[Milsurplus] [ARC5] AN/ARC-2A Available Soon- Dallas Area

Tue May 1 02:27:27 EDT 2018

HueIf the ARR-15 receiver oscillated, it was most likely due to missing aluminum shields over the components of either the RF amp or the Mixer or both.  There is a lot of gain in close quarters and the receiver will not work with missing shields.  See page 5-4, Preliminary Trouble Locating Chart, defective shields are mentioned as a possible cause of receiver oscillations.  
The first ARR-15 I worked on had both of these shields missing and it was prone to oscillating.  When I was flipping the radio over on the bench, my hands provided enough shielding to stop the feedback.  I then found some thin aluminum sheet and fabricated two covers and the problem went away.  See Figure 6-5 on page 6-6 of the ARR-15A manual for shield location.
As for IF transformer coupling, it is by way of C-183 (5 pF), C-181 (8 pF) and C-178 (5 pF) only; there is no magnetic transformer coupling in the IFT. See page 4-14 for a description of the IF circuit.  Changing these three coupling caps to 1 pF or less changed the IF passband from around 12kc to something close to 4kc.  Finding silver mica caps below 1pF is a problem, this is why I mentioned gimmick capacitors earlier.
I am fond of the ARR-15, it is very unique.  You can see the heritage handed down to later Collins designs that started in the ARR-15, the first tunable Collins radio receiver, IIRC.Jim

      From: Hubert Miller <Kargo_cult at msn.com>
 To: "milsurplus at mailman.qth.net" <milsurplus at mailman.qth.net> 
 Sent: Monday, April 30, 2018 11:58 PM
 Subject: Re: [Milsurplus] [ARC5] AN/ARC-2A Available Soon- Dallas Area

Many years ago I owned an ARR-15 and I clipped out the coupling capacitors. The result was that the IF "took off" on its own.
I traded that receiver off and so did not follow up and try to control the feedback. 
Roy Paffenberg ( sp ? ) in his '73 Magazine' article and also booklet on the ARC-2, I think titled "A Modern Transceiver in a WW2
Package" ( people's ideas vary on the "modern" ), recommends the feedback wire to boost selectivity. I think some readers here
are for that idea. I tried it on the 6 - 9 MHz and I can say, yeah, "it works", but it is a pain to use and undesirable. The IF gain 
control then should really be called "Gain & Selectivity" all in one control. Can you see any potential bother in that? Not only that,
but if you look at regenerative receiver design, for smooth control, no one uses a grid-plate feedback capacitor. A Q multiplier,
external, is vastly superior. Of course, in these 2 receivers, the IF is variable. I have suggested a 2-terminal oscillator circuit built
right into the receiver and externally controlled by a DC pot. One example is shown in Pat Hawker's 'Amateur Radio Techniques'
book; this one employs a dual triode, but I think 2 MOSFETs preferable. I think very loosely coupled to the variable IF tuned circuit
thru something like 2-3 pF max, it would not disrupt the tracking that much to render it unworkable. I can't try this myself at the 
moment due to other priorities.
As for the regenerative IF idea, I think if you look at a bunch of reviews of Knight's 'Star Roamer' and Halli SX-140, you'll see some 
reviews less than ecstatic about how it works out.
-Hue 

-----Original Message-----
From: Bruce Gentry 
Sent: Monday, April 30, 2018 9:35 PM
To: Hubert Miller 
Subject: Re: [Milsurplus] [ARC5] AN/ARC-2A Available Soon- Dallas Area

I would like to add a few comments. I have both ARC-2 and ARR-15, and the recivers have quite wide selectivety, I just deal with it. The final IF is tunable, sort of like using a broadcast band TRF receiver after a short wave converter. Coupling between the coils in the IF transformers is both inductive and capacitive. This is probably to keep gain and bandpass reasonably even across the tuning range of the IF.  The way I have heard most often to improve selectivety is to make one of the IF stages regenerative. Some people have used "gimmick" condensors between plate and control grid, trimming them carefully to prevent the stage from breaking into oscillation. All of this reminded me of a method used by some low price receiver builders to improve "Q" and selectivety, as well as provide a makeshift BFO. A variable resistor  is inserted between the screen grid bypass condensor on one of the IF stages and ground. With no resistance between the condensor and ground, bypassing is 
 fully effective.  Adding resistance gradually "lifts" the ground, and allows the tube to approach  or break into oscillation as desired. Has anyone tried this with the ARC-2 or ARR-15? The warnings about delicate wiring are well founded, it is easy to fry a lot without knowing and difficult to find and repair.

      Bruce Gentry, KA2IVY

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