Help Identify blown fuse or trace on a PCB

[edlee]

Hey friends,

I have an analytical scale my late father gifted me when i first started my work, and after a recent move, someone plugged in a power adapter with reversed polarity (center positive instead of the required center negative), and I assume blew a fuse or a printed trace i am having an issue locating.

I spoke to manufacturer and they will unwilling to offer a schematic, and because of age do not offer replacement pcb, i attached a high resolution board short, if someone can help me identify the trace or fuse. Thank you all, i appreciate all your efforts, this holds a lot of sentimental value for me.

I took this image with a 40mm macro, but if there is a more suspect area you may need more detail, i can take more photos with 105mm macro lens

For reference, this is a Sartorius GC 503 Precision Weighing balance

 

[edlee]

After analyzing this some more, do you guys think it might be the pcb transformer that needs to be replaced?

 

[JimW]

I don't think you are going to get off that easily. The 8 pin SOIC on the right side of the PCB is a LT1172 switching power supply IC. So it is going to take the input DC (which will vary a fair amount under load) and convert it to the voltage that the board wants. I would suspect that chip is no longer functional. If you are lucky, that is the extent of the damage. Worst case is the negative voltage input got through to the other components and did additional damage.

You can Google LT1172 circuits and get an idea of where to check your board to look for an output voltage. Fortunately, there are only a few ways that this chip will be used. You should be able to find the electrolytic filter capacitor on the output and see what the voltage is there.

JimW

 

[Mickster]

Someone has done a repair in the vicinity of LT1172 already, judging by the soldering iron burns on the white & blue caps close by, and the piezo buzzer.
Also, there is a TH diode going to LT1172 pin 5, where an SMD diode would have originally been.
So, maybe it's not the first time the polarity has been reversed.

 

[edlee]

Someone has done a repair in the vicinity of LT1172 already, judging by the soldering iron burns on the white & blue caps close by, and the piezo buzzer.
Also, there is a TH diode going to LT1172 pin 5, where an SMD diode would have originally been.
So, maybe it's not the first time the polarity has been reversed.

Wow, you are right, i completely forgot it happened 10 years ago as well, i sent it in to distributor for repair, and i thought they replaced pcb, but i guess they repaired it.

Thank you for pointing this out, i will replace the LT1172 flyback regulator, and TD diode

 

[edlee]

So far i changed the TD diode, and now the board powers up, and its giving me an error code 54, which according to the manufacturer means ' Err 54 ' means that signal from the A/D converter circuit is too low. should i order a new LT1172 and see if that helps?

 

[Mickster]

Going back to the first pic you posted, the electrolytic cap at the top-right corner appears to be starting to 'dome'.
Might want to check or replace that too.
Here's the datasheet for the LT1172:
https://www.analog.com/media/en/technical-documentation/data-sheets/117012fi.pdf
I'll defer to our more knowledgeable members regarding what might likely be damaged downstream of LT1172, prior to replacing it.

 

[edlee]

The power coming out of the board is fine, the issue is the A/D converter needs to be replaced according to the manufacturer

 

[zzyzx]

It's a pretty good idea to replace any electrolytic cap that would have taken a hit when the polarity was reversed. Assume they're bad.

My time is worth more than the price of a new cap. Get an assortment of them. They wear out. Also salvage nice old ones from the goodwill. Laserdisc players and audio equipment have some quality parts inside.

If this thing is good quality it had circuit protection that did it's job im pretty sure though