mjnman wrote:Before with tester I identified the ground on PSU output cable, the cable to connect at the motherboard.
I connected only the ha […]
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Before with tester I identified the ground on PSU output cable, the cable to connect at the motherboard.
I connected only the hard disk drive and floppy drive and powered on the PSU. With tester I tried to check the output voltage ad I found strange values beacause the voltage is always very variables during the readings:
From top to bottom the colors in the photo below:
1° BROWN: From +4.3V to +4.7V
2° BROWN: From +4.3V to +4.7V
1° BLACK: Ground
2° BLACK: Ground
RED: From +5V to +8,9V
BLUE: From -5V to -8,9V
WHITE: From +0,15 to +1,15V
Okay, so your PSU pinout is most likely this:
BROWN: +5V
BROWN: +5V
BLACK: GND
BLACK: GND
RED: +12V
BLUE: -12V
WHITE: PWR_GOOD (power good signal)
The power good signal should rise to 5V and stay there to indicate the PSU has reached correct operating state. In your case it's fluctuating and that's not good but in general the voltages you listed are in the right range. So it could be the load is still too small and the PSU is unable to work properly. These older PSU (well up to ATX era actually) have only one feedback and regulate one single voltage, the rest is "scaled" to that. Usually you regulate the highest voltage (so 12V) to have less error on the other ones but in this case it might be the 5V is regulated because it's the main power draw (and for logic it's important to get clean power).
But anyway - first check if your meter is working correctly. You don't want to be looking for problems in PSU if the weird values are due to your meter battery being low for example.
Next - I assume you've replace the PSU transistor with the same part? If so, and again assuming the PSU unstable voltages are due to not enough load, the question is why it doesn't work with the mobo. Well, maybe the mobo is the problem now. The clicking sound you get is the PSU trying to start but probably tripping on shorted output.
Disconnect the PSU and test the motherboard now. Measure resistance between GND pins and the rest - +5, +12 and -12. You should see at least a couple tens ohms, except maybe the +5V might be down to less than 10 ohms. But not very close to zero - anything below 1 ohm is a short.