Polarity reversed to Ktag... can you fix it?

[Makx]

Thank you for the precious tips. also the certainty that the red led should light up is very helpful.
The diodes I checked are mounted on the card, and I have the impression that (unfortunately) are not skipped.. this is a marked B4 (equivalent ss0540) and read ET GP 518 (but I'm not finding any matches for this code).. I will make other tests on these to see if I turn on the led but I'm still looking for a datasheet and not do other damage bypassandoli before taking them off.
meanwhile, thanks, and I update if I have news

[alex210392]

Thank you for the precious tips. also the certainty that the red led should light up is very helpful.
The diodes I checked are mounted on the card, and I have the impression that (unfortunately) are not skipped.. this is a marked B4 (equivalent ss0540) and read ET GP 518 (but I'm not finding any matches for this code).. I will make other tests on these to see if I turn on the led but I'm still looking for a datasheet and not do other damage bypassandoli before taking them off.
meanwhile, thanks, and I update if I have news

Check that there is voltage on the diodes, could also be Zener diodes, you can try to dissaldarli in order to understand if they are there for protection of reverse polarity or to straighten the input voltage, if you have them for the first cause must operate dissaldandoli if the second cause does not change anything