Artesyn LPT42 SMPS repair

[zkrx]

After 10 years of use, my FAS Axe-FX II died on me. It consists of two boards: an Artesyn LPT42 SMPS that supplies the main board. It had been running fine for 20 minutes when the device turned off right under my nose. The mains fuse blew. I changed the fuse, powered it on again and it blew again immediately.

Artesyn LPT42: https://eu.mouser.com/ProductDetail/Artesyn-Embedded-Technologies/LPT42?qs=oQrZyJBXqWGYKAz4i%252BWkOA%3D%3D

The fuse also blows when I disconnect the SMPS from the main board, so it really is the SMPS board that is at fault.

Considering a replacement power supply will cost me nearly 100 USD, and that components are only ~1 USD, I am willing to spend a few hours on this. But I don't have a lot of free time. I wondered if someone could tell, from experience, what component would most likely fail, what I should measure first, any tricks I could use to diagnose and fix this quickly-ish.

I know that SMPS troubleshooting can be a pain, so I won't be mad if you tell me that I should not waste my time with that.

[asis]

Hi,

There is a similar PSU.
The most vulnerable components are highlighted in red.
If you are not completely sure that all components are in good working order, connect the PSU through a household incandescent lamp (NOT LED).

[zkrx]

Thank you so much. I still have a few incandescent bulbs lying around.

Any tip for testing IC1 (3842) or should I just blindly swap & test?

[asis]

If the circuit matches exactly, then check MOSFET Q1 for a source/drain short circuit.
Acquire the skill of checking MOSFET's.
Release its gate and briefly touch one of the tester's probes to the gate in relation to the source, check the integrity of the structure by swapping the probes, keeping in mind that there is a diode inside the structure.
The charge on the gate after touching it remains for quite a long time and you will have time to measure the state of the source / drain transition.
Thus, by turning on/off the MOSFET you can verify its integrity.
It is possible that the current limiting resistor "R6 0.28E" has burned out.
Capacitor C5 (47mF x 50V) is most often the cause of failure due to its degradation under 24 x 7 x 365 operating conditions.
In order not to assemble a stand for testing 3842, it is enough to check the state of the internal OUT driver (pin 6) in relation to GND (pin5) and Vcc (pin7).
If the structure does not raise suspicions, most likely the IC is working.
The feedback loop in this particular circuit monitors +5V and +12V.
If the circuit has differences, it is better to do reverse engineering.
Everything consists of little things.
--
Follow safety precautions!