Hey there.
So I have an old PS3 here which im trying to fix right now. It was on rebug and broke while switching from dex/cex or cex/dex (its been a long time, I cant really remember tbh).
Everything sounds and beeps normal but im not getting any signal on the TV. I cant even access the recovery mode, after the 2 fast beeps nothing happens.

Is there any way I can still fix this old slim or should I sell it ?
I already tried this tutorial: You must login or register to view this content.
But my e3cr only stays red, not turning blue.

If you have an E3 Flasher. Give it a try, I think its your only chance. If you had a DEX to CEX semi-brick, the ps3 flashes red and beeps like 9 time if I remember correctly (when trying to powering on).

Like I said I already tried that and I cant get it to work.

All you need on the dongle is the "e3crd_rdr_service_mode" at the upgrade folder, right? So I did that, put the dongle in the right usb port of the slim, turned it on + pressed eject. Thats exactly what I did but the E3 card reader stays red and doesnt switch to blue nor beep like it should.

I don't know anything about the card reader, but..

Continuing to what Mitches was saying
If you have e3 flasher and original backups you should be able to fix it..
Since your ps3 doesn't stay on, you would need to solder the tristate to a ground to keep it powered on long enough to flash it. Then flash a good backup to the ps3 in fun flash mode.