[ARC5] [Milsurplus] RBB-6 Bring-Up
David Stinson
arc5 at ix.netcom.com
Wed Mar 20 00:04:58 EDT 2019
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Mark K3MSB
Subject: Re: [Milsurplus] RBB-6 Bring-Up
It's alive! It works! Receives AM, SSB, and CW on all 4 bands. The controls are a bit scratchy, but some DeOxit should help that.
The biggest issue is that on the 3 lower bands, I have to nudge the band switch off the detent for it to receive. Band 4 (covering 80) is fine. The band switch then moves back and no signal.
I didn't see anything in the manual about this. I'm thinking that I'm going to have to remove the individual units in the RF section for cleaning and alignment?
73 Mark K3MSB
--------------------------
Mark, here’s a post from a couple of years ago. The first half addresses
badly mis-matched band switches, which probably isn’t your issue.
But the second half discusses a big gear which, if it’s slipped a few teeth,
will prevent the band switches from being properly aligned at the
band detent. You may just need to align this big gear:
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 Band-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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