ...my plan was to gut the existing mechanical 
>filter housing and insert the ceramic.

Dennis,

Please consider running a little DC into the reworked filter case for any 
needed amplifier. This could be delivered either by a DC supply in the 
case, or external to it, that runs from the radio filament supply.

There would be challenges in keeping "blowby/leakage" and noise controlled. 
Filament lines are notorious for carrying noise and unwanted 
signals.  Notice the filtering in the B+ and filament lines to the 
oscillator module in the HP 606 signal generators, for instance.  These are 
necessary to limit leakage.  The same basic situation exists in the R-390 
IF module.

>  But, of course, I would breadboard the thing first to make sure it works 
> before destroying the already defunct filter.

I imagine a bit of 5/8" copper tubing with end caps held on with screws.

Related note:
A short piece of 1-inch copper tube, two end caps, and a couple of BNC 
connectors make a fine substitute for those often-missing URM-25 
accessories: the 50 ohm load, standard antenna, impedance converter, and 
voltage divider.  Clever workers will mount a Twinax connector on one end 
and incorporate the needed grounding of one twinax terminal, proper loading 
of the generator, and the advisable 10:1 or 100:1 voltage divider all in 
one unit.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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