(the +5 V line gives about 4.75 V no load, and the +12 V line about 12.65 V).

You testing the volts with out a load ? You should always put a few amps load on the 12v and 5v...I use a few old scsi HVD drives for that purpose, they draw lots...
R41 and R42 are minimal load resistors, do not change those.

Horun wrote on 2025-01-05, 18:33:

(the +5 V line gives about 4.75 V no load, and the +12 V line about 12.65 V).

You testing the volts with out a load ? You should always put a few amps load on the 12v and 5v...I use a few old scsi HVD drives for that purpose, they draw lots...

Unfortunately when I use the PSU (with load) +5 V line drops to about 4.67-4.68 V depending on the system's load. I can't rise this voltage (+5 V line) through VR1 due to joint rising of voltage on +12 V line and triggering of some overvoltage protection of the PSU, when +12 V line value gets closer to 13 V (maybe more than 12.7 V or so).

Horun wrote on 2025-01-05, 18:33:

R41 and R42 are minimal load resistors, do not change those.

So, there's no cure for the problem?

On the 5v+ out there is two 2200uf caps in the schematic, if one leaked then both should be replaced if your board is similar. Also they should be near spec whatever was original, using too low esr or too high compared to originals can have odd effects in a psu...sorry is all I got.

Actually, in my first comment I saved some additional explanations on the root of the problem and unsuccessful repair attempt because of the "wall of text" effect. I found an YT-video* from beyond "the (new) iron curtain" in which there's explanation for the very same problem, but the given solution didn't work for me.

In these series of Fortron/FSP power supplies of the time period in question, the core (ferrite) of the output choke (inductor) is made of not very good quality material and changes its ferromagnetic parameters over time, resulting in the observed effects with a change in output voltages due to altered impedance and inductance of the choke. Also, the windings of the choke in question are not factory wound across the entire ferrite ring and the advice from the youtuber was to rewind them with an even distribution across the entire core of the choke, and also removing 1-2 windings from the +12 V winding with the increased output voltage. I did all of this, but for my PSU it did not give any noticeable result with improvement in the output voltage values in the problematic +5 and +12 V lines.

I'm assuming that the output choke of my PSU with its almost 20 years of aging and with plenty of heating and cooling cycles has already reached some new stable ferromagnetic properties of its core and the whole circuit just needs new values for some of the components according to the settled new inductance of the choke.

* - The YT-video is in russian language, so I don't feel the need to post the link here (if anyone understands the language and would be interested in seeing it and hearing the explanations, I can post the link).

Horun wrote on 2025-01-05, 19:34:

On the 5v+ out there is two 2200uf caps in the schematic, if one leaked then both should be replaced if your board is similar. Also they should be near spec whatever was original, using too low esr or too high compared to originals can have odd effects in a psu...sorry is all I got.

Nope, these +5 V rail caps are fine and healthy - I've checked them with my component tester/ESR meter. See the explanation for the root of the problem in my previous comment.

P.S. The circuit diagram is fine - it only lacks connections to PFC choke (and the PFC choke) for my PSU model, see my first post:

analog_programmer wrote on 2025-01-05, 16:05:

(I'm attaching a circuit diagram for FSP ATX-350PNR model, because for 350PNF I couldn't find, but the only difference is missing PFC reactor for the PNF model)

FSP ATX-350PNR - no PFC model
FSP ATX-350PNF (my model) - PFC model

analog_programmer wrote on 2025-01-05, 16:05:

I haven't touched the VR2 trimmer, which is on the main PCB, because I have no idea how it will affect the +12 V, possibly the +3,3 V (this voltage at least is stable - about 3,35 V and not affected by VR1 on the small PWM PCB) and +5 V voltages.

Well, maybe I have to try fiddling with VR2 before this PSU goes back into the closet as a spare parts donor.

analog_programmer wrote on 2025-01-05, 19:40:

and also removing 1-2 windings from the +12 V winding with the increased output voltage. I did all of this, but for my PSU it did not give any noticeable result with improvement in the output voltage values in the problematic +5 and +12 V lines.

