[TMC] SBE-3 & SBE-6 on AM

[Gary Schafer]

Hi Duncan,

I briefly looked at the manuals on the SBE3 and SBE9. The 3 has 17 KC
filters and the 9 has 250 KC filters and I see the carrier phase shift
network in the 9 and it says that it is used to shift the carrier 90
degrees.

I am thinking that the SBE3 with the 17 KC filters end up with little phase
shift of the carrier to audio so that the reinserted carrier is close to
correct where as the 250 KC filters give more shift and the need for
correction when AM is used?

The carrier needs to be in phase with the reference for the DSB signal or
you wind up with an FM signal rather than an AM signal.

On SSB with carrier inserted, it does not matter if the phase is of because
the other side band is not there to cause any cancelation.

When adjusting the carrier level on AM, the meter is a voltmeter rather than
a power meter so 50% voltage level is correct as they say in the manual as
50% voltage is 25% power level.
The 10% carrier levels that they talk about are used for reduced carrier
(-20db) for phase locking a receiver to the transmitted signal.

So if you insert full carrier and adjust the meter for 100 then reducing the
carrier to 50% should give you correct carrier level. Then the audio peaks
should barley hit 100 with modulation.

Be sure that the side band channel switches are both set to channel 1 or
both to channel two, which ever audio channel you are using. Of course the
side band selector should be set to DSB (AM).
This should give you a good AM signal out and you use the output level
control to adjust the drive to the final amplifier which of course needs to
be tuned up, with full carrier inserted, for maximum output. Reducing the
drive to keep levels where they should be.

73
Gary  K4FMX


> -----Original Message-----
> From: tmc-bounces at mailman.qth.net [mailto:tmc-bounces at mailman.qth.net]
> On Behalf Of Duncan Brown
> Sent: Sunday, March 02, 2014 4:16 PM
> To: tmc at mailman.qth.net
> Subject: Re: [TMC] SBE-3 & SBE-6 on AM
> 
> 
> On 01-Mar-14 10:49, John Vendely wrote:
> > Make sure the sideband filters are in good shape.  I've found these
> > old crystal lattice filters (made for TMC by Bulova Electronics), have
> > often aged considerably, and are well out of their original specs.
> > They often suffer from severe passband ripple and poor opposite
> > sideband rejection.
> 
> I did a quick check on the SSB filters in the SBE-6 by manually sweeping
> the audio input frequency from a HP generator and watching the received
> signal level on my Elecraft K3/P3 on 75M.  Both passbands were flat
> within ~2db (resolution of the P3)  out to 4 Khz and unwanted sideband
> rejection was > 55db below 2 Khz and >50db from 2-4 Khz.  (I didn't
> bother looking above 4 Khz.)
> 
> So the SSB filters are probably OK (no big holes or spikes in the
> response!)
> 
> Our original desire to make the SBE-6 work on AM was only because we
> thought it should! Now that we know that it is not supposed to work well
> on AM, we won't expect it to or try to modify it from its original
> specs. (The Museum has other, real, AM transmitters.) As a museum, our
> goal is to preserve equipment (and get it operating as it originally
> did, when possible).  We don't modify equipment just to make it work
> better unless it was an official factory modification.
> 
> That said, I think I will try throwing together the carrier phase shift
> network, just to see what it does for my own curiosity.
> 
> Thanks for everyone's help & comments.
> 
> 73,
> 
> Duncan Brown, K2OEQ
> USASA  31J
> 
> Antique Wireless Association Museum Asst. Curator, Commercial Equipment
> (also Chief TTY operator & repairman)
> http://www.antiquewireless.org/
> 
> 
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