SE/30 PSU Voltage Readings -12V very low

[chrisrueckert]

After the recap of a Sony PSU for my SE/30, the Mac won't start-up (no chime, no fan). The analog board is known working. Here are the readings from the idle PSU:



I wonder why the -12V is so low. Any thoughts on that? Would adjusting "VADJ" at position 251 correct that or just increase it for all other voltages?

- Chris

 

[zigzagjoe]

After the recap of a Sony PSU for my SE/30, the Mac won't start-up (no chime, no fan). The analog board is known working. Here are the readings from the idle PSU:

View attachment 70694

I wonder why the -12V is so low. Any thoughts on that? Would adjusting "VADJ" at position 251 correct that or just increase it for all other voltages?

- Chris

By idle PSU, do you mean with nothing connected? As it is expected for the measurements to be all over the place if there's no load. You should have it connected to the AB and LB before measuring.

That said, no fan indicates something very, very wrong as the fan runs directly off +12v. What you may have done is forget the little electrically isolating thermal sheet between this transistor and the casing. If I recall correctly, without that sheet you have connected ground to +12v, which will test fine when the PSU is not installed in the machine. However, once installed in the machine you'll have earth ground = +12v = psu negative, in effect shorting the +12v rail and causing PSU to protect itself/not power on.

 

[chrisrueckert]

Yes, idle = nothing connected. Thanks for pointing out the missing isolation foil. Adding it didn't change the voltage on the -12 rail.

Any other ideas what is going wrong? I should mention that the PSU was working fine before the recap.

 

[zigzagjoe]

Yes, idle = nothing connected. Thanks for pointing out the missing isolation foil. Adding it didn't change the voltage on the -12 rail.

Any other ideas what is going wrong? I should mention that the PSU was working fine before the recap.

With the pad re-added, does the machine now power on?

The -12v is only used for ancillary uses (sound, mainly) and won't prevent the machine from booting. And again, you'll not get valid voltage measurements without loads on each rail, so voltages being out of whack with nothing connected is expected.

 

[chrisrueckert]

The SE/30 now boots nicely. Not sure if reseating the insulation foil was really the solution. It was seated before. Anyway, thanks for the guidance!

 

[zigzagjoe]

The SE/30 now boots nicely. Not sure if reseating the insulation foil was really the solution. It was seated before. Anyway, thanks for the guidance!

Glad to hear it! If it was missing before, it would definitely cause those exact symptoms I described... that same issue is what led to me posting on this forum in the first place