Hi Roque,
The reason I suggested Flip2.4.6 is that an earlier version of Flip does not work for the AC3 and this does.
Below are some check points which come to mind and I am sure you would have checked, as you have already used the AC2 successfully:

1. EA* is pulled high
2. VCC is stable and clean
3. PSEN* is held LOW well after RESET is released.
4. ALE is present and stable. If available use frequency counter.
5. Try 11.0592MHz crystal (best for Baud rate generation)
6. Start with a low Baud Rate, e.g. 19.2k. I have it working at 115kb.
7. Serial data lines, TXD and RXD, of right polarity and levels.
8. Check that all connections are there.

(Not necessarily in order of importance)

If still no success send a copy of schematic just in case 'fresh eyes' of forum members pick some possible problem.

Regards,
Tony G.