ATTEN APS3005Si: Schematic, pots replacement and meters adjustment [SOLVED]

[Datman]

Hi all

Many years ago I bought a APS3005Si power supply. It works, but a current pot doesn't work well at minimum; also, I don't like the two "coarse" and "fine" pots. So, I'd like to replace them by only two multi turn potentiometers. The problem is that I would need a 55kOhm voltage pot and a 15kOhm current pot for nominal max voltage and current. Then I have to modify the circuit, changing some resistors to use standard value potentiometers.

Does anyone have a schematic of it? I found a schematic, but it is different, because potentiometers have different values from mine: in my PS, voltage pots are 50kOhm and 5kOhm; current pots are 10kOhm and 5kOhm.

Does anyone have also the meters schematic? If I had them I could adjust the meters, because there are only two trimmers, then I can't adjust zero and gain for the two instruments. I have to adjust some fixed resistors...

Thank you all
Gianluca

[Datman]

Searching for the board number SAT7.820.688C I found the schematic
I haven't found the meters schematic.

[Datman]

There are settings for max voltage and max current, then I should have no problems using a 10k and a 50k multi turn potentiometers I bought from Amazon for 26€/2 pairs. They are made in China (they aren't Bourns made in Mexico...), but they seem to be good:
https://www.amazon.it/gp/product/B08FHW13HS
Before doing that, I developed a simple mod to adjust zero on the Ammeter: at low currents, ammeter showed 5mA more: i.e., 15mA instead of 10mA.

1. Pull up pin 9.
2. Insert 10 Ohm between the pin and the track.
3. Solder 4k7 to pin 9.
4. Connect a 4k7~10k "zero adj" trimmer between -5V (pin 1) and meter's Gnd (pin 3 or C10. It's connected to +VOut!).
5. Connect the wiper to the other end of the 4k7 resistor.

6. Adjust the zero adj trimmer for 0.000A with no load.
7. Short output by a reference ammeter.
8. Adjust current limit at about 2A on the reference ammeter.
9. Adjust the original multi turn gain trimmer for same reading.
10. Adjust current limit at about 5~10mA on the reference ammeter.
11. Verify reading; adjust zero adj if needed.

[Datman]

It works!
I also reduced output capacitance from 470uF to only 10uF, avoiding high current pulses when shorting in current limit. I'll check stability in all conditions with the scope.
I tried adding a 100uF between the base of the driver and Gnd, but it slows down the loop and causes relay oscillation.