A friend asked me if I would have a look at his Thurlby bench power supply. It has three separate outputs, one switchable between 3 and 5v and two variable voltage also with variable current limiting. It’s quite a nice piece of kit even though it’s several years old now. Unfortunately, one of the variable outputs had stopped working.
Under the lid is a nicely arranged SMPS (Switched Mode Power Supply) and it didn’t take too long to identify the main PSU blocks and fortunately, the two variable output blocks are basically identical.
After removing the layer of dust that had settled on the PCB topside, I started with some of the obvious checks, looking for damaged, burnt or bulging components but there was nothing obvious.
Removing the PCB from the chassis and an underside inspection looking for broken tracks and dry-joints showed nothing obvious either.
It was only when I was poking at things with my screwdriver tip (I do this to check for things that are loose) that I saw there was a wire broken off from one of the main inductors; looked like a dry-joint had failed. I re-soldered that and powered on. Now the retched PSU refused to start at all.
Using my lovely Atlas testers from PEAK, I checked diodes and resistors. If readings looked odd, I would check the same component on the other channel; the Atlas testers are pretty good but obviously can be confused when testing components in-circuit.
Next, I removed and tested all the silicone devices from the faulty channel but still no faults were found. I then did the same for the three larger electrolytic caps and again, no fault was found but on powering up, the fault seemed to have rectified itself. I can only assume that in the process of removing one of the electrolytic capacitors, testing and then re-soldering it, I fixed a dry-joint I hadn’t spotted.
Now I’ve fallen for this once before.
My new rule of thumb is when presented with a fault; ALWAYS re-solder the joints just to make sure. This is doubly important now my eyesight isn’t as good as it used to be :(
Under the lid is a nicely arranged SMPS (Switched Mode Power Supply) and it didn’t take too long to identify the main PSU blocks and fortunately, the two variable output blocks are basically identical.
After removing the layer of dust that had settled on the PCB topside, I started with some of the obvious checks, looking for damaged, burnt or bulging components but there was nothing obvious.
Removing the PCB from the chassis and an underside inspection looking for broken tracks and dry-joints showed nothing obvious either.
It was only when I was poking at things with my screwdriver tip (I do this to check for things that are loose) that I saw there was a wire broken off from one of the main inductors; looked like a dry-joint had failed. I re-soldered that and powered on. Now the retched PSU refused to start at all.
Using my lovely Atlas testers from PEAK, I checked diodes and resistors. If readings looked odd, I would check the same component on the other channel; the Atlas testers are pretty good but obviously can be confused when testing components in-circuit.
Next, I removed and tested all the silicone devices from the faulty channel but still no faults were found. I then did the same for the three larger electrolytic caps and again, no fault was found but on powering up, the fault seemed to have rectified itself. I can only assume that in the process of removing one of the electrolytic capacitors, testing and then re-soldering it, I fixed a dry-joint I hadn’t spotted.
Now I’ve fallen for this once before.
My new rule of thumb is when presented with a fault; ALWAYS re-solder the joints just to make sure. This is doubly important now my eyesight isn’t as good as it used to be :(
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