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NAOMI1 - 3,3V drops to 2,55V -> no boot

R

Rafo

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Hi all.
This is a tough one... I guess.
Here is the situation.
I have this naomi REV.A mainboard. Higher resolution here
Its looks pretty clean to me except fr some soldering next to BIOS that repairs a cut in the track, nothing to report on the aesthetic side,
Before you ask, yes, the supercap is brand new, although not in the same package as the original (which was fine, BTW)
IMG_20230811_190051662.jpg


It does not boot : no display, no sound. The only thins that happens is when the monitor is already on, and in standby mode because of the lack of signal, when I power up the main board the monitor snaps out of sleep mode but only to get back there after a few seconds.
LED1 and LED2 are lit normally.

Tried another filter board, different BIOSes, same result.

What bothers me is one of the voltage testpoints on the main board :
Code: 
12V     12,11V
5V      5,11V
3,3V    2,55V
2,5V    2,43V
SH IN   2,011V

As you can see, 3,3V is not 3,3V anymore (even on the PSU output) as long as it is connected to this particular main board.

And before you mention my PSU, please note that it powers my Naomi2+NetDIMM and my Triforce Type 3 perfectly. So I think it rules out the PSU for causing this.

As you can guess my electronics knowledge is quite low, but I'm pretty sure this can be fixed and is not a big thing to deal with.
My (wild) guess is something, maybe in the PQ7RV4 area (RG4 on the mainboard), is messig with this voltage and makes it drop : not to zero, but low enough to prevent 3,3V powered chips from working.
The RG4 itself is already replaced, as i had a few around (the original one did not seem in good shape to me, so I thought this was the one causing this.... but it seems it's not (or not alone).

I'm not sure what causes this 3,3V getting lower, someone here has quite an experience of this kind of issue to help finding out what is happening and/or how to fix it. Again, I'm sure it's nothing big.
Of course, I'm ready to answer any additional question or perform additional required tests which could help to narrow the research down.

Any clues ?
 
Last edited:
Addition to the first post :

Here is a closeup of the beginning of the power supply circuitry from the filter board. Yellow is 5V, Brown is 3,3V.
Higher resolution here
This looks like twin circuits to me, maybe there is a component on the 3,3V side that is faulty.
Do you think anyone of these that could cause such a weird issue ? (as a reminder : 3,3V is downed to 2,5V when supplied to this main board and this one ONLY).
I'm not familiar with MOSFET chips, so I can't tell if it can do this when its faulty.

IMG_20230812_172830525.jpg


What do you think ?
 
Looks like someone added solder to FET3 and the capacitor below it may not be connected
 
ekorz said:
Looks like someone added solder to FET3 and the capacitor below it may not be connected
Click to expand...
I noticed that too. I'll check the capacitor. As for FET3, this does not look like it's wrong as long as it connects to the right point, compared to FET4.
I have a FET from another Naomi mainboard (rev D though, where 729MTD1302 becomes 905MTD1302, searching about this cannot make me tell a significant difference), so I'll try replacing it if it can do the trick, and keep you informed.

Of course any idea coming inbetween is welcome.
 
ekorz said:
The FET may indeed be fine, I just take it as evidence that someone possibly reworked the area before.

The first digits are a batch number. The MTD 1302 is the company and part

https://www.alldatasheet.com/datasheet-pdf/pdf/5434/MOTOROLA/MTD1302.html
Click to expand...
Yes I could find it.

Anyway,
The 220µF (C100) Cap was replaced along with the FET3 chip, with no success : 3,3V is still around 2,5V (slightly above the actual value of the 2,5V Test Point).
This hope vanishes then, back to point zero. Right now I have no idea where to look.

I noticed something though but I don't have another REV.A mainboard to compare :

On the 5V side, there are 2 resistors :
R346 and R347. Both are marked "102" which means 1000Ohms (please correct me if I'm wrong)

On the 3,3V side though, there is a slight difference, which seems odd (even if they don't look like they have been touched) :
R318 and R319 : R319 is marked "102" too, but R318 is marked "103", which means 10 kOhms.

I don't know if it's supposed to be this value, maybe somebody can confirm it or not.

And while we're at it : each voltage (3,3V and 5V) is equipped with 2 transistors marked M5 and L6.
Does anyone know what they are ? What do these inscriptions mean ?
Man, some schematics for NAOMI mainboards would really be nice, at least to show to some contacts of mine, which are much more familiar with electronics.
 
nem said:
Surely if voltage is going down so low, there should be a component that is getting very hot?
Click to expand...
Back at this issue : the answer is no : no component is hot, not even close.
I ordered a hot air solder station to be able to solder/unsolder SMD components more easily.
A friend of mine which is pretty good in electronics tried to hand-make a schematic of the power supply circuitry and we noticed the 3,3V goes to 2 different directions, so he asked me to remove FET3 and see if 3,3V is still dropping of not, to narrow the search down.
As soon a I have the needed tool I do it and test it right away.
I'll keep you informed.
Meanwhile, any idea of where to search is welcome.
 
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