[Milsurplus] [MRCA] HRO RAS AVC Issues

[Nick England]

I have posted the CKB-50142 RAS noise limiter manual here if that is of any
help -
https://www.navy-radio.com/rcvrs/ww2/ras/ras-limiter-man.pdf

Nick England K4NYW
www.navy-radio.com


On Thu, Aug 12, 2021 at 3:38 AM Robert Downs via MRCA <mrca at mailman.qth.net>
wrote:

> 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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