Repair a Metrix AX502 bench power supply

[Lycias]

Hi,

I’ve recently acquired a Metrix AX502 bench power supply.

Documentation is available: https://catalog.chauvin-arnoux.com/fr_en/ax-502.html

This power supply has two variable outputs (0 to 30V, 2.5A):  “master” and “slave”.
The “slave” output is working as expected. However, the “master” output is faulty.

When rotating the potentiometer, the output varies from 150mV to 1.65V approximately, instead of 0V to 30.5V, according to the specifications. Also, the screen doesn’t display voltage and current as expected, but only the decimal dots.

I’ve started to investigate the issue. So far, I haven’t noticed any faulty components, but I’ve noticed the voltage is incorrect at various places. I have yet to disassemble the two circuit boards.

Unfortunately, I failed to obtain schematics from the manufacturer (Metrix / Chauvin Arnoux).

Would anyone familiar with linear power supplies could provide some advice about the possible causes of this issue?

Failing "master" output, potentiometers to minimum:



Failing "master" output, potentiometers to maximum:



Working "slave" output, potentiometers to minimum:



Working "salve" output, potentiometers to maximum:



External views of the power supply:









Top view of the power supply:



Thanks.

[Lycias]

Here is a description of the different parts of the main circuit board:

[shakalnokturn]

First start checking that all the DC voltages after diode bridges are present.

Considering there is no display on the master panel meters I would look at what IC's are used for that function and if they are getting correct power, this and the no output symptoms would make me expect that you're lacking a +/-12V supply somewhere.

Your photos show a bare / burnt / corroded trace on the front PCB behind the slave output. Have you checked on that? If it is corrosion take a look at the hidden side of the front PCB.

[tunk]

Disclaimer: I don't know much about PSUs.
That said, it looks like they are (almost) identical:
- are the outputs from the transformer the same.
- are the resistances of the potentiometers the same.
- there is a device under a folded metal heatsink which
  could be switching between the transformer windings.
  Does it work? (Don't ask how to check.)
Also, if you connect e.g. a 100ohm resistor, does it still
show 150mV to 1.65V?

[goaty]

Maby also check (low voltage) power supply for display and opamps (like 7812 linear regulator) which could be gone bad.