7905 Negative voltage regulator on location U198 is located near A4000 keyboard connector. When you locate it, you may notice that Commodore designers have put a copperheadspreader area under it and a hole to the motherboard very likely with the common solution in electronics of putting a screw and a nut through the holes in the regulator and the motherboard, but this solution was never used in A4000 motheboards and I can imagine two reasons for this:
1) Commodore assembly line managers thought manually assembling a screw and a nut would slow the maufacturing process too much and so manufacturing would become more expensive and less profitable.
2) Commodore designers may have realized (in prototypestage or later) that a copperheatspreader is so small that it has not enough mass to cool such a component enough and it would just make the heat spread through PCB material to other nearby components and 'cook' electrolytes out of those nearby capacitors.

No matter what was the reason I think they made the right decision for not bolting this regulator to the motherboard, since leaking capacitors will do more damage than broken regulator, but anyway adding a heatsink to should increase the component life and make it less likely to delivering inadequate -5VDC. If the reglator has already gone bad it may be cause following problems:

Get TO-220 heatsink, M3x6mm screw and M3 nut.

Locate U198

Lift the regulator carefully to about 90 degree angle, so adding the heatsink, M3 screw and M3 nut is possible. Do not lift and lower the regulator more than once or twice, otherwise the regulator feet will break in their weakest point and you need to resolder them.

Lower the regulator now tightened to the heatsink by the screw and the nut. Notice that most heatsinks are so long they might reach nearby resistor solders near location ER372 and shortcircuit them. So it is important that the nut is serving also as a feet between the motherboard assemblyside and the heatsink backside and to keep the regulator slightly lifted.

After installing the heatsink the temperature of U198 was as low as 33 degrees celsius after 24h idle running and case removed. At least 63 other motherboard components had higher temperatures ranging from 34 - 48.5 degrees celcius! So believe me it is worth heatsinking this. With closed A4000D case and under software load temperatures will surely go higher.