HMP4040 Schematic

[Murphy86]

I run small business assembling and maintaining ATS (automated test systems).
Goten quite few RMA-s of failing HMP4040 PSU-s. Some are under warranty some too old for that.
I have excluded AC side issues as some older systems have Agilent A66000A and they have ZERO issues regarless of age.
Failing channel(s) differ but its always LTC3703 circuit that seems to be pre regulator before linear stage. 
R&S wont respond ofc, allready complained about low quality screw terminals at back(they sometimes wont give connection even if wire is tight under screw) but no response.
Hoping with schematic i could maybe find out design issue and maybe replace parts with different(say example mosfet ratings are designed too close to failure).
PSU is never run close to capacity, usual loads are, under 5V and 2A(unless DUT has fault). So overheating etc is not an issue.

[coromonadalix]

thread w pictures

https://www.eevblog.com/forum/metrology/hmp4040-calibration-and-adjustment/

[Harry_22]

Hi!
Let us see the behaviour of several HMP.
Only two pictures each. Switched off outputs and switched on.

[Murphy86]

Sadly have no picture/video to offer for symptoms.
have one in RMA at the moment,  that one is waiting parts(correspondig channel removed). But symptom is output on no voltage at output.
Diagnosos: 2x 7A fuses in channel AC input will blow and atleast one mosfet will be dead. 
Transformer measured 44VAC(should be around 61VDC after bridge) and mosfets are 100V(IPD180N10N), it should be fine. Yet they burn.
Only thing i can think of is in ATS outputs are turned on and off many times. Probably over thousands of times per day. But i mean, its LAB PSU its expected to have this use.

[Harry_22]

When you solder fuses hold them with tweezers all the time.
Even the original ones can swell.