[R-390] Lankford Filter Mod

[Tisha Hayes]

Charles,
Thanks for pointing out the power reduction from the heaters being turned
off and the diodes replacing the rectifier tubes. I was just about to point
out the same thing.

Any sort of solid-stated-ness additions for specific enhancements could
easily draw power off of that circuit. You could either down-regulate the
24 VDC supply that drives the antenna relay or even stick in a tiny power
transformer to isolate the AC side and create a tiny little DC supply to
run whatever project you have in there. I cannot see any sort of digital
addition drawing more than a half-amp from the DC supply, more than enough
headroom is available after you pull the rectifiers and PTO/ crystal
oscillator heaters.

The most I could see would be if you did something like a solid state audio
mod or maybe some sort of amplifier to compensate for insertion loss on the
filter (milliamps for that one at the most).

-------
I do have a Collins helical filter as a roofing filter mod in place on my
prime receiver. Perry had asked me to provide the winding details on the
ferrite transformers but it was so long ago that I lost the files I had
that described what I had come up with. Originally I tried to be all
professional-engineer about it but ran into problems that you could not
make any assumptions about the actual impedance of the RF or IF deck.. much
less that stupid mini-bayonet/ coax setup. I had already modified my radio
to use RG-174 coax and SMA connections so anything I had done was useless
for y'all to copy, unless you did all of the cumulative mods I had in place.

I found it best to wind ferrites and measure insertion loss, then pull one
winding off of the ferrite transformer, measure again and plot it, pull
another winding off, measure and plot it... until I found the sweet spot. I
do not remember what I ended up with. Without my little spreadsheet that I
plugged all of that data on to I cannot reliably give you numbers to work
from and as I said, they will be different with the other things I have
already done to the radio. It turned out that empirical data from
experimenting gave me better results and was actually more fun.

Perry and I spoke a few months ago about the roofing filter mod and what
sort of improvement I saw. It is not an earth shattering type of
improvement. I can detect it with a sweep generator and a spectrum analyzer
on the IF output.

This is also complicated by the fact that my best IF deck had Clevite
ceramic filters instead of the coil, disk and vibrating needle mechanical
filters. My spare IF decks use the old sort of filters but they do bench
duty on my second radio or are working spares on the shelf.

I have even dinked around with the stagger tuning on the IF deck and
increased shielding between stages on the IF deck. Added PI filters between
stages on certain lines, fingerstock on the chassis. Added the squelch
facility, made audio mods. It is all rather complicated and only someone
crazy would do what I have done with that radio.

You want to know the rub? I have not made such a big difference in the
radio that I would wholeheartedly endorse someone else going through all of
the bother. Is it better than it was? Yes. Enough to justify the cost and
time? No. I can always go power up an RF-590 or a Cubic, Racal or Watkins
Johnson if I want to go listen to electrons getting intimate out by
Jupiter. The improvements are all subtle little things that are apparent on
a bench full of test equipment and are as easily nullified by one tube
getting a bit soft.

-- 
Ms. Tisha Hayes. AA4HA

*""It is not because things are difficult that we dare not venture. It is
because we dare not venture that they are difficult." -Seneca"*