Hello, everyone.
I'm new to this forum and new to working with cell phones.
I have (rather - created) a problem - my former Sprint Galaxy S4 (L720T) probably bricked: the phone shows "Firmware upgrade encountered an issue, please select recovery mode..."
This happened after I tried to downgrade from OA3 firmware to MK5.
I can still get to download mode by pressing Vol-/"center button"/power and start Odin (1.82 or 3.07 or 3.09) and start flashing (thru PDA), but the process freezes and Odin stops responding.
what led to it?
I'll try to explain everything I did before.

I have two phones (galaxy s4 sprint) and for both I obtained MSL 6-digit numbers (from sprint).
I unlocked first phone (which had ...MK5 version) to use over GSM network: it was rooted, I added superSU, Titanium backup, went through *#0011# command and changed settings (found on this forum), virtual terminal (did something through it, too) and, then, "applied" SSU.apk to actually unlock the phone. It didn't go as fast and smooth, because some things did not go as described in the tutorials and instructions. The phone works.
The second phone has ...OA3 version and I did as much (and relatively in the same order) work as with the first one. EXEPT, SSU.apk did not unlock the phone in this particular version (it was designed for MK5).
So, I thought that downgrading from ...OA3 (Android 4.4.2) to ...MK5 Android 4.3) is a sound solution. and I tried, using Odin(s) and file "L720TVPUAMK5_L720TSPTAMK5_L720TVPUAMK5_HOME.tar.m d5" (through PDA button in Odin.)

The odin took long time to process it and it seemed to finish the job (I really don't remember what exactly happened that time) and I either turned off the phone or unplugged usb cord, and remember seeing blue "status bar" on the phone below the logo and message below it.
I realized that I screwed up after seeing the "dreadfull" message: Firmware upgrade encountered an issue, please select recovery mode...
I turned on phone in download mode and used odin (versions 1.85, 33.07, 3.09) to try to download firmware (MK5) ( I used buttons "bootload", PDA) to no avail.
I also tried to get into recovery mode (Vol+/center button/power) - nothing happened.
Now I'm stuck - I don't know how to re-install the original firmware (MK5) on this phone (or OA3).
I was looking for a solution (and still looking), but I found no answer so far.
Thus, I turned to you, knowledgeable people, to help me and lead in (point to) the right direction.
I know almost nothing about Android programming and the smart phones. I know veeery little about Linux (for PC - as an end user), so bear this in mind when you reply.
Thank you in advance.
Meanwhile I'll keep looking for solution anyway.