See the schematic at BAMA first:
http://bama.edebris.com/download/drake/2a/2a.djvu After recapping, checking resistors and aligning, I could not get the same performance on both sides of the passband tuner. Since I'm trading the rig for a REALLY REALLY nice one, I felt I owed him a perfectly working 2A.
So, I took the passband filter and tested and tweaked and prodded, and still found that one side had way more bandwidth than the other side. So, then I opened the case up and charged forth...
It is a clever thing, using four 'coupled' slug tuned coils. BTW, these coils are soldered into position. The only adjustment Drake provided was an offset adjustment screw, to set the center frequency.
Connecting the tuner to a sweeping signal generator, it was easy to see the issue - there is a lack of response in the middle of the 2.4 kHz passband. Because of a mistake in my test setup, I was momentarily convinced that something was wrong with a component, so I check each part, including those 31.3 pF caps (and they are spot on!) I changed my test setup around.
Here is what I have gotten so far.
A signal generator (with 50 Ohm output) is sweeping the filter (40-60 kHz) and producing a marker at 50 MHz for me to trigger a scope to. The signal generator is connected to the OUTPUT of the filter, on the 68 pF cap, where it would normally lead to a grid. I have an oscilloscope (with a regular 10x probe) connected to the INPUT of the filter. I adjusted the now-free slugs for the sharpest and highest peak. See the photos.
I made sure to number the slugs to keep them from getting mixed up - though I think they are all the same material.
How does this look? I've got some improvements to make to my setup. The reason I flipped the input for the output was to use the 330K resistor on the input on my high-impedance scope, reducing 60 Hz noise and to put that 68 pF output coupling cap on the signal generator.
