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You can use a 7906 with a two-resistor divider circuit (for adjust to 8v) to his ground pin.
Sorry, a lot of periods are requiered only there
.!---------!
-! 7906 !---------*---- out
.!---------!...........!
..... !...!--------!...!
......*--!..........!--! .220 Ohm
......!...!--------!
......!
....!---!
....!....!
....!....! Potenciometer (100 Ohm)
....!---!
......!
-----*----------------- gnd
That 7908 is useless in your case, no way to modify it.
If you insist of using 7908 and need 8.0V you have to buy some 7908 and pick out the right one since the 7908 has the output voltage within the range from -7.68 to -8.32V, typically -8.0V.
But I do not understand why you need -8.0V exactly and with 7908, as in generic requirement of a electronic circuit, the needed supply voltage maybe vary within the range of +/-5% or +/-10% of the nominal value, and the line regulation of the IC is more important than its voltage tolerance, it means -8.2V is good enough for every common use.
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