[K2] AF Gain Control

[KBG Luxford]

Hi Everybody,

I am updating K2 S/N 01762 from A to B prior to installing options.  My
questions regard the AF gain control.  The AF gain control has no effect
whatever on the level of signal in my headphones.  No, I am listening to
signals on the ham bands, not just the CW sidetone which I understand is
at constant volume.  Was there a change to the AF gain control circuitry
between the Rev A RF board and the post S/N 3000 B boards?  Will the
change to the AF circuitry that I have yet to make fix this problem?

Thanks in advance,
Kevin
VK3DAP / ZL2DAP
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[David Woolley (E.L)]

KBG Luxford wrote:

> at constant volume.  Was there a change to the AF gain control circuitry
> between the Rev A RF board and the post S/N 3000 B boards?  Will the
> change to the AF circuitry that I have yet to make fix this problem?
>

I'm surprised you haven't had an official reply.

Looking at the circuit diagrams, assuming that the rework fully produces
the new design, there was a significant change.  The original just had a
variable resistance across the inverting and non-inverting inputs; I'm
not sure what forms the top of the potential divider.  The result is
fully balanced.  The current revision has the one input of the amplifier
connected to the counter-clockwise end of the control, but the other
connected to the slider.  An extra pin (#3), on P1, carries that signal.
  The current version is slightly more conventional, but only balanced
at full volume.

If the front panel changes were made without any corresponding
motherboard ones, you would get constant volume, because the control
slider would no longer be connected to the clockwise end.

--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
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[KBG Luxford]

Problem solved (I think)

I believe that the front panel PCB underwent a change somewhere along
the line - incorporating the changes recommended to the AF gain control.
  The K2 I am working on has a low serial number - 01762.  However, the
original owner who was building it struck trouble with the front panel.
  When I got it, I tried repairing it, but gave it up as a bad job and
got a new front panel kit from Elecraft.  On plugging in this (new)
front panel, I found I could not vary the AF gain.  I swapped the front
panel for another from my other, post S/N3000 K2, with exactly the same
result - no variation in AF gain.

I have proceeded with the A to B mods and have completed the AF gain
changes on the control board and also the RF board.  When I came to the
front panel, it appeared as if the recommended mods had already been
applied.  So I buttoned up the K2, applied power, and voila, the AF gain
control worked perfectly.

Thanks, David, for your explanation of what was going on.

73
Kevin
VK3DAP / ZL2DAP


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