[R-390] Filter Repair, etc.

[quartz55]

OK, I've been playing with these filters for about a week now.  I took the 8K filter and tried to run it down in gain but I could only reduce it about 3 dB or so.  I've got the 82pf and a 6-120 variable on the output side.  The input I'm feeding with a 50 to 100K ohm filter I made up in a Pamona box.  It appears that reducing the gain of the output filter with the var cap does change the response somewhat, but I cannot get a 20 dB difference in gain.  As far as terminology, I'm using what the Collins filter people are using, "response variation", yes I know the word ripple.  Here's a trace of the 8K filter with various settings of the var cap. http://i251.photobucket.com/albums/gg287/DogTi/R390A/R390A%20Filter%20Repair/filter8Kchanges_zpsc3c04b2b.jpg  I'm learning a lot about making charts in Excel.

 

I didn't do the whole response because I have to step through 100 hertz at a time, write it down and transfer it to Excel and make a chart with the numbers.  No automation here.  But I seem to be getting consistent results, even over time so I'm pretty confident my measurements are fair to good.  I've even discovered I can get the isolation of the in/out to almost 60 dB without an amp, so that helps.  It just took a bit of playing with the setup.  Of course the filters that are already 20 dB down, I don't get that large isolation in to out with those filters.

 

I'll plug away here and put these 4 filters back in the module that I've l already put orange drops in, but I didn't change the 5000pf bypass which I should have done to start with, but the only thing this IF module was having problems with was the AGC and I'm pretty sure it's the low resistance (relatively, like 15-30 Meg ohms) from the coils to ground.  But when you add a few 15 Meg in parallel, it starts adding up to real resistance.  Plus the 2 and 4K filters were shot.

 

I have another IF module I may play with when a few other things get done, at least I've got the filter process down.  I've got ceramic caps for that module,  those orange drops are just too big to put in the IF module in general, and the filters all work in that module.  The ceramics are very small compared; they look like large surface mounts with wires attached. Plus they're a nice blue color. Check it out with the red arrow here next to the OD, that's a .1uf, 630V  http://i251.photobucket.com/albums/gg287/DogTi/R390A/ceramic_zpseae5d080.jpg



Dave
N3DT