Repairing power supply for vending machine

[Kobliha]

Hi, I'm having trouble fixing this source. In the beginning, the power supply did not give any voltage, so I found out that the VCC capacitor for the IC controller was dry, so I replaced it and the power supply started to give voltage, here the second problem occurred, namely that the output should be stable, but the output still oscillates (without load) between 25-27 volts and should be 28 volts. Under load, the output voltage drops by 1-2 volts and keeps oscillating and is not stable at all. The board contains an IC member Viper 22A 8 DIP, 2 control mosfets f12nm50n and a control oscillation controller Lb598 16 DIP.
Here are more photos of power source https://drive.google.com/drive/folders/1g2I6PAmJGAuG9QPe78S0CVPgL3ARrden

[SeanB]

Would look at the secondary side, and replace all the electrolytic capacitors there, as they likely are all worn out. 105C low ESR capacitors of the same value and voltage, or next higher value or voltage if not available.

[Kobliha]

i did that but the problem is still the same

[DavidAlfa]

Replace all the small caps, check the large 400V one .

[Kobliha]

You think replace all SMD capacitors? or all THC

[DavidAlfa]

The small ones (1...100uF). I saw no smd parts in the pictures?

[Kobliha]

Yes, I replaced all the electrolytic capacitors with new ones and took out the others and measured them, they were fine. sorry I forgot to upload the bottom photo of the board to google drive

[johansen]

is it actually this?
https://www.st.com/resource/en/datasheet/l6598.pdf

is it stable under load?

[Kobliha]

Yes, it is exactly what you sent.
when connecting a load to the output, the volts drop by 1-3 volts and it constantly fluctuates unstable by 0.5-1 volts. so the output is not 28 volts but 25-26 volts

[johansen]

Yes, it is exactly what you sent.
when connecting a load to the output, the volts drop by 1-3 volts and it constantly fluctuates unstable by 0.5-1 volts. so the output is not 28 volts but 25-26 volts

toss the whole board in an ultrasonic cleaner.

[uli12us]

Try to replace the optocoupler, thats  the 4 pin part between the big capacitor and the coil, transformer. ISO4 is the name iirc.

[fzabkar]

Does the Viper 22A chip perform an APFC function?? If so, check that the boosted APFC voltage is stable (400V ?).

Edit:

I'm just wondering why there are two power controller ICs. The datasheet for the L6598 shows an L6561/2 being used as an APFC controller, so that's why I asked about the Viper chip. In retrospect, I think I'm wrong.

https://www.st.com/resource/en/datasheet/l6561.pdf
https://www.st.com/resource/en/datasheet/l6562.pdf
https://www.st.com/resource/en/datasheet/viper22a-e.pdf

Is the machine rated for dual supply voltages (120VAC and 240VAC)? If so, then perhaps the Viper IC is pre-regulating the supply by boosting it to 400VDC??


Edit:

Is there a standby supply? Would that be why there are two power controllers?

[Kobliha]

The source is supposed to give two outputs, namely 28V and 5.5V. At the beginning, it did not give either of them, after replacing the capacitor in the viper22 IC controller, the voltage increased. The 5.5 volt output works fine even under load (probably the output for the display and other controls). Unfortunately, the 28 volt output gives only 26 volts, and even when the power is connected, it fluctuates between 25-26 volts (it is not stable). This output should be for powering the motors and the CPU control unit for vending machine.
Power supply for this vending machine is 240V AC

[Kobliha]

I tried putting the board in an ultrasonic cleaner and that didn't help, now I'm waiting for the optocouplers. After measuring, I found that on a board that works (I have a working one here and I am fixing a non-working one). On the function board, the voltage on the TL431 is 1.8 volts on one pin and 17 volts on the other between the reference. With a broken board, there is 2.5 volts on one pin and only 1.8 volts on the other.

[fzabkar]

On the function board, the voltage on the TL431 is 1.8 volts on one pin and 17 volts on the other between the reference. With a broken board, there is 2.5 volts on one pin and only 1.8 volts on the other.

I would think that 17V and 2.5V would be present on the working PCB. :-??

[Kobliha]

So I also tried replacing both optocouplers and TL431 and neither of them helped..

[Kobliha]

I found that if I calculate the values ​​of the resistors for TL431 and replace them with those that are there, the output voltage should be 28V, then the output is a stable 28V without change even under load, then I don't understand where to look for the problem that when I have resistors that should eventually give an output voltage of 25V, when connected, the board gives 26.7V and when the load is connected, this value is unstable, the voltage drops and keeps moving between 25-27V. So do you think there is something wrong with it a mess? or that it's normal for sources like this.
Resistor R2 for Vref has a value of 2000 ohm, Resistor R1 has a value of 18k ohm.