[AccessD] limit drop down from another field...

Hi,
There are three fields into the same form

[result_zscore_ostiki_piknotita]--> decimal
[result_tscore_ostiki_piknotita]--> decimal
[simperasma_1] --> combo box with record source
SELECT [0simperasmata].ID_symperasmata, [0simperasmata].simperasma, 
[0simperasmata].tip FROM 0simperasmata ORDER BY [0simperasmata].simperasma;

into [0simperasmata].tip are saved three different values "z", "t" and "zt"

what I need is to limit the drop down list the based on 
[result_zscore_ostiki_piknotita] or [result_tscore_ostiki_piknotita] values

I tried:

Private Sub result_zscore_ostiki_piknotita_AfterUpdate()
If Not IsNull(result_zscore_ostiki_piknotita) Then
    Me.simperasma_1.RowSourceType = "0simperasmata"
    Me.simperasma_1.RowSource = "SELECT ID_symperasmata, simperasma, tip 
FROM 0simperasmata " & _
    WHERE(([0simperasmata].tip) = "z")
Else
End If
End Sub

but it doen't work

any help please?
thank's
/kostas