Trying to improve an old generic chinese PSU

[ElectronSurf]

I have a Dazheng PS-305d PSU for almost 12 years now, over the years of abuse it has never failed except the POTs that I had to replace once. the only modification I did was adding a temperature controller circuit for the fan because it was too loud.

A few days ago one of the potentiomers failed again, so I opened it to replace the potentiometer but this time I was really disgusted by the crappy old analog board with 741 opamp and TL431 reference voltage, I could replace the reference voltage with something better like LM4040 but there was no substitute for the op amp that I could find.

The potentiometers are REALLY unstable they drift a lot and takes a long time to stabilize, I also don't like the coarse and fine idea, I want to have a range switch instead.

Also the output capacitor is too big, the sparks on short circuit are fun tho...

What I plan to do:

1. Redesigning the analog board from scratch with faster opamp and better Vref.

2. Replacing one of the voltage and one of the current adjustment pots with a switch to switch between 30V/5A and 10V/1A.

3. Replacing the potentiometers with anything better than what it has! single turn.

4. Adding a switch in the middle (it has the room but no switch was added ) to set the PSU output on/off and while it's off also show the set current.

5. Adding a temperature controlled fan circuitry.

I'm not going to touch the V/A meter, it's good enough.

I have not designed the voltage and current set front panel board...

ps. In my schematic the op amp supply is upside down!

[daisizhou]

It seems that the transformer in the middle is a reduced version. Can the current actually reach 5A?

Potentiometer You need to choose the type of potentiometer shown in the figure below,It is a metal resistance wire wound type.
It turns out that the type is a carbon film potentiometer, which has a short service life.