Can't connect to fortigate console via rs232

[Chuuung]

I recently got fortigate 3140b on market, and first i tried to connect with management port(lan) but it not worked (maybe ex-admin changed it?) so next i tried to connect with rs232 console port with putty (

9600 8N1 No Flow Control) and it shows text well(like bootup bios message, login message) but the tx (which i press on keyboard) not sends well. if i type 'a' to keyboard, i must enter 'a' more than 5 times and eventually i can't access it. i asked seller if he changed serial settings but he says he doesn't know.

How can i access this fortigate? or reset it?

[AlexC-FTNT]

Ideally, the first thing to do after such a purchase is a clean install (format Boot device, install new firmware). You can also try to run a hardware test, maybe the unit is damaged.

[Chuuung]

i really want to format this device, but without accessing cli it's impossible to do that .. maybe serial hardware damaged?

[ede_pfau]

Maybe you should try all available serial speed settings

9600, 19200, 38400, 57600, 115200

to be able to access the boot menu. There, you can set the serial transmission parameters. Note that they will only take effect after rebooting.

* https://community.fortinet.com/t5/FortiGate/Technical-Tip-How-to-change-the-baud-rate-of-the-FortiGa...

I wonder if you can get access using the default credentials then...

[Chuuung]

actually i pressed keyboard many-many times and successfully changed baudrate in bios to 9600, but same issues

[AlexC-FTNT]

Have you tried to leave the unit powered on for 10-20min before connecting to the console? Maybe it needs time to load up everything, as it is quite a big and slow device.

If you know the status of the unit or have the previous configuration, you can try to connect via SSH to one of its regular ports (where SSH is allowed). In this case you need the logon information. From there, you can check the flash, boot from another partition, or even remove some of the configuration. 
The console port should not be slow as you mention, but if there's a hardware failure, I doubt it is related to the port itself, so you may experience other strange behaviors.