[K3] RF Main assembly VFWD & VREFL trace cut mod?

[Bob Cunnings NW8L]

I'm about to install the REMIOUPDG replacement KIO3 digital i/o
assembly. The instruction sheet (Rev B) includes a step which involves
cutting 2 traces on the main RF board leading to pins 11 and 12 on
P51, VFWD and VREFL. The instructions state that this "ensures the
digital-to-analog converters (DACs) always receive the proper signal
levels". A look at the schematic indicates that this mod is breaking
the connection between these nets and U6 (analog mux) on the RF board.
These signals seem to be the forward and reflected power outputs of
the SWR bridge and are inputs to U8, a ADC78H90C ADC on the DSP board.
So the instruction should perhaps read "analog-to-digital converters
(ADCs)" unless I'm missing something.

Indeed, the latest schematic warns "Vfwd/Vrefl traces @P51 must be cut
on rev A PCB".

Is the purpose simply to prevent saturation of the ADC input an thus
inaccurate Power/SWR computations?
Or is there more the story?
What happens if it's not done?

Just curious, I've been running these 2 K3 for over 2 years now
without apparent trouble, and this change appears to have a purpose
independent of the main purpose of the REMIOUPDG update (negative
going ALC and band data output pullup mods).

Bob NW8L
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[KK7P]

Yes

> Is the purpose simply to prevent saturation of the ADC input...

73,

Lyle KK7P/7
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[Bob Cunnings NW8L]

Thanks. In the absence of any mention of the trouble indication being
addressed, I will assume the mod prevents problems with a feedback
system such as KAT3 tuning, SWR protection for the finals, power
control etc. which rely on the integrity of these signals. It's nice
to have an understanding of the operational benefit of the mod before
taking an Exacto knife to PCB traces.

Bob NW8L

On Sat, Jul 10, 2010 at 3:01 PM, lyle johnson <[hidden email]> wrote:
> Yes
>
>> Is the purpose simply to prevent saturation of the ADC input...
>
> 73,
>
> Lyle KK7P/7
>
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html