Acculab ALC-80.4 analytical balance repair

[sw2022]

Hope some experts can advise on this one, even if not familiar with the equipment. Just started trying to fix an Acculab high-accuracy lab balance. Cannot find a schematic or service manual, just the user instructions. Bit of a sad history, vendor originally advertised for sale working so I offered and bought. Vendor then came to test pre-dispatch and found non-working, totally dead, with a loose centre pin in the PSU input socket thought to be the cause. The socket was replaced or repaired, not sure which. It has a lot of hotmelt around it now anyway. Balance then was reportedly found still dead on retest. I then negotiated a large discount with a plan to attempt repair. Balance arrived here without any PSU. Reading the specs it is supposed to have a 15V centre negative supply via the jack, range 12-20V. I reconfigured an existing wall wart (12.4V off load) as centre negative but found the balance was indeed dead on attempted power up. Opening it and powering the board again revealed a hot smell in the area of the boards power regulation components, see my image, this is the right part of the PCB pictured. The inductor felt hot, and being stupid I tested the LT 1172 switching voltage reg with my finger. The test was positive for overheating - it burnt my finger.  So, looks like a fried switching reg. My guess is that their “engineer” may not have had the correct PSU to hand as they failed to supply me with one. The jack is the common type found on 12V wall warts, though usually connected centre positive, my hypothesis is that they plugged what they had to hand in and applied reverse polarity, frying the reg IC? The current wiring of the jack seems correct in that pin 1 of the LT1172 has continuity to the negative input, it is supposed to be grounded. Interestingly pin 5 does not have continuity to the positive supply, measured 33M, it’s supposed to be V in.

I can get an LT 1172 from Mouser, not a cheap reg, may not be the exact variant (can’t find one with the 426 additional marking) but should I guess be compatible. I am currently wondering if this is worth trying. I don’t know a great deal about switchmode circuits. What is the expert  opinion on likely failures all over the board if reverse polarity was indeed applied? Any hope for this board now? Equally I suppose the reg could just have randomly failed, better scenario maybe.

I checked the misaligned component attached to pin 5 btw, it does indeed connect to its pads and is not shorted.

Sorry no schematic so there is a lot of guesswork involved here. Pictures of the balance PCB with the discoloured LT1172 are attached

[vindoline]

A couple of things. First, 426 marking on the 1172 is a date code. So don’t worry about it. The ‘misaligned component’ needs to be addressed. It looks like a diode to me and it may be there for reverse polarity protection. It looks like things got hot enough that the solder melted and the diode moved. If you’re expecting a positive voltage at pin 5, it currently looks to be backwards to me. You might check to see if the + 15 V is present there.

[dhillman]

Having recently repaired a similar vintage Acculab balance, ensure that you replace and thoroughly clean under and around all of the surface mount electrolytics. 

While this may not be the main issue, it will help with unstable readings, and possibly power-on. 

[graphtronics]

Acculab was the low end brand of Sartorius.
Even when new, some like me avoided them like the plague.
Some received them as Out of the box failure (yes brand new)

Sartorius did not provide replacement parts, except for AC adapter and other.
After years they began to sell the keypads and other small parts.
Not aware of the schematics ever being released.
The division was closed long ago.

Conlusion
Even if you fix it, there is no guarantee its operation to be accurate.