I guess you are talking about L1 in the schematic. This inductor is meant to reduce cross-regulation issues, and does not affect 5V-to-12V that much if both rails are loaded "similarly". So I guess you can have one of two issues:

1. You are loading the supply in a way that cross-regulation compensation is required, i.e. at your load on +5V and +12v, L1 is supposed to push some energy from +5V into +12V. If the core of L1 failed and the coil does not transfer energy as expected anymore, this could explain your issue.
2. L1 is working fine, but the transformer outputs (from T1, the main transformer) are bad. The basic ratio between +5V and +12V is set by the turns ratio in T1. That ratio does not change by component aging, but you might need to check whether all rectifier diodes are still working OK.

mkarcher wrote on 2025-01-06, 17:27:

I guess you are talking about L1 in the schematic. This inductor is meant to reduce cross-regulation issues, and does not affect […]
Show full quote

analog_programmer wrote on 2025-01-05, 19:40:

and also removing 1-2 windings from the +12 V winding with the increased output voltage. I did all of this, but for my PSU it did not give any noticeable result with improvement in the output voltage values in the problematic +5 and +12 V lines.

I guess you are talking about L1 in the schematic. This inductor is meant to reduce cross-regulation issues, and does not affect 5V-to-12V that much if both rails are loaded "similarly". So I guess you can have one of two issues:

1. You are loading the supply in a way that cross-regulation compensation is required, i.e. at your load on +5V and +12v, L1 is supposed to push some energy from +5V into +12V. If the core of L1 failed and the coil does not transfer energy as expected anymore, this could explain your issue.
2. L1 is working fine, but the transformer outputs (from T1, the main transformer) are bad. The basic ratio between +5V and +12V is set by the turns ratio in T1. That ratio does not change by component aging, but you might need to check whether all rectifier diodes are still working OK.

On the schematic the big output inductor (choke) is L1.x (L1.1, L1.2, L1.3...). I rewinded it (according to solution from the YT-video), so that its new windings are spread more evenly over the ferrite core. Unfortunately there was no visible result in my case.

Ok, I'll find and post a link to the YT-video with the explanations and solution for exact same problem, hopefully the automatic-YT-subtitle-translation will help with the language...

Here it is the video with the solution, that did not help in my case 🙁

P.S. And this is the first video with the explanation of the off- +5 and +12 voltages problem for the same PSU.

Agree should check the diodes. Also do you have a oscilloscope ?
Am wondering what the ripple on the +5v line is compared to the +12v line under small load, might help pinpoint something.
In a pinch can use a good digi voltmeter on ac (depending the dvm, some do not read well above a few hundred hertz)

Horun wrote on 2025-01-07, 03:14:

Agree should check the diodes. Also do you have a oscilloscope ?
Am wondering what the ripple on the +5v line is compared to the +12v line under small load, might help pinpoint something.
In a pinch can use a good digi voltmeter on ac (depending the dvm, some do not read well above a few hundred hertz)

No, I don't have any oscilloscope, they're pretty expensive even at second hand market. What's the point in using my digital multimeter in AC voltage measuring mode on a low voltage DC line?

Which diodes do you mean (BD3, BD4, BD5 ?) and what should I check them for? If BD3 is not operating as two diodes, the PSU will not supply any voltage close to +5 V DC. The same goes for BD4 and BD5 on the +12 V line.

I'm still using this PSU when testing old PC configurations, but I don't like the picture: 4.75 V <- +5 V ; +12 V -> 12.65 V . I think, these voltage deviations cause some kind of stability issues with tested systems in some cases.

P.S. As far as I understand russian language (and I understand it no worse than english language), the words "типичная неисправность" in the titles of the only two videos on whole YT platform, that give an explanation on the very same voltage problems with pretty same Fortron PSU model, mean "typical malfunction", and "типичная поломка" mean "typical failure/damage". And there's nothing related to failed diodes or main transformer. There is also a third video for Fortron PNR model PSU (the one like mine, but without PFC) on the same YT-channel, in which the guy directly replaces the problematic output inductor with a similarly sized choke taken from another model PSU, and then +5, +12 V lines voltage values immediately get into the normal range.

