[ARC5] R-10 in the receivers.

[Kenneth G. Gordon]

On 24 Sep 2014 at 14:21, AKLDGUY . wrote:

> The cathode resistors of the RF amplifier and first IF amplifier (R-1 and R-9
> respectively, both 620 ohms) are not grounded, but are brought out to the
> "Gain Control Line" which goes to both the rear connector (pin 3) and the front
> connector (pin 1).

Yup.
 
> The line is decoupled with two capacitors, 3uF (C-5) and 0.05uF (C-6B).
> It is obviously intended that a gain control resistor or pot can be remotely
> connected to set or vary the receiver's gain.

Yup.

> R-10 feeds the B+ to the line so that when the pot is at anything other than
> zero resistance, the gain varies much more sharply than it would if R-10 were
> not present.

Ah! I didn't know that.

> It works like this: With the pot at zero resistance, DC via R-10 is shorted
> and only the cathode resistors are in circuit and gain is maximum. As the pot
> is rotated towards the other extreme, not only does more resistance appear in
> the cathode circuit but the increasing positive voltage appearing at
> the voltage divider junction of R-10 and the pot tends to bias off the tubes
> more quickly.

OK. Makes sense.

> The 3uF capacitor is large enough to absorb voltage jumps caused by the sliding
> contact in the pot, and smoothes out any such variations. I understand it has a
> working voltage of 25V.

No. According to the Table of Replacable Parts in ALL the manuals, it is 3 
ufd 300 VDC. I have never yet found one of those to be bad either, which is 
another mystery to me.

I have always called that capacitor "the RF/IF-gain buffer capacitor", which 
works to limit the noise from bad or scratchy controls.

> The usual mod is to remove R-10 and ground the cathode resistors/Gain Control
> Line, allowing the receiver gain to run wide open.

The Surplus Destruction Manuals say to do that ONLY to the RF stage. I 
won't do that at all.

> That is probably justified
> for an amateur using a poor antenna, but for those using a really good, well
> placed antenna, R-10 should probably be retained along with some value of
> gain-setting resistor or preferably, pot.

Yes. I use a 50 K log-taper (audio taper) pot for the RF/IF gain control. That 
seems to give a much smoother control operation: all the gain doesn't 
suddenly appear near one end of rotation, but is spread out a bit more.

> The resistances of R-10 (360K) and the associated pot have not been arbitrarily
> chosen.

Oh, undoubtedly NOT. As I have insisted for many years, the designers of 
the ARC-5 receivers were sheer geniuses. I have yet to find anything they 
DIDN'T think of.

Thank you, Neil.

Ken W7EKB