I asked this in another forum so just copying and pasting here:

 

Hi guys,

I obtained a used Helix 9 Mega SI from ebay but I'm having issues with Mega and high frequency imaging. Sorry for the glare-y picture, it was the best I could do. Essentially Mega imaging has a very dim almost non existent picture. And non Mega imaging also has some issues. 

The bottom half is when I have Mega imaging on for DI and SI. The top is when I switch it to 800khz and below. I have to switch both DI and SI to 800khz from Mega. Also, you can see the right side is noticeably dimmer. Has anyone encountered this? It's not a power issue as I tried my bow power source which works. It's not a software issue as I had just did a software update per humminbird instructions. The transducer has some nicks and scratches on the right side but the seller assured me it was working perfectly. The seller has a 100% rating on ebay so I don't think he's trying to pull a fast one on me. Before I send it to Humminbird for repair I was hoping one of you guys could provide some insight.

Make absolutely sure the transducer is pushed home fully into the back of the unit, might just be a connection issue. If you know anyone else with a working unit try plugging your head into their transducer and power lead and see whether that improves it, could be the transducer being partially broken. If it doesn't work on someone else's set up, then it's likely the head unit. Try running it with your motor trimmed up a bit too, sometimes the lower unit gets in the way if the sonar beam, but as you don't seem to have a down image I doubt that's the issue.

Unfortunately I don't have a friend that has a working Mega SI unit. However, I wonder if I could plug in my Mega DI transducer to my Mega SI head. That shouldn't break anything right? 

As long as the plug is the same you should be OK. You might have to tell the unit which transducer you have, not sure.

Don't hesitate to use the Humminbird e-mail customer service.  They have been very good and fast for me.