[R-390] Stuff that's not in the book

[Bill Hawkins]

Many thanks to Roger KC6TRU for a great write-up of a little
help with sensitivity. He mentioned the alignment of the
balanced antenna input as being "not in the book."

I worked on an R-390 with a very scratchy local audio pot.
Had a 2.5 K pot from ATC, so assumed it was for audio, but
decided to check the taper. The ATC pot taper was linear -
turn it to half rotation and measure half the pot resistance
to the cold end. That didn't seem right. The existing R-390
pot measured less than 1/4 of the total to the cold end, more
like what you would expect for an audio (log) taper.

So I removed the original pot after dropping the front panel,
bent the cover tabs and opened it, sprayed it with regular
contact cleaner (Caig can empty) then air, closed it up and
reassembled. Had to bend a handle a bit in a vise to get the
threaded studs to line up. You wouldn't think you could bend
the handle by hand, but it didn't take much.

Oiled and re-set the bushings for KC and MC shafts, then found
that the MC knob clamp was slipping even though it was tight.
Used some cleaner to get the oil off the external shafts and
cleaned black gunk out of the knob shaft bores so there was no
lubricant between the knob and the shaft. Much better. Wonder
how many clamps have been broken because of the overtightening
required when the shaft is slick.

Is the pot taper mentioned in the manual? Looks like the RF
Gain pot has a reverse log taper.

How about stuff in the manual that is wrong? The instructions
to drop the panel want you to loosen the clamps between the IF
deck shafts and the panel. It is much easier to leave the clamps
alone and remove the knobs. There's no retainer that keeps the
extension shaft from going through the panel bushing.

This is not true if you want to remove the IF deck without
dropping the panel. Then you have to have a long Bristol wrench
to loosen the clamps, or long-nose pliers and plenty of patience.

Regards,
Bill Hawkins