[Milsurplus] RBZ vs RBZ Special? Info From A Complete RBZ Special Manual

[Mike Morrow]

Mike T wrote:

> I've been trying to research what the circuit differences are between
> the RBZ and RBZ Special. The difference on paper is that the RBZ covers
> from 2.0mc to 5.8mc whereas the Special covers from 5mc to 13mc. 

>From the recent reports, it is obvious that most of the RBZ (Special) 
instruction books held are incomplete, being simply the USN book with
the "2 to 5.8 Mcs." on the title pages being black out and a rubber stamp
of "5-13- MC" rubber stamped nearby.

However, there should also be a printed 24-page supplement dated March 27,
glued into the front, and a blank "Unsatisfactory Equipment Report" War
Department form folded with the manual.  The schematic should show an
unlabeled resistor added in parallel with R5, and a note stamped stating:
"R5 IS SHUNTED WITH A 100,000 OHM RESISTOR".

The RBZ (Special) was procured by the War Department on Order No. 18390-Phila-45.
Electrical differences of RBZ (Special) from RBZ are these:
a.  L1, L2, L3 are different.
b.  RF amp and IF amp screen voltage drop resistor R5 is shunted
    to half the effective resistance found in the RBZ.

------------ BEGIN QUOTE FROM SUPPLEMENT ----------------------------

Add the following note between "Electrical" and the beginning of the
text of paragraph 2-4:

Note:  Model RBZ (special) radio receiving equipment differs from
the standard equipment in that Type CEX-46203 radio receiver has
been slightly modified.  Tuning coils L1, L2, and L3 have been
changed to change the tuning range of the set, and R5 has been
shunted with a 100,000-ohm resistor in order to change the operating
point of V3.

------------ END QUOTE FROM SUPPLEMENT ------------------------------

In addition, the supplement changed the high end alignment point from
5.88 MC to 12.9 MC.  The check point for IF image is changed from
6.79 MC to 13.81 MC.

There are a few other changes in the alignment steps related to signal
generator level.  The majority of the supplement is standard Army manual
boilerplate, such as administrative reports and the "FITCAL" process.

> From the limited information I've been able to find it seems like
> the conversion was a field conversion that involved little more than
> realigning the receiver and changing the dial face for a new one.

That it definitely was not.  A totally new tuning inductor (L1/L2/L3)
assembly was required.  However, a shop alteration would not be too
hard with the required parts.

> I spent some time today attempting to do just that on my RBZ and
> found that the RF and OSC trimmers would only give about +/- 300khz
> of adjustment.

No surprises there.

> I'm not sure where they found an extra 7mc or so. Does anyone have
> any information or ideas?

Just what's above.

Mike / KK5F