[R-390] 4kc filter

[Bob Camp]

Hi

This is turning into two different threads .....

The filters on the 390 come from the land of vacuum tubes. They have=20
very high input and output impedances compared to modern Collins or=20
ceramic filters. That isn't to say you can't make the solid state era=20
filters work, they just won't work quite the same way. If you can get=20
filters that are set up for vacuum tube era stuff it will be easier to=20=

drop them in. Since they built a run of 390's with ceramic filters it=20
is possible to get either type. Finding OEM ceramic parts would be=20
tough though.

The PTO issue ....

There is a well documented issue with the main coil in the R-390 PTO's.=20=

Collins figured out the problem and changed the design on the next=20
generation of radios. All of the subsequent Collins designs used the=20
newer coil rather than the R-390 one. It's not at all clear why=20
somebody didn't put he newer coil into a 390 PTO ... I suspect that=20
Collins kept quiet about what the fix was for competitive reasons. The=20=

guys that figured it out still treated as a big secret thirty or forty=20=

years later.

A couple of things happen to the coil over time:

First the core material that tunes the coil changes value over time.=20
Depending on the material this could go either way in terms of=20
inductance.

Second the coil form it's self shrinks very slightly. This puts the=20
windings of the coil closer to the core. The change in length is=20
insignificant. When the turns get closer to the core the inductance=20
goes up.

The net result is that the tune range of the coil changes as it ages.=20
Since no manufacturing process is perfect the coil turns are not quite=20=

spaced evenly over the length of the coil. As the value of the coil=20
changes the relationship between the non linearity and the tune setting=20=

changes.

The nice thing about the gear train is that until you loose a tooth off=20=

of a gear it keeps doing about what it always has done. The play goes=20
up a little on the counter  as things wear with time but the rest of=20
the system stays pretty tight.

I have no problem at all with doing a linearity fix on a PTO. It takes=20=

a lot more patience than most of us have. The thing that puts most of=20
my 390's off frequency is the crystals. Getting to the point that they=20=

are not a problem also takes a lot of patience and a very big bucket of=20=

crystals. If you wanted a radio that you could simply tune to frequency=20=

that would be the way to go. No calibrate each time you switch the MHz=20=

knob. Of course you could revive one of the Manson Labs. =46rom talking=20=

to the tech's who built them that would be a bit of a chore though.

	Take Care!

		Bob Camp
		KB8TQ



On Apr 11, 2004, at 6:52 AM, D. ball wrote:

> well getting the filter apart was easy, i used a hot air rework=20
> station to do it, I could find nothing to indicate what the failure=20
> was. i even re assembled the filter, sweep it, then decided to take it=20=

> apart again determined to find the fault but ended up breaking one of=20=

> the little wires spot welded inside.. this filter case will get=20
> stuffed with ceramic filters, or maby a modern collins filter..  =20
> people have said the most used filter goes bad, ??   something took=20
> out my filter over time, not sure what it was..  sure was no foam to=20=

> go bad, this collins filter had rubber supports inside.
>
> as far as the PTO. it really make me question what causes that..  if=20=

> you cal the PTO with the dial adjust at .000 and then move 300kc you=20=

> notice you are off right?  ok, I move up to .700 and readjust, same=20
> result, dead on fo about 300kc either way.  because of this i question=20=

> the the gear train, I think a pencil mark on the shaft of the PTO will=20=

> tell me what is going on, it is very possable that this is the gear=20
> box and that the adjustment  you made in the PTO makes up for that.. =20=

> just a thought..  something like this does not bother me.. I can=20
> always re-cal the pto then tune in. but the filter really bugs me, the=20=

> most amazing thing about the r390a is it's ability to hear a station a=20=

> few kcs away from a powerful one, i have never heard a radio do that..=20=

> not like this thing, plus you can hear the collins filters, it really=20=

> sounds good, I have found pleanty of replacements, just bums me out=20
> the 4kc filter is gone..  i just bought this radio..   great shape.. =20=

> just out one filter..
>
> ok,
> thanks
>
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