AVCOM SDM-42A tuning/manual

[rfclown]

I just aquired an AVCOM SDM-42A Spectrum Display Monitor which turns out to be an 10.7 MHz spectrum analyzer of sorts. Google didn't turn up any manuals. AVCOM site doesn't show it (old instrument). I could post a question on their website, but haven't yet. I fed in 10.7 MHz from my sig gen (FM signal shown in picture). The fine tuning knob, input sensitivity knob, and center frequency trim adjustment all respond as expected. The span knob responds correctly, but... the center frequency moves alot with span adjustment.

The only reference I found to AVCOM manuals was a comment in this forum topic about them not being available.

This thing is made to visually monitor your radio's IF to see signals that you might be missing. I'd think that you'd want a center frequency that didn't vary with span for this to be useful. Any suggestions on finding help adjusting this thing before just opening it up?

Aaahhgg! After posting this, I went to open it up, and the center fastener on each side is a rivet.