Modifying output voltage of an Old Dell laptop charger

[Vampy_1110]

I'm trying to turn this old dell laptop psu into a variable power supply (5-19.5) as i have already modified a few power supplies before and now i want to do the same with this but it's a bit complicated
The pic below has an op-amp ic (Z103WA) that does the CC/CV adjustments and a double diode 3 pin ic at the far right
As you can see there's no TL431 or Switching ic self-regulations that i can tweak easily.
Any help would be appreciated

[stj]

i see an optocoupler,
so on the secondary side you need to look how it works with no tl431
maybe it has a zener diode

[MegaZAC]

$3 step down dc-dc board from Aliexpress will keep the psu and save you time. And will have a voltage screen, handy connectors, and be used with othe psus when this one dies.

[Vampy_1110]

i see an optocoupler,
so on the secondary side you need to look how it works with no tl431
maybe it has a zener diode

The opto coupler is directly connected to the double diode 3 pin ic
Removed the sticky glue at the top side and no I don't see any zener
I attached a schematic from TLW03WA ic (similar to mine) from it's datasheet

[Vampy_1110]

$3 step down dc-dc board from Aliexpress will keep the psu and save you time. And will have a voltage screen, handy connectors, and be used with othe psus when this one dies.

$3 dollars there are 30$ here (Inflation sadly sucks here)
Plus i'm not going to spend money on this Psu as it's a decade old one and i'm just trying to find a use for it

[harp]

So you have only few resistor on right side of opamp, how about to paralel with each of it solder 100k potentiometer and monitor the output while rotating?

[noppa]

To set the voltage reference 1% resistors are usually used. I would try modifying R70 or R73 and see what happens.
Can't read the marking on D74, but this could be a zener or a 431.

[stj]

i see a 2.5v reference in that schematic

[redwire]

The TL103 operates like a TL431, so you have the usual two resistors going to pin 2 for voltage setpoint.
R77 27k+RX71 4k7, R85 5k6 (to GND) my math give a 16.6V output.

[Vampy_1110]

Modifying R76 (472) does change the voltage a bit but the supply makes a ticking noise and goes into some sort of protection when the voltage drops below 18 volts so i can change voltage between 19 and 18 volts 😂....
There must be a preset resistor that does the UV/OV protection or ig R76 ain't the one im supposed to change

Edit: Putting a load on it stops the ticking and stabilizes the voltage but any no load voltage below 15 volts causes it to go tick tick mode
R76 is the most effective at changing voltage, the rest have little or no effect at the right side of op-amp in the image with red with no effect yellow with little and green with the maximum effect (all the way from 25 to 10 volts but with ticking)

[stj]

switching psu's arent supposed to run without a load, maybe consider a load resistor.

[Vampy_1110]

switching psu's arent supposed to run without a load, maybe consider a load resistor.

It works fine between 15-19 volts but starts to go into protection on off on off mode below 15 volts with no load
I think it has to do with some sort of short circuit protection because low voltage no current causes it to go into protection but im scared of touching RX85 and RX94 current sense resistors because this psu can go upto 120 watts so it will nuke itself at lower voltages

[MegaZAC]

I have similar Dell laptop supplies that have some sort of power saving standby, wo when you just turn it on it gives 19.5V, but a few second later without load it drops voltage to just above 0. But as soon as you connect anything taking load it will instantly give you 19V again. And certanly there is a low voltage protection too. The datasheet that you refered to tried exactly 15v in testing: Short-circuit to GND VCC+ = 15V.

[Vampy_1110]

I have similar Dell laptop supplies that have some sort of power saving standby, wo when you just turn it on it gives 19.5V, but a few second later without load it drops voltage to just above 0. But as soon as you connect anything taking load it will instantly give you 19V again. And certanly there is a low voltage protection too. The datasheet that you refered to tried exactly 15v in testing: Short-circuit to GND VCC+ = 15V.

Just need to bypass this low voltage protection ig