[R-390] Filter Repair

[Steve Hobensack]

Tuning a new mechanical filter or a replacement filter can be a little tedious. There is usually a parallel silver mica capacitor in parallel with the ceramic trimmer. I use a very small variable cap in parallel with the ceramic trimmer cap. I adjust the variable until there are two peaks on the perimiter of the ceramic trimmer cap. I remove the midget variable, measure it with my cap meter, then install a fixed mica cap of near the measured value.

Steve N8YE 

> Date: Sun, 19 Jan 2014 18:48:47 -0500
> From: "quartz55" <quartz55 at hughes.net>
> To: <R-390 at mailman.qth.net>
> Subject: [R-390] Filter Repair
> Message-ID: <8B0F2B7B970746058DB8ED9C2CD214A0 at DAVE>
> Content-Type: text/plain; charset="Windows-1252"
> 
> Well, I've installed the repaired filters in the IF module that had the 2 and 4 K ones dead, and rebuilt the other 2 while I was at it. I'ts now working just like the other IF that appears to have good filters. Measuring the R at S503 rear pin 8 which should be the AGC line at the filters, I measure ~300 Meg ohms on both IF's now (out of the RX). That's a pretty good trick, pulling out those filters and replacing them and it works first time, if I do say so myself. Of course I did a bunch of measurements compared to the other IF before I put it in. I guess I'm anal enough. I notice the 14MHz band is dead though, must be a loose crystal.
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> The carrier meter drops off with the 4 and 2 K filters, much expected as they have almost 15 dB more loss than the 8 and 16. Now I need to tune it up.
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> Anyhow, I've gotten most of my write up done for 'filter repair' and since I don't have a web page, I'll post it here as text and links to my photobucket account for the pictures. If it can be added to the 'Pearls', I don't mind.
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> It was a project; I'm not sure how many hours I had in it, maybe a week or 2. Was it worth it? No, but so what, maybe it will benefit someone else.
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> Dave
> N3DT
>