[TMC] SBE-9 question

[Nick England]

So last night I went belowdecks (Navy radio gear is in the basement)
to see what was happening with this AM isssue and the carrier phase
shifter. Found the phase control but tweaking it just caused a
reduction in carrier insertion and no phase change - huh? So started
hooking up the scope to the components in that circuit with more
puzzling results. Ended up unsoldering the little add-on board and
found out that at some time the transformer had been replaced and
miswired. So I wired it back and now I get some shift but there is
sizable 250kc signal loss here - maybe I guessed wrong about the xfmr
leads. The replacement is a (homebrew?) toroid, where the manual has a
TMC pulse xfmr part number. Maybe it never would work as "repaired".
So time to do a little scrounging and head-scratching, as I do want to
try this SBT-1K out on AM even though as John's analysis points out it
won't be great. I'm still looking for an AN/FRT-24 (4-1000 modulated
by pair of 4-400s) for AM some day.

Another note - FWIW perusing manuals showed me that the SBE and SBE-2
have a different signal generation scheme from the later SBE models -
therefore different VMO/crystal frequencies. So be careful which
manual you're looking at!

your partner in boatanchory,
Nick K4NYW
www.navy-radio.com

On Tue, Jan 24, 2012 at 7:31 PM, John Vendely <jvendely at cfl.rr.com> wrote:
> The existence of the carrier phase shift network on the SBE-10 is
> interesting, another feature of which I was unaware, and which was not
> present on the SBE-3.  This would certainly improve AM operation, which
> was truly dismal on the SBE-2 and SBE-3.  However, even with this
> improvement I would not expect great AM, due to differential delays
> across the USB and LSB filter passbands which will upset the
> all-important phase relationships between the two sidebands.  This is
> certainly no way to produce high quality DSB-AM.  However, TMC was,
> after all, a "sideband company" and they undoubtedly considered AM to be
> very "passe", when these exciters came out.  The SBE series exciters
> were really aimed at the more important sideband market.
> Interestingly, TMC did AM "right" in the MMX-2 series exciters, which
> featured a separate DSB AM modulator to avoid this problem...
>
> 73,
>
> John K9WT
>