Performa 475 power problems

[Oberlehrer]

So I got myself a cosmetically very nice Performa 475 without a hard drive and without VRAM. It doesn't really turn on and I'd like to find out if this is a power supply problem or a faulty logic board.

When switching on the machine it clicks/pops continually from the speaker; the LEDs in the keyboard blink with the same frequency. All voltages are very low (roughly 50% of the nominal voltages). The power supply seems to work when not connected to the logic board; I'm getting +5.3V, +11.1V and -4.5V.

The power supply is a Dyna Comp model; I don't know if the Performa 475 came with other supplies as well.

My thinking is that a short on the logic board would probably only pull down one of the voltages (and not all three). Neither logic board nor power supply are recapped though both don't show signs of leakage.

Any ideas?

 

[MrFahrenheit]

These LC475 computers have issues with needing capacitors redone on both the motherboard AND the power supply.

That is very symptomatic of a motherboard recap being required.  Also, did you change the PRAM battery?  These require a good PRAM battery to turn on.

You can 'jumpstart' one of these with failing capacitors by turning it on, letting it sit for a few seconds, and very quickly switching it off and on (so fast it doesn't really turn off).

 

[Oberlehrer]

The Performa came without a PRAM battery. I was under the impression that without the battery it would chime but not produce any video; and that the "jumpstart" method would work for the missing PRAM battery (but not failing caps).

I'm mostly irritated by the voltage breakdown; it's obvious that the computer won't boot when instead of +5V there are less than +2V.

Anyway, a complete recap of both supply and logic board is probably in order.

 

[Oberlehrer]

...and as soon as my desoldering pump made contact with the power supply board I knew how wrong I had been about the caps probably not having leaked. A familiar smell permeated the air...

Anyway, after recapping the power supply I got a chime; and with the "jumpstart" method I also got video. So far, so good. But of course I got one cap with the wrong height so the lid of the supply won't close.

I'll probably recap the logic board as well while I'm at it.