Nintendo 64 PSU - Secondary side dead

[raider988]

Hey guys. I have this PSU with the secondary side completely dead and I need some help in troubleshooting and I couldn't find a schematic for it. I'm still a beginner in electronics so please bear with me.

There is no visible damage to any component, the PSU is reasonably clean with no burnt smeel whatsoever. The secondary side seems completely dead, I couldn't find any voltage anywhere, not even at the transformer's terminals. It seems as if the transformer is not doing its job, maybe because it's bad or because it's not told to switch on .

Here's what I checked on the primary so far (without desoldering, images attached):

Filtering and rectifying: ok
Fuses: all ok
Caps: not shorted
Resistors: all ok
Inductors: continuity ok
Diode: OL and 0.5
Zener (delsoldered): OL and 0.72

With the power on, I checked the power mosfet's gate and it was 0v, I don't have an oscilloscope so there's really no way for me to tell if the voltage is oscillating, I tried measureing as AC and DC with the same result. The gate is not shorted to ground, so I concluded that no signal is reaching it.

The part of the PSU responsible for switching the power mosfet (the small board on the side) seems to be the culprit, it has has 3 pins connected to the primary side and communicates with the secondary through 2 optocouplers.. Of the 3 pins, one is ground, one connects to the mosfet's gate and the last one connects to the transformer through an inductor. I figured that must be where the board is getting its power. I measured the voltage there and it was 0v. I tested continuity between that transformer pin and ground and it tested positive. I think it should be normal, that side of the transformer supplies low voltage and so should have a very small impedance.

At this point I think either the problem is on the small board, the tranformer is bad or I'm missing something. But, assuming i've been right so far, how can I safely test which on of the two options is the correct one?

[stj]

Re: Nientendo 64 PSU - Secondary side dead

if there are any capaitors under 100uf, change them.
psu's usually have a small cap that starts them up.

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

On the primary there's only a chunky 400v 47uF electrolytic cap, 2 class Y and a small film cap, which ones do you mean?

[stj]

Re: Nientendo 64 PSU - Secondary side dead

neither of those,
something between 4.7 and 47uf at maybe 50v
what's that small one between the controller pcb and the transformer?

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

Hey thank you for the quick answer! It's a small 472k 250v (looks like a class y to me)

[stj]

Re: Nientendo 64 PSU - Secondary side dead

no, the small electrolytic between the variable resistors.

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

Yes, it's a small 22uF 50v. I replaced it and since there are only 5 electrolytic on the whole board, I just replaced them all. No results. I also retouched all the connections, again just in case. Still nothing happened.

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

I started desoldering some components to test them. On the primary, there's a transistor marked D1302 s .71. It's NPN, I checked the datasheet and I'm not sure if the transisor is bad or the pinout is wrong, because it makes no sense that its base is connected directly to ground. This are the values I'm reading (following the pinout from the datasheet):

BE: OL
BC: OL
CB: OL
EB: 0.69v
EC: 0.69v
CE: OL

Edit: On the board, next to where the "base" connects, it's marked with E. So I guess the datasheets I found where not the correct ones and the transistor works.

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

Solved. The 27ohm sense resistor failed short, I initially thought it read low because I measured in circuit, but once i desoldered it I found out it was damaged. I temporarily replaced it with a 1/4w (I don't have a 1w at hand) and the power supply came back to life. I guess what happened is that the PWM was sensing too much power was beeing pulled and shut down the current to protect the components. Thanks for the help STJ

[R_J]

Re: Nientendo 64 PSU - Secondary side dead

I think your emitter and base are reversed. the emitter goes to ground, the base is connected to the fet source via 220Ω, collector drives gate. Base is also connected to module, for feedback (is there an optocoupler on the module)

Are you sure that resistor is bad, I doubt it failed shorted, it is likely a 0.27Ω resistor
It could be similar to this:

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

Hi RJ, the NPN reads correctly, I found out that there are 3 types of to-92 BCE configurations, so I concluded it was my fault in getting the wrong datasheet. On the board, the image of the transistor and marked location of the emitter matches with the reading I get, under the assumption that I mistook the base for the emitter.

The resistor reads R27J, so i thought 27Ω with a 5% tolerance. I tested the power supply with a 27Ω resistor, it outputs the correct voltages, but very low current, not more than 40mA on the 12v rail, nothing compared to the 800mA it should be able to deliver. On the other hand, another google search showed that R27J means 0.27Ω with a 5% tolerance like you pointed out, but the resistor I removed measured 0.6Ω, more than double, do you think that might be a problem?

I guess I'm back to square 1. I'll draw a schematic of the primary side, from what i see it seems similar to the one you attached, maybe it'll help in understanding what's actually going on.

