[R-390] Filter Repair

[quartz55]

Hopefully some good news tonight.  I mentioned I removed an 8K filter to try to get rid of it's high loss to ground resistance (about 20Meg).  It was about a 2 day project.  I thought I could get done in a day, but things cropped up.  Mainly one of the bobbins had resistance from the wires to the insulators of about 10 Meg which meant I had to insulate the whole bobbin from the copper cup, which involved removing the bobbin, wrapping it in mylar tape (clear shipping tape) and re-installing it hoping I didn't crush the magnetostrictive wire, which was successful.  However, I found that grease or contaminants on the glass feed throughs in the end caps would produce resistance readings from ground to the coils too, so I had to be careful to clean everything which meant I had to remove the end caps and clean them thoroughly. After all that I now have an 8K filter that I can't measure the resistance from the coils to ground (I've discovered the Fluke 189 will read to 500 Meg) and the coils still measure  ~50 ohms after soldering the caps back on the tubes.  Tomorrow I'll check the filter on the test set to make sure it's in spec and then hopefully re-install the filters back in the IF module and the AGC will work right.  The 2K filter had other issues which I hope to resolve quickly.

 

It would have been a 1 day project if the one coil hadn't had resistance to ground.  I'm assuming there must have been a crack or open in the magnet wire enamel that produced the 'short'.



Like I said previously, I'll do a photo explanation of the process when I get done.  I've got some good pics and I've figured out how to get the filters apart easily and solder them back together without destroying them.  If it hadn't been for the problem with one coil, I could have fixed it in about 5 hours, which is still a project, but worth it if you want filters/AGC that work.



Dave

N3DT