I see that here I'll not be able to find some electronic's way to reduce +12 V line output voltage slightly under 12 V, so both problematic +5 and +12 V lines voltage values can be adjusted close to their nominal values by VR1. Ok, let's try the electrician's way 😀

Maybe not the smartest idea, but what if I add some 35+ A power diode on the +12 V line after the scre*ed-up output inductor? Its V-drop will reduce the output voltage by 0.6-0.7 V, which will be enough for my case:

The problem here is, that I can't find any not so bulky power diode for more than 35 A current. The label on the PSU "says": +12V1 = 18.0 A; +12V2 = 16.0 A. Any suggestions for a suitable less bulky diode?

That is a good idea. My thought on diodes is for BD3 sbl2045 schottkys, as they age their voltage drop (Vf) can raise, listed at 0.5v @10A, which could account for a lower +5v output. Noted the 12v are sfa1603G of 1.0v Vf @10A..just thinking aloud...

Horun, now I've figured out the idea with the power double diodes measurements. It really makes sense if the V-drop of the double diode BD3 on the +5 V line has increased over time. On the other hand, there is no reason for the voltage on the +12 V line to increase with time due to similar aging effects for BD4 and BD5.

Of course I can dismantle the heatsink and measure V-drop for the BD3 diodes, just to be sure if they have some deviation from the nominal value. But If I replace BD3 with brand new substitute and still +12 V line stays about 12.5 V at +5 V line set to 5 V, this will not help me much in general.

I was hoping to find some electronic's way to adjust +12 V line voltage below 12 V, so I can use VR1 to set both +5 and +12 V lines to more acceptable values.

analog_programmer wrote on 2025-01-08, 17:22:

I was hoping to find some electronic's way to adjust +12 V line voltage below 12 V, so I can use VR1 to set both +5 and +12 V lines to more acceptable values.

Your idea of simple high current diode is probably the easiest. You could use 2 lower amperage in parallel to get higher max Amps.
Something like two of these 25A max types (with Vf of .7 @1A, .8v @10A and axial leads): https://www.digikey.com/en/products/detail/di … P2500A/22191581
You could try first no load and then small load and try to set the +5v near 5.1v and see what the 12v looks like....hopefully near 12.0-12.1v

Horun wrote on 2025-01-08, 18:04:

Your idea of simple high current diode is probably the easiest. You could use 2 lower amperage in parallel to get higher max Amps.
Something like two of these 25A max types (with Vf of .7 @1A, .8v @10A and axial leads): https://www.digikey.com/en/products/detail/di … P2500A/22191581
You could try first no load and then small load and try to set the +5v near 5.1v and see what the 12v looks like....hopefully near 12.0-12.1v

Actually I was also thinking of two or three smaller diodes in parallel, but maybe I should pick them from a pile of diodes, so that their parameters are nearly equal - I don't want to get an overload by current on one of them.

Thanks for approving my simple electrician's idea and for your suggestion for diodes I may use. As long as there are no suggestions for changing the original circuit's components, I think I'll just take "the path of least resistance" i.e. the easiest way to fix the problem even if it's a quick and dirty fix 😀

Horun wrote on 2025-01-08, 18:04:

Something like two of these 25A max types (with Vf of .7 @1A, .8v @10A and axial leads): https://www.digikey.com/en/products/detail/di … P2500A/22191581

I can't find any similar in size and current rate of 25 A (or more) like these P2500A at my nearest local electronic parts shop, but I found FX2000A rated at 20 A/50 V (Vf < 0.82 V at 5 A, < 0.94 V at 20 A, max Tj = 150 degree C, axial leads) for about EUR 1 each piece. 40 A for two in parallel, I think they'll do the job. What do you think?

Yes think those should work well.

Horun wrote on 2025-01-08, 21:12:

Yes think those should work well.

Great! My last concern is their operating temperature and cooling, since I can't attach any heatsinks to them and they will only be cooled by the airflow from the PSU's fan. On the other hand these diodes will not actually work in rectifying mode. I just don't have an idea what amount of heat they will produce in that case.