Samsung Galaxy US Cellular R530U can/will brick while on 4.3 MJA if tried downgrading by Odin or recovery or by flashable zip MB1/MD4 firmware by SD card. Firmwares contain sbl1, sbl2, tbz and some other components fatal to the bootloader. The handset will be dead completely leading to the only option of JTAG Riffbox. It is also seen than pin-outs of JTAG to R530 are not easily mappable. Here is an emergency restore to boot into download mode by which you can re-flash the stock R530uvxubmja tar odin file.
Things needed are:-
1. A 16GB class 10 SD card (for 16GB handsets, 32GB for 32GB handsets), a 16GB class 4 of Transcend or Sandisk also works..Tested !!
2. Download this file in a folder http://www.4shared.com/zip/l6N9muH_ce/win32diskimager-v09-binary.html
3. Insert the SD card into the PC, we are making image partitions, SD can get corrupt or unusable, card reader will get damaged.
4. Download this resurrection file extracted by JTAG https://app.box.com/SCH-R530U (a 32GB JTAG mmblk0 file will be uploaded soon by me
5. Open the unzipped application of the extracted folder and select the JTAG bin file and write to the SD card
6. Remove the SD card and put it in the R530, remove the battery, insert a good power adapter, re-attach the battery
7. Phone will start charging in off state, get into download mode and download this tar file http://upload.teamuscellular.com/5D1?d=1
8. Your phone is saved
Edited:- more details added.