Power supply relay board troubleshooting - Shimadzu AA Spectrometer

Thread Starter

ChemistSam

Joined Jan 19, 2021
2
This circuit board powers hollow cathode lamps for a analytical chemistry instrument. The lamps run at approximately 20 mA.

F1 intermittently blows and I can't figure out what is causing it. R36 and R37 were blown and replaced, (I did not identify why; I thought it was due to power fluctuations) but this didn't solve the issue. All the surrounding resistors, capacitors and diodes were removed and tested ok. Both transistors on either side of the fuse were replaced as well as a few of the diodes because of the surrounding discoloration I noticed, but the instrument works for a short time and then the fuse blows again. It's beginning to happen more frequently.

Does anything in the photo jump out to any of you experts or does anyone have an understanding of this board to help guide my troubleshooting? Thanks in advance.
 

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Thread Starter

ChemistSam

Joined Jan 19, 2021
2
Hi Eric,

Thanks, good eye. There appeared to be continuity, but I cleaned up all the connections anyway and will see if that was the issue. Both the burnt resistors are on the base of each of the transistors. I don’t think they amplify the same circuit but I’m trying to figure out the common connection. In any case I don’t know what would cause both to fry. I’ll post what I figure out.
Sam

hi Sam,
Hard to tell the cause of the basic problem, but the Red marked joint on the PCB, linking the Transistor and the burnt resistor looks 'Dry'.

EView attachment 228252
 

sarahMCML

Joined May 11, 2019
39
Hi Eric,

Thanks, good eye. There appeared to be continuity, but I cleaned up all the connections anyway and will see if that was the issue. Both the burnt resistors are on the base of each of the transistors. I don’t think they amplify the same circuit but I’m trying to figure out the common connection. In any case I don’t know what would cause both to fry. I’ll post what I figure out.
Sam
F5 also appears to be blown.
 
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