PlayStation 2
Modbo v5.0 - No display - Part 3
July 15, 2023
Fuse Fiasco: Diodes, Differences, and Dead Ends
In Part 2 I worked through tracing back from the Multi-AV port, through a number of the video components chasing a signal, only to make my way all the way back to the Graphics Synthesizer and coming up empty. After realizing that I had another console with the same GS-15 mainboard I decided I would try some diode testing to to compare between the working and non-working systems to see if I could narrow down a problem region.
The PS2 has a number of fuses on the board which help to protect various regions of the board in the event of something going off the rails. PS2-Home.com has a great thread with resources for fuse locations, the following picture is taken from that post.
I started by checking to make sure all of the fuses on the board were good, and then I took diode readings from the fuses on both boards. Four of the fourteen fuses tested showed diode readings that were about 100mV higher on the bad board. Not a slam dunk, like something being OL on the bad board compared to getting a reading on a good board, but without anything else to go on it seemed worthy of looking into.
PlayStation 2 Fuses - GH-015
PS4, PS6, PS8 and PS9 were the four fuses which were each reading about .231 on the bad board and .139 on the good board. Referencing the details from the fuse map, it looked like all of the fuses were related in some way to the 5V rail. It would make sense that if there was a problem on the 5V rail this could cause a widespread problem like I was seeing. I checked the voltage at each of the fuse points and they seemed to be reading at 5V, but I wondered if maybe there was a lot of switching noise or something else happening.
Looking at the schematics I began following the flow of the 5V rail it looks like switching regulator IC409 outputs 5V which then trickles down to a few other regulators including IC407 and IC413 to be further stepped down to supply other voltages. I checked the 5V inputs on the downstream regulators and then seemed to be a decent 5V, but again I had not checked noise yet. When the 5V line comes out of IC409 it goes through fuse PS006, and into two MOSFETs, Q407, Q408. It was when I started diode checking the MOSFETs that I seemed to find the difference in diode readings.
But, on closer inspection I noticed that on the good board, one of the MOSFET was a different IC than on the bad board. Quite likely a slightly different construction between the MOSFETs was responsible for the small reading differential. Since the 5V signal looked OK, and using an oscilloscope I checked for excessive noise, it seemed like I had hit a dead end with the 5V rail theory.
Where to now? Head over to Part 4 to see!