[TMC] SBE-9 question

[Nick England]

Hi gang -
I've just started to work on getting my SBT-1K operational. I have an
SBE-9 exciter for it and today had a little head-scratching fun
tracking down some hum in the USB audio path (re-tightening ground lug
screws fixed 95% of it). Note to self: do this throughout these TMC
units.

But in the process I noticed a difference betwen my unit and the
schematic I was using -
On the inputs to audio amps V-122A and V-123A there evidently was a
field change to add a 25:1 voltage divider (8200 ohm series & 330 ohm
shunt to ground) . It does not show up in the Army AN/URA-28 TM or in
the original SBE-3 and SBE-9 manuals on K4OZY's web site, but it does
show up in the field changes in the front of his SBE-9 manual. Well,
it shows up on a conveniently undated schematic there but not in the
Field Change instruction sheets.

In my SBE-9 someone didn't like the audio gain reduction this gives
(or there was a later FC I don't know about) and they cut the 330 ohm
ground lead, essentially bypasssing the voltage divider.

Does anyone know why this mod was done or have any other info on it?
There is something odd about such a drastic change in AF input level.
The other thing I have noticed so far is that the alinement (it's an
Army TM) procedure I'm using calls for a 0.05v audio line input
signal, but that really overdrove the USB and LSB sections. Like maybe
that 330 resistor needs to be hooked back up. But that manual's
schematic doesn't show the voltage divider so maybe the field change
did two things - gave more stage gain somewhere else but attenuated
the input signal.

I'm just guessing in the dark here and it really won't matter to
getting this rig running, but I always like to understand why/what is
going on when I come across modifications.

OK, ramble mode off - if you were here, I'd just point underneath the
chassis and show you the scope.
cheers,
Nick K4NYW
www.navy-radio.com