[ARC5] R-10 in the receivers.

[AKLDGUY .]

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).

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.

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.

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.

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.

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

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

73 de Neil ZL1ANM



On Wed, Sep 24, 2014 at 12:49 PM, Kenneth G. Gordon <
kgordon2006 at frontier.com> wrote:

> A friend who is working on is first BC-453, which was murderfied before he
> got it, and which he is trying to get back in operation, has called my
> attention
> to R-10, a 360K 1/2 watt resistor which is connected between the
> gain-control line and B+.
>
> The table of replaceable parts describes that resistor as, "H.V. Bleeder to
> Gain Control."
>
> I am somewhat mystified concerning its true purpose.
>
> Can anyone enlighten both of us?
>
> Kenneth G. Gordon W7EKB
>
> "Courage is being scared to death but saddling up anyway."--- John   Wayne
>
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