I've a got an LED light string with an integral controller that offers several modes; full on, flashing, fading, and so on. It was super cheap after XMAS one year.

Anyway, it quit. I tore open the little controller box and, low and behold, found a spot where a resistor used to be. What replacement value would YOU try?

Oh, and it looks to me like the LED strings 1 - 4 are powered by rectified AC, under the control of Q1 thru Q4, Vmax at terminal 0. Agree?

The bottom of the case. Love the tamper proof screws.


The board. AC mains on the left, light strings on the right. Two terminals under ZD1 are an electrolytic cap on the opposite side of the board. There's nothing else on the other side except a darkened spot behind where R2 used to be. See R2. It's gone!

 

UPDATE
This morning for an experiment I soldered in a 5100Ω, 1/4W leaded resistor to the R2 pads. Full function restored! But my substitute R2 got hot to the touch. I think both R1 and R5 were also hot.

I don't understand why there are 3 resistors (R1, R2 and R5) that appear to be in series. Wouldn't one resistor with a higher wattage rating make more sense?

While running I get about 36V across each of those 3 resistors. That works out to 7-8mA of current and 0.27W of dissipation. That sounds like a problem! There's 4.6V on the cap and across the zener.

Could the capacitor be the problem?

 

Could the capacitor be the problem?

Just replaced it. No change. The zener is not hot and since there's a voltage across it, it doesn't seem likely to be the problem. I'm running out of options.

 

I decided the main problem is poor design. Running a few mA through those small surface mount resistors is just too much heat for them. That's probably why they used 3 instead of one, but that wasn't really a good enough solution.

I bypassed the three 4.7K resistors with two 10K 1/2?W resistors from my junk box. It works fine. The resistors still get quite warm but I can hold my finger on them. I'm going to declare success, put the light string back up and move on. We'll see. There are two strings in the room, so the unmodified one will probably give up next.

 

Yes the leds look like they're run on rectified AC with fets or bjt pulled to ground, the reason for the 3 resistors in series and not one resistor,is the pick and place machine is limited for components or the extra cost of buying a 15K resistor.l, ive seen this many times on smd crap!