MAC SE PSU Sony CR-44M Question

I actually wish the POT in my SONY maxed out at 7V so I could pull it down to where the voltage at the motherboard would be 5.05v or there about. My max "no load" voltage, as set via the blue POT, is 5.49v, and you can see that on my spreadsheet here (created when I did the Baby Face PSU review video). The implications of that is that if you have the stock main wiring harness and power-hungry upgrades installed, there's a large voltage drop across that harness, and you always end up with 4.90v or less when measured at the motherboard.

However, your stated "7.0v" is quite high.

You said dropping the voltage via Blue POT brings the 12v line down to 10.45v, but what is the 5v rail voltage, measured at the motherboard, when the 12v rail is 10.45v?

I ask because if your 12v rail drops to 10.45v when you set the POT so the 5v rail measures 5.00V at the PSU (not at the motherboard), that's the wrong way to go about it. You always need to measure at the motherboard due to the large voltage drop across your main wiring harness. The more stock your motherboard, the less voltage drop it will be. But on my machines with upgrades, the voltage drop is always rather large. Or at least, it used to be. I now upgrade my main wiring harnesses in my SE and SE/30 machines using thick 16AWG wires, and you'll find links to RED and BLACK versions of those 16AWG wires in the lower right corner of the spreadsheet page ("SONY PSU" tab) I linked for you above.

You also need to verify STABILITY of your PSU's voltage. For example, does it stay rock solid at 5.0 voltage or does your meter show fluctuations? A scope is far, far better to check stability than a meter because a meter often doesn't update but once every 200ms or so. The scope shows you ever little dip and spike over time.

I’ve never heard of the blue “RV251” POTs going bad, and I’m not saying yours is or out of spec either, but if you aren’t making any progress based on what I said above, you could desolder the POT and measure it out of circuit to see if it is indeed still a 1KΩ POT, as per the attached schematic.

_nec6678 said:

… checked my voltages without the motherboard or analog board connected. From black to yellow and red, I read about 13.35 and 13.85 volts. From black to both oranges I got 5.32 volts. And finally, from black to green, I read -8.22 volts

Click to expand...

Normal for a no-load test. If you twist the blue POT to max, they probably will go slightly higher. My measurement of 5.49v with the blue POT maxed out is given in my spreadsheet linked earlier.

But through it all, I’ve never measured a no-load voltage as high as 7v on the PSU’s 5V output rail, and that is at the heart of the issue being discussed.

I'm not sure what to say about the Ground and the 20V reading. As was the case with the oddball 7v measurement, it's clear there's something going on that needs to be figured out, either in terms of proper measuring (probes, contact the probes are making, or the meter itself and/or its battery) or in terms of something possibly being wrong with the PSU in question. But I am glad you seem to be measuring something closer to 5V now, rather than 7v.