[TMC] SBE-9 question

[Sheldon Daitch]

Nick,

The change adding that voltage divider appears also to be associated with
the change in the T101 and T102 secondary loading resistors from 560 ohms to
22K ohms.  Interestingly enough, sheet 2 of the change notice calls for 
R101 and
R102 to be 2.2 megs, while the schematic calls for 22K ohms at those points.

Also note that the two gain pots (R168 and R169) were changed from 5000 
ohms
to 250K ohms.

This change might not have any impact on the line input levels needed to 
produce
a certain RF output in the SSB mode, but the audio test procedure using 
the 0.05V
calls for the test audio into the mike input, right?.

The higher resistance pots would not load down the output from the V101 
mike
amplifier and that would require require some attenuation at the grids 
of V122A
and V123A, to keep all the levels about right.

Looks like with the changes in R101/R102, R168/R168 and the addition of the
R289/R290 and related R291/R292 voltage divider, it would seem the line 
levels
would pretty much remain the same for a given SSB RF output, but it 
certainly
seems to change the way the mike input levels would work.

As to why the change?  Maybe a bargain on 250K pots?  The sales volume 
of exciters
doesn't seem to warrant the addition of four resistors - the trade off 
in the cost of the
pots vs the additional resistors, so economics might not be the 
explanation.

Another comparison is in the specs between the SBE-9 specs and the SBE-3
audio line input levels is that the -3 just has the lower limit (-20dBm) 
for full RF
while the spec with the -9 also has a high input spec (+10 dBm), so 
perhaps the
changes were associated with this spec change in some way.

What do you think?

73
Sheldon




On 1/21/2012 1:58 AM, Nick England wrote:
> 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
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