How to safely test the power supply of a 9600/300 Power Mac (pin-out)

Walter
If you don't propose to tangle with the insides of the power supply, the issue is rather safety for the computer. The outputs required by the 9600 circuitry are +12V, -12V, +5V and +3.3V. Although the 9600 uses 5-V RAM, the +3.3-V supply is for the PCI slots.

A fairly innocuous place for a crude measure of the power and logic voltages, assuming that your question is not, 'Do these supplies reach every place that they should?', but rather, 'Are the required voltages present?', is at some accessible connector such as the 4-pin connector for power supply to the HDD (for +12V and +5V). As you look at the rear of a drive, the power input pins are numbered 1 to 4 from right to left. The corresponding female socket must supply +12V, 0V, 0V, and +5V from left to right (as you face it). If you boot from a floppy disk you can safely disconnect the drive's logic and power cables before startup.

If however, you wish to make a thorough inventory of the power supply's outputs, something like this will be of use to you, because you will then need to measure at the power supply's cable connectors.

Take due note of the disclaimer at the head of the article.


Apple IIe; 15 x 68K; 7 x PPC; 5 x G3 System 6.0.8 to OS 10.4.x

Walter,

The link that Denis gave has a nice photo of the Molex connector that goes from the power supply to the motherboard. Because it has low voltage power out to each pin, you can safely disconnect that plug and test for power out of the PS.

If you do not get a voltage reading from any of the pins (disconnect the plug from the CPU before turning on the PS), you will have to open up the cover to the PS and take all the normal precautions for working with exposed 120 volt AC wires.

There is a fuse in the PS. The white lead into the power block should be neutral. Or, you can clip to the green grounding wire with the negative lead from your multi meter. Test both sides of the fuse. If you have power at the fuse but not at the stepped down power leads to the Molex connector, your PS transformer is shot.

Continued luck and care,

Jim

With the AC main cord plugged in, you should be able to find the 5 Volts "always on" Trickle Power signal at the power supply connector. (You can usually just push the probes in the back end of those connectors and hit the back of the crimp contact to get readings "in place".) If you do not see that 5 Volt supply on a distinctive-colored wire going to the main connector, your power supply is not working and you have no hope of powering up.

That little voltage is what keeps the PRAM alive -- the battery is the backup for it.

Walter,

I check with recyclers to save Macs from untimely trashing. If a computer does not work, the power supply might still be worth saving. Check around and you may find something. A Seattle recycler had G3 MTs for $10. They are just one option.

Grant mentioned trickle power. Think of a VCR that has a digital clock. Even though you turn off the VCR, the clock still shines. That is running off power that does not go through the main power switch.

The other reason for checking every lead is that some power supplies only turn on the main circuits after a relay gets a signal from the keyboard. Grant's reference to finding the one lead that is always has power is so that you do not assume that the power supply is dead when it is only dormant.

The ultimate test is to pop the top off the PS and carefully trace the 120 V. current from the AC cord, through a fuse, if any, and then to the transformer output leads.

There are homemade remedies posted out on the web and if you are up for modifying your Mac, you can take a PC power supply and adapt it to work in a Mac. Happy hunting.

Jim

You looked for +5 and +12 on connectors that are live After the fans come on. Now look for something more fundamental.

Trickle Power is the name of a standard tiny 5 Volt supply used to maintain parameter RAM and PCI slot card parameters. It is NOT connected to the Hard Drive connectors. It is available from a working power supply whenever the AC cord is plugged in, even if everything else is off. It will be on a wire different in color from the rest, and you want to measure against any black ground wire. The Main DC harness can be connected or disconnected, it does not matter.

If it is not present, this power supply cannot be used to start up your Mac. The reverse is not necessarily true.

Walter,

I opened up an 8600 and tested the leads. The main molex connector does not get power until the power switch circuit is triggered on the motherboard. It is a momentary switch so the circuit is only closed temporarily, long enough to signal the relay in the power supply. If the large Molex connector is disconnected from the motherboard, the relay in the power supply will give an audible click. Every time. Pushing the micro switch on the motherboard is preferable to pushing the spongy switch on the face of the case.

The power supply will not activate if the large molex is not connected to the motherboard. I did not trace wires to see exactly which lead(s) trigger the relay. On and ATX PS for a PC, I would tell you to jump the purple and black (I think - don't quote me on that!).

As for color coding: The trickle charge ( trickle only because that is the limited draw placed upon it, not because it is any less 5 volts than any other 5 volt connector) comes from the yellow lead in the Small Molex connector. Or, the trickle charge is not 5 volts and comes from the purple lead.

Using this link as a reference,

http://www.macgurus.com/products/motherboards/mbppc8600.php

here are the relevant specs:

Small molex
There is a green wire, a purple/lavender wire, then a yellow wire coming from the upper right hand corner of the small molex connector labeled in the diagram as the 3.3 v. Power Supply. The green reads zero. The purple reads .548 volts. Note, that is less than one volt, not a misplaced decimal point. The yellow reads 4.99 volts. That is your 5 v lead. This is irrespective of the existence of any PRAM battery and does not depend on the large molex connector being plugged into the motherboard. As long as the AC power cord is plugged into the power supply, you should get 5 v at the yellow lead to the small molex.

IF the large molex is connected to the motherboard, and the power supply works, the machine will start up, even if the PRAM battery is physically removed from the building you are in. (PRAM batteries are important and computers work better when they are fresh but I have never needed a PRAM battery even near any macintosh computer I have ever tried to start up. Ever. The advice to have a fresh battery is good but does not apply to your case. A warm boot gets me around a dead or missing battery every time.

So, You either have wall power or you do not. Tracing it from the wall outlet, into the power supply out of the transformer to the yellow lead going to the small molex connector will tell you if the relay will ever turn on and trigger voltage to the rest of the motherboard. Anything else assumes that the power will come from positive vibes floating through the atmosphere. : )

Bonus link:
http://www.eserviceinfo.com/downloads/13992/Apple_Power%20Macintosh%208600/9600/ WorkStation9650.html

Good luck as always.

Jim

Walter,

Did you decide to give up on that machine?

Jim