First of all, please try solutions (1), (2) and (3). It is easy.
(1) Check and clean connections between SIMMs (ROM-SIMM) and SIMM (ROM-SIMM) slots, and restart.
(2) Remove PDS card(s) such as an accelerator card if installed, and restart.
(3) Check the battery. Replace it with a new one if voltage is low, and restart.
If nothing above works, try solutions (4), (5) or (6) AT YOUR OWN RISK. You NEED soldering and desoldering skill.
(4) Replace electrolytic capacitors C6 (1µF 50V) and C7 (47µF 16V) on the logic board with new ones. Surface mount chip type capacitors are soldered on the original logic board. But you can use miniature radial lead type capacitors instead.
Anyway, be carefull about desoldering and soldering procedures not to break circuit patterns on the board. Make sure the polarity of electrolytic capacitors before soldering.
If you find liquid leak or corrosion of other capacitors, it is a good idea to replace them or all electrolytic capacitors with new ones. Macintosh SE/30 was introduced in 1989 and discotinued in 1991. Electrolytic capacitors have been degraded after years.
(5) If solution (4) doesn't work, pull up the sound IC chip (UB11). You can see letters "SONY 0F09G 3430045B APPLE85" or "SONY 9A04 3430045B APPLE85" printed on the chip. To pull up the IC chip, connect pin 7 to pin 15 of the chip with a 1 kilo ohm resistor.
(6) If solution (5) doesn't work either, replace the network filter RP2 (Bourns 4120R-601-250/201).
Also see 9.
Bright Monitor with neither Arrow Pointer nor Startup