[Milsurplus] HRO RAS AVC Issues

[Robert Downs]

Ray,

 

I found a description of the NL kit in SHIPS 275 and the later NAVSHIPS
version but neither place has the modified schematic of the NL.  I also
determined that the RAS was built up through RAS-5 but the basic receiver
was only built as CNA-46080 and that CNA-46080-A was just 46080 with the
wide band IF transformers installed.  But nowhere have I found a schematic
with either or both kits installed.  If you do find either modified
schematic, please let me or the lists know.

 

I do have a reprint of the original instruction book for the RAS-1, RAS-2
and RAS-5 available.  But as I indicated above, it does not have either kit
installed.

 

I once had a complete RAS-3 but donated it to USS Texas (BB-35).

 

Robert Downs

 

From: milsurplus-bounces at mailman.qth.net
[mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Ray Fantini
Sent: Monday, August 9, 2021 09:05
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] HRO RAS AVC Issues

 

Noticed that my HRO RAS was starting to be less sensitive then some of the
other radios around the shop so decided that I would pull it apart and see
what's going on. This is the first time in about ten years that I have had
the covers off.

The Navy RAS is a copy of the National HRO jr receiver with several changes
that include having a lower IF frequency of 175 kHz and a 6V6 audio output
tube unlike the civilian version.

Most Hams did not like the RAS because of its broad IF and poor image
rejection due to the low IF frequency but being I just use that receiver for
AM work I do not have those issues and the old RAS is just about as
sensitive as most modern receivers and has a low noise floor that makes it a
good radio for AM work

Years ago, I changed out all the old wax paper capacitors due to high
leakage and did the alignment and all that sort of stuff. Been using the
receiver ever since.

Found the big reason for decreased performance was connected to having a
couple tubes that were not up to what the TV-10 says they should be. But
there is one issue I still have. The AVC action of the receiver is poor and
nowhere near what I think it should be. The receiver overloads on strong
signals and you have to back the RF gain down to get the receiver out of
distortion with the AVC turned on.

The RF gain control on the RAS works by ganging all the RF and IF cathodes
together and having a variable control to set overall gain of those stages.
The AF gain control just varies the level to the final audio amplifier
(6V6). The AVC system on the RAS appears way different then the regular HRO.
The AVC bus is connected to the control grids of the RF and IF amplifiers
but it appears that they have a AVC amplifier and the AVC is derived somehow
from the last audio stage as opposed to the standard diode system used on
other HRO receivers.

All the capacitors on the AVC bus have been replaced and the grid resistors
are all somewhat in tolerance, being seventy-five years old they are close
enough and prefer to keep the original components as opposed to installing
new resistors just to be within ten percent. And I can see AVC action when
using signal generator and watching the AVC bus but just don't seem to be as
good as it should be.

I think the AVC should be able to be switched on and the RF gain control
turned all the way up without any distortion. But with this receiver have to
back down the RF gain to reduce any distortion on strong signals. With thing
like broadcast stations have to run the RF gain control at around half or
less to avoid distortion. 

The question is how is the AVC is supposed to work? One of the problems is I
do not have the original manual also I have the Noise Limiter sub assembly
installed in the receiver that takes the place of the original detector and
AVC amplifier. I think the noise limiter is directly installed in the
receiver without any modifications or changes to the receiver and that's how
I installed it.

Can anyone point me to information or schematics of the RAS with the Noise
Limiter installation? Or have any ideas why this radio has such poor AVC
action? Or why that receiver has such a different AVC system than any other
HRO?

 

 

Ray F/KA3EKH

 

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