[MRCA] HRO RAS AVC issues

[Ray Fantini]

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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