[Milsurplus] Navy RAS receiver

[Ray Fantini]

A week or so ago I decided to remove the HRO RAS and correct a couple little problems, mostly stupid things like dirty pots and the like. Now that the receiver has been striped down, cleaned, had the gear drive disassembled and lubricated and a number of other test I have been trying to resolve just a couple little things. First is the issue of the Noise Limiter modification and the other issue is what I feel is a poor response curve for the AVC action  and what I consider to be poor performance in the receivers ability to deal with strong signals. 
First: The Noise Limiter Modification (CKB-50142) Looks like no one has the schematic or any information on this kit, thanks for those who sent what they have but apparently no one has the book that came with the kit so I have decided to reverse engineer it. Background, the CKB apparently is put in place of the detector AVC amplifier tube (6F8) that tube is a dual triode with one triode being an AVC amplifier and the second triode being a detector. The CKB has two tubes, a 6SN7 and a 6SQ7 the AVC part is simple being that pins 5, 6 and 8 from the receiver are carried to pins 4, 5 and 6 on the modifications 6SN7 so there are no changes to the AVC circuit. That leaves pins 2 and 7 that are the filament  on the receiver that are carried to pins 7 and 8 for the 6SN7 and 6SQ7, why are they the only two octal tubes I have run across that use 7 and 8 for the filament and not 2 and 7 like every other octal tube in the world?  That leaves pins 3 that was the plate of the detector and pin 4 that was the cathode of the detector. Pin 1 was never connected to anything to begin with and the output of the last IF was connected to the grid via the cap on top of the tube. The CKB kit has a transformer with a place to connect the IF output to, a little mock grid cap.  I am slowly tracing out the modification assembly but it’s a slow process not added by all the internal wire being tightly bundled and all the same color. One thing I know for cretin is that the circuit appears to be very similar to what was used in the HRO-5 series only simplified. Pin 3, the plate and audio output to the radio goes directly to pin 6 the plate of the 6SQ7 with pin 4 the cathode of the old detector being connected to the ground of the modification chassis and the common point of that assembly. The 6SQ7 functions as an audio amplifier with its two internal diodes being connected together to form the detector and the second triode of the 6SN7 having the grid and plate connected together to form a diode, the weird thing is there is only one diode with no apparent way of developing DC bias for the limiter.
 So here is a question, being there were no instructions with this when I got it I always assumed you just pop out the 6F8 and stuff it in the socket and go. But looking at the design and how everything in the modification is grounded to pin 4 and pin 4 in the radio is held high by the old cathode resistor R132 (25K) and bypass capacitor C134 (10uf) do they get removed and a ground jumper installed? There is a clamp with an attaching screw on the bottom of the modification that snug around the base of the tube shield and maybe that all that’s needed for a ground connection? Being that the tube shield is already grounded and the chassis on the modification is common maybe that’s all that’s needed? Other then that the limiter is working and definitely an improvement over the original circuit that produced painful static burst and other noises.

Second: Poor AVC performance, I assume that good AVC performance to be that when the AVC is on you would not be required to touch the RF gain control and when tuning across the band when going from strong to week stations the RF gain control will remain in the same place. This receiver exhibits AVC action but not a lot. Do not know if this is because there is an issue still not resolved in the AVC chain or if maybe that this is just a characteristic of the HRO series? All of the grid resistors are in tolerance and all of the grid bypass capacitors have been replaced. I do see AVC changes and action but maybe it’s just the nature of the RF gain control circuit in these receivers that limits the AVC window of correction? The only capacitor I have not replaced is one of those old six dot capacitors that couples the last IF transformer to the grid of the AVC amplifier (C140) don’t ask my why but for some reason when I changed out all the old wax capacitors I did not change any of the little six dot mica jobs, first because I don’t know how to read them and secondly am assuming that they never fail.

So far this has been a fun project, it amazes you sometimes when you pull a radio that you did years ago and go thru it again some of the things you find. I corrected a couple mistakes that I did years ago and improved the receiver’s performance a little. The RAS is just as sensitive as any of the radios I have and although it suffers the same image issues that all low IF receivers do find it to be a good radio. Admire its basic simplicity and ease of use and somehow reminds me of the BC-348 in that inherent goodness that was found in eight or nine tube signal conversion receivers.
Ray F.