bench DC power supply : relays click at ~7.5V without charge

[pbig]

hi everyone,
I come here once again begging for advices. I have an old PRO POWER PS3003 30V-3A bench DC power supply I have recently used for charging my LiIon battery but the result was not great :
- It was difficult to set the voltage:  very light rotations on the fine potentiometer knob lead to strong voltage variations
- I could not limit the output current with the appropriate potentiometer knob
- The relays are clicking when changing voltage around 7.5V for up and down variations even without a charge attached to the power supply
- no burnt smell
- the circuit board is AQF7.820.7120, no fan but large heat spreader. The unit is very similar to the one presented in this thread on badcaps:
https://www.badcaps.net/forum/showthread.php?t=82534
What would you advise me to check at first from these symptoms? Thank you
Pierre

[Eraldo]

- The relays are clicking when changing voltage around 7.5V for up and down variations even without a charge attached to the power supply

This one is easy to explain. Basically the relay is changing the taps of the transformer to reduce the power dissipation of the transistors. If you were to need less than 7.5v it switches taps  to eg 10v rms (maybe higher) so that the linear regulating transistors heat less. The bigger the voltage difference between the collector and emitter, the bigger the power dissipation of that power transistor.

Something like this (not my design)

As for the current limit, that needs more investigation on your part. Maybe you're using it wrong or maybe some component in the current circuitry is burnt.

[Swainster]

Hi Pierre,
Unfortunately PS3003 is a generic name used for various low cost power supplies, and bad caps keep their pics behind a 'registration wall', so posting your own pics will help establish the identity of your supply. That said:
1) difficulty to adjust the voltage maybe due to a faulty potentiometer - sometimes just excercising the pot may improve its behaviour. If that doesn't work then probably best to replace it.
2) Could be same as 1), but not clear what the exact symptoms are. Are you saying that turning down the current limit has not effect? Did you try with the output shorted?
3) Important to know what model you have, but typically, relay switching point is based on output voltage, not load, so no need to have anything attached to hear them click.

As far as lithium ion battery charging goes, I'd highly recommend that you make sure your that battery has its own protection circuit. You dont want a spontaneous fault or accidental user error with the power supply causing a fire.

[pbig]

hello Swainster and Eraldo,
thank you for your answers. I will disassemble the device to spot its chip reference and give it to you.
I ssupect Swainster has the correct answer as a fiurst guess : the coarse current potentiometer (W148) is probably faulty. when I short-circuit the output, turning the coarse pot, current is ok up to ~1A but jumps immediately from 1A to its max ~3A.
both coarse pots are 6.8kOhms while fine are 1kOhms. I will replace the 4 but 6.8k pots are no longer manufactured. Is it best to replace them with 5k or 10k?
The Li ion Lenovo laptop battery has its own BMS protection circuit and limits the current. I feel like one of the cells is dead : 5 yers old is venerable.
Sincerely,
Pierre

[Swake]

Difficult to guess pot size without schematic. Try the 10k first.

That said, with a multimeter you should be able to confirm if the old pot is still working or indeed dead. Pots rarely go bad but often go dirty and oxidize. A good clean of the inside is probably the best answer.