I own a lowrance hds 7 and it doesn't power on. Have any ideas what to do? Where to send for repair?

Question edited for clarity.

The first thing to check is the fuse to the unit. The could be a fuse block for accessories on the boat, or it could be an inline fuse to the unit itself or both.

The pull all the plugs from the rear of the unit and check for corrosion. Spray with WD40 and replace the plugs carefully and tighten the fixing screws evenly.

If that doesn't fix it, it may be the transducer, but you should still get a picture for chartplotting. There is no way to check them for correct operation, but try this

https://sportfishingbuddy.com/how-to-tell-if-transducer-is-bad/

so if all that fails, you will need to return it for repair. There are some troubleshooting ideas here. This easy fix is on the Lowrance site

*Screen goes dark?: If your display turns on, shows the blue Lowrance screen, and then goes completely dark, most likely you have accidently shut your backlight off while powering off your unit.
To resolve this problem, once the unit is powered on and the screen is dark, simply press the "Power" button a few times until the screen brightens back up. Do not hold the "Power" button down, just quickly press and release the button.
To prevent this from happening again, when powering off your display, simply press the "Power" button one time and then select "Power Off." This will save all settings and features and safely shut down your display.

https://www.lowrance.com/help-and-support/?guideTitle=Display-Problems&guideId=003-043

https://www.google.com/search?q=lowrance+HDS+12+repair

..

Got you covered.
Here is the link to your owners manual.
Hope this helps.
-Steve
http://www.lowrance.com/Root/Lowrance-Documents/CA/HDS-5-7-5M-7M_OM_EN_988-10041-001_w.pdf

Lowrance has started using the same power cord on their Elite units and their HDS units. The yellow wire on the power cable is an accessory wake up wire designed to be used in conjunction with the structure scan modules for the HDS.
You can simply tape this wire off during your installation ND ignore it.

most the time its a bad key pad when unit wont power on and what it cost to repair you can buy another one check the pins in the power cord see if there not broke or bent yes they break take a 12 volt test light see if you have power going to power in the plug that connects to back of unit if you have power . . .bad key pad

The manual says 10 AMP on page 29, but lowrance says to use 3 amp. I have fried 2 hds-5's using a 7 amp which made me call lowrance of course they said that the documentation is in correct. Probably not the the thing to be incorrect about.

My HDS gen 2 touch did this and I decided to open it up since Lowrance wouldn't help (out of warranty 2yr). The ribbon cable from the motherboard to the display came loose. Connected it back up and now I'm in business

You can download the user manual for this unit from this links.

http://www.lowrance.com/Downloads/Manuals/

http://safemanuals.com/user-guide-instructions-owner-manual/LOWRANCE/HDS-5-_E

http://www.retrevo.com/support/Lowrance-HDS-5-GPS-manual/id/23603ag817/t/2/

Make sure it is completely powering down and you are not just dimming the brightness.
Are you powering off by holding down the power button until shutdown, or are you choosing power off from the popup menu and pressing ENT. Go with pressing ENT on the power off option. If you are completly powering it down for sure, there is probably a malfunction and you'll need to get it taken care of.

Unplug the power cable, and then plug it back in...does the HDS-7 turn on by itself now? If yes, there is a probably short in the keypad, likely due to water intrusion. You'll need to contact Lowrance support at 800-324-1356 or you can email them by going to http://www.lowrance.com/en/Support/Product-Support/Contact/ to get it taken care of.
Give them your serial number and name and address and phone number in the email to hopefully speed up the process.

You need to purchase a EP 60 fuel flow meter. Once you have it installed in your boat, you need to monitor it. You set it up under NMEA DEVICES on your setup menu,

You may just need to update the softwear on the GPS, you can download it directly from Lowrance.com along with instructions on how to install it.