I have an LED light bulb in which there are 16 LEDs in series powered by a constant-current supply.
One of the LEDs burned out so I just replaced it with a blob of solder and it worked for a while, but then started flickering.
I measured the voltage drop on each LED and they all had a voltage of about 5.5V except one which had 5V. I bypassed this LED as well (putting a blob of wire parallel to it), but this did not help with the flickering at all.
The next thing I did was to add another capacitor across the LEDs. I kept the already present 2.2uF cap and added a 4.7uF parallel to it. This did not help either.
How do I go about solving this problem? Of course I could just buy a new light bulb, but I don't care about having a working lightbulb, I just want to know how to troubleshoot such issues in general.
Datasheet of the driver (MT7828C): https://datasheetspdf.com/pdf-file/1260493/MaxicTechnology/MT7828/1 ‒ the very last paragraph indicates a possible the cause of this problem:
The VDD (Pin1) de-coupling capacitor (typically 1uF ~ 4.7uF ceramic capacitor) must close to the pin (Pin1) as much as possible. Better not exceed 5mm. This can greatly improve the system noise immunity.
On the actual PCB, the distance of this capacitor from the VDD pin is about 5mm.
The flickering is mostly gone after I re-soldered the wires connecting the PCB to the LEDs (which are mounted on a sheet of aluminum). During normal operation (i.e. no flickering), the total voltage across all LEDs stays near 69V. When the flickering happens, the voltage drops to around 61V. I shall now determine whether there is also a drop in current (in which case the fault is most likely in the power supply) or not (in which case the fault is in the LEDs).