Here is a guide from dospiedras1973, roughly translated: elotrolado.net/hilo_downgrade-3-7x-para-slim-y-fat-s-con-nor-flash_1659475
PS3 3.7x Downgrade for Slims and Fats with NOR Flash
Hello all, I got it at the end we have a downgrader for slim consoles this time is different than it has done with consoles fat, I've got the two flashers progskeet Teensy++ and Al nougat!
We need:First dumped our NOR flash, the file must accurately measure "16,777,216 bytes" neither more nor less. Draw out several to be absolutely sure of what you do.. cojemos the dump "example josejuan.bin" and we have to introduce it in a file flowrebuilder to make it readable (bytereverse) the option is called bytereverse dump and extract rules. what we do and we will create a file but the extension ends in. bin.REV
Open it with the hxd and took our personal data to the console EID, BOOTLOADER, CSID and METLDR no need to put any more. take them out as follows:
Choose for example in this case to stick our METLDR prepatched to downgrade the image attached to this tutorial
Inside the folder where we put the flowrebuilder. rev also has created another folder called "nombredeldump.EXT" then there are our personal archives of our console and we need to introduce them to catch some pre-patched image that I have since opened the hxd and open the file and metldr downgrade.bin is inside the folder asecure_loader.
Choose the tab on the hxd metldr and copy the entire contents into HEX and enter the downgrade.bin and press control + 820 g write position is that of METLDR. right-click on the first line of position 820 to give to "stick type" and so the same way we introduce others to enter the files are:- METLDR: Position "820" size "EDE0" (Note: Correction by dospiedras1973: Correct Offset "810" size "E960")
- BOOTLOADER_0 Position "FC0000" size "40000"
- EID: Position "2F000" size "10000"
- CISD: Position "3F000" size "800"
Then we take the downgrade.bin with saved changes and we put in the option flowrebuilder bytereverse dump and extract rules PROGRAM THIS TIME WE WILL GIVE AN ERROR but is a normal error, if error occurs is that okay done and you create a file named downgrade.bin.REV will be the file you have to enter in the console "flash"
If all went well to finish writing you kindle the console and will press the button or English ps push button NO NOTHING PULSE IS apagais the console and put it in factory service mode once this need to put the correct file system for 3.55 jaicrab with lv2diag of cfw without reader and a special:Kindle the console with the stick with these two files on the usb device into the far right of the console and shut down one to 10 / 15 minutes, kindle the console without any usb connected to verify that you correctly leads to xmb If all went well apagais the console and you put your lv2diag FILE2 of this pack: http://www.mediafire.com/?b07qrb96iz99ibb
The console will turn on and 20 seconds will turn itself off and CONGRATULATIONS you have your console in functional kmeaw cfw 3.55 100%
Thanks:
DiGiTaLAnGeL (Tester with progskeet)
Glevand & mfw builder team (cfw)
NDT (Assistant) is a very good person 
JaiCraB (lv2diag without reader)
Robs1 (my guide with the normal flash)
EussNL (his great support in the wiki that I use every day PS3DevWiki.com)
Defyboy (to create PS3DevWiki)
A whole channel #irc-hispano.org darkps3 for their support and many hours of testing we have hit eh!
DemonHades (because if you had not put the cover on your website with the lie that contastes on me, I had not met or uf6667 DigitalAngel and these two helped me a lot)
Download: PS3 Dospiedras1973 Downgrade Pack / PS3 Transplante v1
From dospiedras1973: by psicoleo, uf6667 and I have made an application (PS3 Transplante v1 above) to automate the use hex editors to modify the downgrade.bin bytereversed you put your flash in donor and recipient and will downgrade.bin in the process hxd without... 
you not need hex edit anymore... and "If at any time during the downgrade DISPLAY RED (RSOD) STOP PROCESS IMMEDIATELY I WILL RELEASE A FIX SOON" Please be patient
PS3 Models: NAND 256MB / VFLASH 16MB (NOR)
PS3 Fat:- CECHA = 256MB
- CECHB = 256MB
- CECHC = 256MB
- CECHE = 256MB
- CECHG = 256MB
- CECHH = 16MB
- CECHJ = 16MB
- CECHK = 16MB
- CECHL = 16MB
- CECHM = 16MB
- CECHP = 16MB
- CECHQ = 16MB
PS3 Slim:- CECH-20..A = 16MB
- CECH-20..B = 16MB
- CECH-21..A = 16MB
- CECH-21..B = 16MB
- CECH-250.A = 16MB
- CECH-250.B = 16MB
- CECH-250.A = 16MB
- CECH-250.B = 16MB
- CECH-251.A = 16MB
- CECH-251.B = 16MB
Update: New Method - PS3 Downgrade v2 Supports all NOR Flash via TwoStone: darkconsoles.com/foro/viewtopic.php?f=7&t=16
Download: PS3 Downgrade v2 / PS3 3.70 to 3.55 Downgradable Model List / Progskeet QT Flasher for PS3 Test and Localized Versions / ProgSkeet PS3 NOR Downgrade v2 Patcher
I updated entire patch method with new v2 with some fixes, now be compatible with all consoles except 2504b DATECODE 1B and 3000 consoles. Write a new method consisting of 6 patches, this time we will use to edit the dump hxd original of your PS3 (make backup)
Steps:
First check if our standards Dump bytereversed, to start patching the original dump.bin file we must first ensure that the beginning of the flash at offset 200 is readable "IFI" if instead you see "FI.I "Your rules need to patch it bytereverse before (a and go back to him to put it in bytereverse the console to be read again" FI.I "when we patched the files and offset's a patch are these
Code:
patch1: OFFSET 0C0010
Patch2: OFFSET 7C0010
patch3: OFFSET 80000
patch4: OFFSET A0000
Patch5: OFFSET 40000
patch6: OFFSET 60000
Easy, normal and cojemos the patch1 for example, copy content and paste it patch1 0C0010 writing and so the rest (the file size does not vary after the stripe)
The rest of the downgrade is the same as before, factory mode, and pup jaicrab lv2diag nochecks.pup of the previous release.
Originally Posted by EiKii
This method doesn't need a dump from 3.55, nor do the other downgrade methods, and for 3.56 service mode, most likely yes, as you patch 3.55 cos and then are able to leave service mode
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PS3 3.7x Downgrade for Slims and Fats with NOR Flash 
