Original Xbox 1.0 PSU: Foxlink FTPS-0002 Rev. B. — 12 V low, 5 V high — transformer?

[ohren]

Hello!

I'm looking at an original Xbox PSU: Foxlink ftps-0002 rev. B. The 12 V output is too low, and the 5 V output is too (?) high. Unfortunately I don't have an oscilloscope but only a DMM for diagnosis.

Measured voltages with no load:

After rectifier diodes
12V: 6.17 V
5V: 5.67 V

Secondary transformer pins AC measurement (really don't know if this says anything)
12V: 2-2.5 V
5V: 1.04 V


I also have another (working) Xbox psu from Delta. The same measures there, in order, being: 10.79, 5.36, 8.9-11.9, 6.3-6.9 V (depends a bit on probe polarity apparently — due to DC component?)

Also, measuring AC on transformer input pins just confuses me greatly and I imagine a DMM is the wrong tool. The sick PSU measures ~10 V (really?) The healthy PSU measures ~150 V. Again, I'm not sure it says anything to measure presumably square wave pwm at hundreds of kHz with a DMM, and also seems to depend on probe polarity as well as the specific voltage setting on DMM. I guess I don't have the theory or technique down for AC measurements.

Observations and details

* I'm not aware of any service manual or schematics for this PSU
* I don't really have any easy load to measure with (Xbox will not stay on)
* 5.67 V for 5V rail is out of spec assuming a 10% tolerance. Assuming a 5% tolerance would put the Delta out of spec too.
* Bulk cap stable at 315 V
* Switching transistor seems fine
* 12 V cap healthy (1014 uF 0,06 ohm ESR) according to cheap component tester and my eyes
* All twin rectifier diodes healthy (symmetric 150-200 mV drops which I understand is ok for Schottky). All their joints were resoldered for good measure.
* 5/12 V rails share transformer T1 while 3.3V uses T2, so I assume a single feedback for each transformer (two optocouplers and two switching transistors in total)
* 3.3V output stable
* I assume 5 V being too high is because 12 V is too low (?) Both 5 and 12 V go to the control board so I'm assuming both levels are taken into account for feedback
* 5 and 12 V rails are separated by 36 ohms in accordance with heatsunk resistor of corresponding color code
* I can't really get at the vertical PWM control board (tl3845) during operation, and only half of the board otherwise. It's wedged between the bulk cap and primary heat sink. I did figure out what's connected to it although I fail to deduce the transformer topology from it, since I'm not that experienced with different types of smps designs.
* I can get at the vertical control board only with difficulty. It's covered by all the output cables.
* I hear a buzzing from the transformer which doesn't sound entirely healthy (see attached audio)
* Earlier, when the sound changed momentarily the output voltage jumped up momentarily, a volt or so. It stopped doing that.
* Measuring one pair of the three T1 pins on the primary side (in 750 V AC mode) changes the sound — it becomes quieter
* Percussive maintenance doesn't produce an effect
* I suspect maybe some mechanical trigger to the problem: it appeared after I took the PSU out to inspect and resolder the (previously repaired by me) AC input jack. This was in response to stepping on the plugged in end of the power cord and hearing a crunching sound. Since there was no visible damage on the board I attributed the crunch to 15 year old hot glue coming loose when the AC input jack bent downwards. Still, some mechanical stress is put on the board when pulled out of the Xbox and my inspection was probably the cause.


I strongly suspect the transformer. How do I go about confirming that?

Am I jumping to conclusions before checking everything properly? Did I miss something obvious?

What should my next step be in troubleshooting?