[Yaesu] FT-900 with 'motorboat' audio on SSB

[David Willmore]

Hello, all.  Long time reader, infrequent poster. :)

I have an FT-900 which I bought from my elmer a long time back with the
intention of using it as a general coverage receiver--I was a no-code
tech at the time and had no HF TX privledges.  So, it's served that role
just fine for a long time.  Now, I've upgraded and I'm trying to use it
as an actual tranceiver.  But, I've got a problem.  When I put a receiver
next to it, and transmit in SSB, I have an odd modulation on top of my
voice.  It sounds like a 5-10Hz or so AM modulation--sort of a wavering.
I'm going to run the RX into a spectrum plot so I can better see what
the issue is, but I'm stumped as to what may cause this problem.

I tried FM and AM modulations and they work fine.  That may help to
eliminate possible causes.

Does anyone have an idea what may be the issue and/or what I should
do to further diagnose the problem?

Cheers,
David n0ymv