-5 volts needed for...?

[johnklos]

Happy holidays, 68KMLAers!

 

I have a nice Quadra 605 motherboard which I've recapped and did the resistor change to run at 33 MHz. Partly since the plastics have become so incredibly brittle, and partly because I don't have the capacitors needed to recap a power supply, I've decided to build this motherboard in to a small 1U rackmount case.

 

I noticed that the whole motherboard takes 12 volts, 5 volts and -5 volts. I guessed that the -5 volts is probably only needed for the serial ports, so just to see I wired up 5 and 12 volts, and the system booted fine.

 

There are two things I'm wondering:

 

1) I don't get any audio out of the speaker at all. This could be damage from the bad capacitors, it could possibly be damage from the recapping work, or it could be that it needs -5 volts for audio. Does anyone know if it needs -5 volts?

 

2) I plan to replace power supplies for a Quadra 610 (with 260 megs of memory and a QuadDoubler ) and a Quadra 630 (with 196 megs and a QuadDoubler), and ATX power supplies have -12 volts, but not -5 volts. I see from some projects, both here and elsewhere, that the Q630 doesn't require -5 or -12 volts and that the Q610 takes -12 volts. Can anyone confirm?

 

I may just get some LM7905 regulators and feed them from -12 volts. It's always good to have working serial ports!

[Bolle]

Pretty sure the audio amplifier for the internal speaker needs -5V and +5V to operate.

[IlikeTech]

It has -12 v, so you can use an LM7905 I think to get -5v.  If the system uses and OP AMP for amplification, It may need -5v.

[techknight]

it does indeed need -5V for the audio circuit. I found this out the hard way on an LC475 when I was servicing one.