[R_J]

Re: Nientendo 64 PSU - Secondary side dead

when you measure low ohm resistors like that, you need to consider your meter lead resistance, so short your meter leads together and note the value, IE meter leads shorted read .33Ω checking a .27Ω resistor would result in .6Ω.
It does seem odd they would use a resistor with the value marked on it and not color bands
R27 should be 0.27
2R7 would be 2.7
Also increasing the value of the source resistor would supply a higher voltage to the base of the drive transistor so maybe there is a fault with the transistor, but it does look like the rest of the circuit works.

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

This is the schematic, T1, T3 and T4 are the transformer's pins, from left to right. Could it be that there's something wrong with the feedback/PWM circuit?

Edit: sorry, vertical image. I checked the internal resistance of my multimeter and was about 0.3, thank you for the trick!

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

I checked the transistor, it still reads fine, later I'll swap it with a new one, just in case.
I still don't understand how increasing the resistance of RSENSE would make the supply deliver the correct voltages. As far as I understand, the purpose of RSENSE is to signal PWM when too much power is being drawn, which would explain why, with a higher resistance, hte PSU couldn't deliver more than 40mA onthe 12v line. But it still doesn't explain why the PSU doesn't work to begin with.

I'm getting more and more convinced my problem is not with the primary side at all, rather with the FB/PWB board. I'll start desoldering it and see if I spot anything suspicious.

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

I replaced the the transistor and nothing changed.
I noticed a coupled things, even without delivering any output voltages, the PSU uses about 400mW. Since I monitor the 400v bulky cap after unplugging, I noticed that when it discharges to about 100v, the PSU emits a tiny nise, like a small short and the big cap gets discharged instantaneously to about 15v. I removed the fuse shown in the datasheet I draw and masured the breakdown voltage of the zener, which was about 15v as well. Maybe a bad zener is creating the problem?

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

Hey guys, finally I'm getting closer to solving the problem. I'm a little ashamed to admit that I cheated my way out of it. I borrowed the same power supply from a friend and the circuitry was exactly the same. I swapped the small daughter boards and the power supply started working again. Then I combined the FB of the working board to the PWM of mine: the PSU worked.

So my problem is on the feedback side, I replaced the optos and checked every resistor and diode, comparing the results of the two boards, everything seems good. At this point, the only options left are the 2 ceramic caps (they are not shorted, but at this point who knows anymore) the LM393 and 3 small ICs marked 5H and Z_R. My money is on the LM393. I'm not even trying swapping them, the moment I remove an SMD component it's basically ready for the trash. I will simply replace them all. Anyone can help me identify 5H and Z_R?

Picture attached, in the bottom right there are two resistors that seem burned, it's just black silicone, they measure fine.

[R_J]

Re: Nientendo 64 PSU - Secondary side dead

5H seems to be a Panasonic XN04501
Z_R seems to be a 2SD601A-R

[raider988]

Re: Nientendo 64 PSU - Secondary side dead

Thanks R_J! I have a new update.

While waiting for the components to arrive I thought that I might as well retin the SMD components. You wouldn't believe it, the power supply came back to life. I don't know why I didn't think of doing that before, since I already retouched all the through-hole components. I attached an LED strip as load to the 12v line and it outputs up to the 800mA as it's supposed to. Since the daughter board is next to the heatsink (the closest part is the right side of the last image I attached) of a bulky diode (MA10799), something must have come loose due to the heat and age. It's 20+ years old supply so I guess I should have expected something like this to happen.

Just for the sake of it, I re-replaced all the original components and the PSU still worked. Now, in all fairness, how likely is something like this to happen? I also cannot exclude that an open circuit on the FB side might have caused some minor damage elsewhere that's just not yet bad enough to be noticed, I'll keep an eye on this supply and update the thread if something comes up. I re-replace the new caps, transistor on the primary side and optos. When the LM393 and the SMD transistor arrive I'll replace them as well, just for good measure.

Thanks again for the help STJ and R_J, by the way R_J, how do you know how to identify those SMD transistors? I found a database of IC markings but it wasn't much help.

[traianvui]

Hi. I know I'm reviving an old topic, but I really need help.
I also have a n64 power supply with a faulty daughter board.
I've found the faulty parts and replaced them with ones from a working power supply.
But now I need to identify them, so I can put them back in the working power supply.

1. Maybe someone can help identify the 3 diodes in the picture.
2. XN4601. Absolete part. I'm planning to replace it with BC846B+BC856B. I hope that is ok, because i already ordered them.
3. the 2 optocouplers are busted, too. Model TLP721F. Can I replace them with PC817C or PS2501-1?