Q605 / LC475 Half dead psu can make you think sound is dead.

[uniserver]

The PSU that i normally use to test  LC's is a re-capped one.

but i forgot, in messing with my personal machines a week ago the good one got swapped with one that was not already serviced.

Witch caused me some issue, but, just another tid bit to toss in the bucket.

 

[Elfen]

Interesting.

 

[uniserver]

if i had to guess i would say, the -12 rail died.

 

[Elfen]

12V? More interesting, I thought 12V was mostly for the hard drive and RS-232 Serial Protocols, but 5V for the chips.

Teach me more!

 

[uniserver]

yeah the negative 12 volt rail.

 

[uniserver]

Well i was close… the LC475 / Q605 does not have a -12 volt rail…  It has a -5v rail…   and if that goes out you get no sound and maybe other stuff as well ( serial ports )

And as elfin mentioned before, He had situations where the metal in his LC power connector was rotted away.  That can also be bad, looks like -5v is on the end right near C129, so i bet if cap go made it to that terminal, could get rotted out. Killing your -5v rail as well.

 

[techknight]

Well when I am testing Mac II, SE/30, I dont need any of the negative rails. 

I need the 5V, and the 12V. Thats when I get sound and everything. But youll loose communication, and I am unsure of floppy drive requirements. 

The machine wont run without 12V, and it definitely wont run without 5V. The 12V i assume is part of the reset management circuitry, which explains why it needs it. And its for sound as well.