Why volatge is changing? Fixing power supply.

[nadona]

I am fixing the power supply of LeCroy LC584AL oscilloscope.

It is a switch mode power supply which outputs several voltages.

The problem is -15V output is changing to -21V in about 5 minutes and and keep going lower about 1mV per 10 seconds. I turned off after 10 minutes. If the scope is turned off when it gets over -21V then I have to wait a few hours to turn on the scope again. If I turn on the power supply right after it starts a few mV below where it turned off.

What component behave in the switch mode power supply this way when it goes to bad?

Thank you very much in advance.

[Edit] Incorporated the definition of "moment" & "slowly"

[c4757p]

Define "moment" and "slowly".

[nadona]

Define "moment" and "slowly".

Sorry.
Incorporated the definition of "moment" & "slowly" into my original post.
Thank you for pointing it out.