[R-390] 390() vs 390(A)

[Barry Hauser]

Roy wrote:

> The auto tuning motors in the R-391 take MUCH more current than the tuning
> motor in the R-389.  In the R-391, both the KC and MC setting mechanisms
> are moved by (separate) Motors and associated position setting
> mechanisms.

Not so .. only one motor in the R-391.  I have two of the beasts.  Each one
has a single motor made by Hamilton Beach -- and when they're running, it
sure sounds like a Mixmaster run amok.  (OK, Mixmaster is Sunbeam or
whatever.)

The motor is located behind the panel at the lower left and drives a single
shaft with three worm gears on it that runs horizontal across nearly the
entire lower front.  There are five major components -- The MC and KC
positioners, the control module , a sealed relay, the single motor and an
essential but not so major channel switch.  The MC and KC positioners are
identical except that the MC one has a detent on it.  The Control Module
includes a couple of wafer/rotary switches (some other stuff) and the
indicator wheel with "1" through "8" on it which shows through the hole in
the panel.  There is a rather involved description in the manual of the
logical steps.  The whole cycle is triggered when the setting of the channel
switch is changed from equal to the control module indicator to not equal.
There are mating worm gears for the two positioners and the control module.
The positioners consist of a stack of disks with indentations and mating
spring-loaded pawls -- look like the guts of some kind of combination
lock -- or maybe an old mechanical adding machine.

Basically, there are 7 "memories" and one extra.  When you manually tune the
receiver (keys must be loosened/unlocked), you are changing the MC and KC
settings for the channel that's showing in the window.  One rule is to never
operate the autotune with the locking keys unlocked.

There is no electrical connection between the autotune system and the radio.
The whole business meshes up to a standard R-390 RF deck.  The aluminum
casting behind the panel is special to provide mounts for the autotune
components, motor gearing, plus protrusions in the casting with sintered
bronze bearings ("oilite") for the worm drive shaft.   In an R-390, there's
mostly open space there.  Originally, they hired Rube Goldberg to design the
thing, but he couldn't work in such a small space.  No room for the sliding
pond, bass drum, pulley system -- let alone the chicken. ;-)

The R-391's have the same round 4-pin power connector on the back panel as
the R-390.  Two are the AC, one ground, and the fourth one is for 24 vdc for
the autotune motor.  As I recall, it needs 3 Amps continous, but surges to
something like 5, so a heavier supply is needed, and 7 or so isn't a bad
idea -- maybe necessary.   It may need more current if the disks and pawls
are gummed up, the contacts in the sealed relay are sticking, and the thing
tries to tune past the 10-turn stops and wreck the RF deck.  It's possible
that the original power supply was rated higher to provide DC to more than
one R-391, or an R-391 and a DC malted mixer.

Anyway ... one motor -- whole bunch of other stuff, but just one motor.

Barry


  The DC supply for that is external to the radio, and if I
> remember correctly, is about the same size as the radio itself.  In the
> R-389, only the single tuning knob is moved, so the mechanical work
(torque
> times rotations per minute) is much less.  There is no presetting of
> frequencies in the R-389.  The motor is there to assist the operator in
> moving the many many turns (up to 50) from one spot in the frequency range
> to another.
>
> Roy
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
>
>