[Milsurplus] [Boatanchors] RBC- The Bear Be 'Rasselled

[Raymond Cote]

Is 3890 on the low end of its range?



Raymond Côté

Ex US Navy ET1 (SS)

Submarine service



Ex Univ of Hawaii

Oceanographer

Seismology

underwater volcanology

Ham license -KD9CCZ
Commercial radio phone license PG-21-3809

On Sep 30, 2017, at 07:11, David Stinson <arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com>> wrote:

I done whooped that RBC Band switch.

The radio is playing pretty on all bands.



This receiver was "refurbed" by San Francisco Navy Yard in 1960 and

I think I know why it remained in such very nice, pretty condition since

that time:  It was never installed, because it could never have worked.



Here's a view of the bottom of the rig with the two bottom shields

removed (put these back on- rigidity helps keep the LO stable).

Sitting on its left side:
 https://photos.app.goo.gl/qqyeSRsKO9ZF9o1z1



The four removable boxes contain from one to three big, beautiful

ceramic Band switches.  A long, square shaft is inserted from the rear

of the set and runs through all the boxes, connecting them to the

mechanical gears for Band-changing.


The switches were all incorrectly aligned with the shaft- that

Band One was working at all was just chance.  It seems someone

kept fiddling with them until they got a band working, but that

was not the root of the problem.  The issue was the mechanical

part of the Band Switching mechanism.



I removed all the boxes, opened them and manually set all

the switches to the "Band 2" setting.  Re-installed the ANT

and 1st RF boxes (to check alignment and leave room to work)

and carefully re-installed the shaft without moving the switches.

However, once I got the shaft through the boxes, I found the coupling

to the mechanical Ban- Change gears was off by one band- the front

displayed "Band 2" but the coupling was in position for Band 1.

This coupling is pinned to a large gear, so the gear has to be moved

to change its position.   Here's a photo, rig on its left side:

https://photos.app.goo.gl/NZq2oRaBk7peWKb92



As you can see, the shaft coupler and its gear are mounted in a

kind of two-piece bracket.  Near the bottom silver cylinder, there

is a "locator pin" protruding from the lower black bracket which

engages a hole in the chassis, assuring the bracket is set in the

proper position for the gears to properly mesh.



Completely loosened the bottom screw and loosened the top screw

enough to move the assembly.   Used a flat screwdriver to carefully

pry the lower bracket up and dis-engage the locating pin.

The bracket and gear are disengaged by swinging the bottom part

of the bracket forward about the diameter of the pin.



With the Band switches properly set and the shaft carefully inserted

to prevent moving them, the shaft can then be used to properly align

the coupling gear.  Takes a couple of tries to get it just right.  Pushed
the

bracket back into place, tightened the screws, engagde the shaft and
carefully

rocked it to check for binding.  OK, so checked for proper rotation and Band


changing.  All was well, so pulled the shaft back, installed the remaining

boxes and ran the shaft back to the coupling.



With the Band switches all in the right places, the LO problems disappeared

and the set aligned without trouble- other than the "bear wrestling"

I've already written ;-).



It may be big, but it is a sweet receiver.  The Narrow IF selectivity for CW


with the Audio filter is about as good as a mechanical filter.  The audio on
AM

is superb and it drives a lo-Z speaker just fine.   Very sensitive and I
love those

"Steam-punk" meters.



There is, of course, one more problem to puzzle-out (ain't there always).

An intermittent drop in sensitivity which appears to be in the 1st RF and

just might be a problem with the Tuning Capacitor.  That's going

to be challenging.  But for now, I'm taking my coffee out to the barn

and listen to the 3890 Texoma AM Trader's Net.



GL OM ES 73 DE Dave AB5S



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