Zombie Samsung UN55NU6900F dead / no standby voltage - randomly came to life after hours of troubleshooting then died
This is my first attempt at troubleshooting a board like this, I know I could just buy a new board but I'm curious to figure out how this works. I've been testing different components in the evenings this week and I think the problem is with the IC failing to start up for some reason. When the board is plugged in there is no standby voltage.
Then today it randomly sprung to life like a zombie but shortly died thereafter. I have no idea why it would have done that and its the first time I've seen it running. I discharged capacitors on the board last night, then it was sitting today. I had the board out of the chassis bench testing, poking around testing continuity across different components, then plugged it in and one of the lights for optical audio on the board came and you could hear a faint noise which had never happened. The standby voltage pin was reading 12.8V (supposed to be 11.7 but I figure it was just because I didn't have anything plugged in), which is the closest to running I've seen it. Then about 30 seconds later the light on the board went off and it went silent back to it's dead state.
I have no idea how it could have come to life like that. The only thing I can think of is that the caps were all fully discharged and sat idle for the whole day. I had also just cleaned off RM819 (2.2ohm resistor connected to the VCC pin on the IC) with rubbing alcohol to get the silicone off of it.
What I've tested:
Datasheet:
Potential issues I've noted:
Board Top
Board Back
Hot side close up
VCC Loop to VS
Optocoupler Readings
IC - Optocoupler Continuity
Then today it randomly sprung to life like a zombie but shortly died thereafter. I have no idea why it would have done that and its the first time I've seen it running. I discharged capacitors on the board last night, then it was sitting today. I had the board out of the chassis bench testing, poking around testing continuity across different components, then plugged it in and one of the lights for optical audio on the board came and you could hear a faint noise which had never happened. The standby voltage pin was reading 12.8V (supposed to be 11.7 but I figure it was just because I didn't have anything plugged in), which is the closest to running I've seen it. Then about 30 seconds later the light on the board went off and it went silent back to it's dead state.
I have no idea how it could have come to life like that. The only thing I can think of is that the caps were all fully discharged and sat idle for the whole day. I had also just cleaned off RM819 (2.2ohm resistor connected to the VCC pin on the IC) with rubbing alcohol to get the silicone off of it.
What I've tested:
- Power cord / outlet tested ok
- Fuse on the board tested ok
- No visible caps / components burned
- Diodes all appear to be working based diode test
- Rectifier appeared to be working properly, 170V DC between +/- and diode test seemed ok
- No components I've tested were shorted out
- There's 120v AC coming from power cord
- MOSFETs seem ok in diode test
- See voltages on the IC in the table below
| FA6B20N Readings | |||||
| Pin | Reading 1 (V) | Reading 2 (V) | Reading 3 (V) | Reading 4 (V) | Assumed Datasheet Function |
| 1 | 97 | 97 | 97 | 97 | VH (High voltage input) |
| 2 | 0 | 0 | 0 | 0 | NC (No connection) |
| 3 | 0.4 | 0.4 | 0.4 | 0.4 | STB (Standby signal input/ output) |
| 4 | 3.8 | 3.8 | 3.8 | 3.8 | FB (Feedback input) |
| 5 | 0.5 | 0.5 | 0.5 | 0.5 | MODE (Operating mode setting for OCP) |
| 6 | 0.1 | 0.1 | 0.1 | 0.1 | CA (Input power sensing capacitor) |
| 7 | 0 | 0 | 0 | 0 | IS (Resonant current detection) |
| 8 | 0.1 | 0.1 | 0.1 | 0.1 | VW (Winding voltage detection) |
| 9 | 0.1 | 0.1 | 0.1 | 0.1 | CS (Soft-start and burst operation setting) |
| 10 | 10.5 | 10.5 | 10.5 | 10.5 | VCC (Power supply) |
| 11 | 0.03 | 0.03 | 0.03 | 0.03 | LO (Low-side gate driver output) |
| 12 | 0 | 0 | 0 | 0 | GND (Ground) |
| 13 | 0 | 0 | 0 | 0 | NC (No connection) |
| 14 | 3.7 | 3.4 | 0.5 | 0 | VS (High-side floating ground) |
| 15 | 3.9 | 3.4 | 0.5 | 0 | HO (High-side gate driver output) |
| 16 | 9 | 7.1 | 3.6 | 1 | VB (High-side floating power supply) |
Potential issues I've noted:
- The standby circuit reads 0V at the test pin instead of the noted 11.8V
- Tried unplugging everything to see if the pin tested properly but that didn't work
- RM819 connected to VCC noted above doesn't read 2.2ohms, it seems hard to get proper contact with the probes because its so small but readings range from 10Mohms to ~4ohms.
- RM819 also doesn't beep for continuity, again its hard to place the probes because its so small, but other ceramic resistors seem to beep just fine
- When there's power, VCC reads a stable 10.5V. The pin on RM819 connected to VCC also reads 10.5V but then the other side of RM819 is unstable. Sometimes it reads as low as 3.5V.
- The two diodes near the standby pin don't immediately go to OL when probes are reversed in a diode test, they start around 1V then climb to 2V over about 2-3 seconds then read OL. Not sure if this is an issue or not, all other diodes immediately read OL when the probes are reversed.
- The optocouplers gave odd readings in circuit, I've attached a picture below. Some weren't showing as open on the emitter/collector side as they should in a diode test. Some were also showing voltage readings with probes on either the anode or cathode which isn't normal. I haven't been able to test them out of circuit so I don't know if that would affect the readings in some way.
Board Top
Board Back
Hot side close up
VCC Loop to VS
Optocoupler Readings
IC - Optocoupler Continuity
Attached Files
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