Problem with PCB with no 5V power

Thread Starter

jb361800

Joined Feb 28, 2020
11
Hello,
Any help would be greatly appreciated. I have a small refrigerator that has some small blinking LED lights. The refrigerator is powered by 12V AC and has a small PCB board that converts a power wire to 5V and feeds the LED lights. I am not able to get power from the PCB to the lights. I have used a multimeter to test the 5V plug on the PCB and it is not reading 5V. Correct me if I'm wrong, but that tells me that whatever piece on the PCB that reduces the power from 12V down to 5V must not be working correctly. In the first attached photo, I have a flathead screwdriver pointing at the 5V plug that I tested. Can anyone tell me by the photos I have attached which piece is possibly failing. Again, any help would be greatly appreciated. Thank You! 20200228_183748.jpg20200228_183806.jpg20200228_183815.jpg20200228_183824.jpg Can anyone tell me by the photos I have attached which piece is possibly failing. Again, any help would be greatly appreciated. Thank You!
 

jpanhalt

Joined Jan 18, 2008
11,087
There are clearly scorch marks around a few components. The 10 uF/50V capacitor has some. From the side, the picture to too fuzzy to tell whether it is puffed up. If so, that is one sign of failure..
 

Thread Starter

jb361800

Joined Feb 28, 2020
11
There are clearly scorch marks around a few components. The 10 uF/50V capacitor has some. From the side, the picture to too fuzzy to tell whether it is puffed up. If so, that is one sign of failure..
I did notice the scorch marks on it. Maybe I should try replacing it? I attached better photos of that capacitor.20200228_191208.jpg20200228_191253.jpg
 

jpanhalt

Joined Jan 18, 2008
11,087
Capacitor leads are often made of plated steel. They can rust. On closer inspection, maybe some of what looks like scorch is rust. It might be worth removing cleaning, and resoldering/replacing. If it is rust, it will be difficult to resolder.
 

LesJones

Joined Jan 8, 2017
4,174
Are you sure that the board is supplied with 12 volts AC ? Looking at the the two capacitors C1 & C2 which I think are connected in parallel and are rated at 4,7 uF and 400 volts working I think it is more likely to be powered directly from the mains supply (120 / 240 volts AC ?) Have you checked the the AC input fuse ? It looks like the AC input is rectified by a surface mount bridge rectifier and the ouput from this is smoothed by C1 & C2. This DC voltage is then fed to a switch mode power supply. I would expect the DC voltage across C1 & C2 too be about 1.4 time the RMS input voltage. Be VERY careful measuring this voltage. If this voltage is OK then the fault will be in the switch mode power supply. I think the 8 pin surface mount IC is the controller mode power supply for the switch. What is it's part number ? (I can't read it from the pictures.) If you trace out the schematic of the board that would be a great help in trying to diagnose the problem. NOTE Switch mode power supplies can be difficult to fix as very often more than one component has failed.

Les.
 

Alec_t

Joined Sep 17, 2013
14,280
Capacitor leads are often made of plated steel. They can rust.
Wouldn't rust on the pcb be limited to just around the pins? It looks like the mark is a circle right around the cap edge. My vote is for a scorch mark.
I note the smd labelled MB109? under the pcb looks blistered. Google hasn't heard of a MB109. Perhaps its a B109 power NPN?
 

LesJones

Joined Jan 8, 2017
4,174
Hi Alec,
I think the item with MB109 marking close to it on the board is the input bridge rectifier. It is difficult to see anything properly with the flaking coating on the board. I think jpanhalt is correct about rust. In the pictures in post #3 there looks like rust around the wires from C5 (The blue capacitor that looks like a varistor.) where it enters the board.

Les.
 

bwilliams60

Joined Nov 18, 2012
1,442
I agree, I think there is mains voltage coming onto this board and that 10uF cap looks dome shaped. Maybe the rust color is from the dielectric? Looks like a simple board though. Reverse engineer a diagram and post. That will get you your answers.
 

Thread Starter

jb361800

Joined Feb 28, 2020
11
Are you sure that the board is supplied with 12 volts AC ? Looking at the the two capacitors C1 & C2 which I think are connected in parallel and are rated at 4,7 uF and 400 volts working I think it is more likely to be powered directly from the mains supply (120 / 240 volts AC ?) Have you checked the the AC input fuse ? It looks like the AC input is rectified by a surface mount bridge rectifier and the ouput from this is smoothed by C1 & C2. This DC voltage is then fed to a switch mode power supply. I would expect the DC voltage across C1 & C2 too be about 1.4 time the RMS input voltage. Be VERY careful measuring this voltage. If this voltage is OK then the fault will be in the switch mode power supply. I think the 8 pin surface mount IC is the controller mode power supply for the switch. What is it's part number ? (I can't read it from the pictures.) If you trace out the schematic of the board that would be a great help in trying to diagnose the problem. NOTE Switch mode power supplies can be difficult to fix as very often more than one component has failed.

Les.
Les Jones - you are correct about the power being 120 volts. My original thread stated 12v by accident.
 

Thread Starter

jb361800

Joined Feb 28, 2020
11
Are you sure that the board is supplied with 12 volts AC ? Looking at the the two capacitors C1 & C2 which I think are connected in parallel and are rated at 4,7 uF and 400 volts working I think it is more likely to be powered directly from the mains supply (120 / 240 volts AC ?) Have you checked the the AC input fuse ? It looks like the AC input is rectified by a surface mount bridge rectifier and the ouput from this is smoothed by C1 & C2. This DC voltage is then fed to a switch mode power supply. I would expect the DC voltage across C1 & C2 too be about 1.4 time the RMS input voltage. Be VERY careful measuring this voltage. If this voltage is OK then the fault will be in the switch mode power supply. I think the 8 pin surface mount IC is the controller mode power supply for the switch. What is it's part number ? (I can't read it from the pictures.) If you trace out the schematic of the board that would be a great help in trying to diagnose the problem. NOTE Switch mode power supplies can be difficult to fix as very often more than one component has failed.

Les.
The part # for the 8 pin surface mount IC is EE13F-2.0MH
 

Thread Starter

jb361800

Joined Feb 28, 2020
11
Have you checked the fuse? Are you comfortable working with 120VAC?
dl324 - I have not checked the fuse yet. I have a nice multimeter so I can check it if need be. But before I do, let me ask a question. As I originally stated, this PCB goes to a smaller size refrigerator. When everything is hooked up and plugged in, the compressor on the unit works normally. Wouldn't that mean that the fuse is working properly?
 

Thread Starter

jb361800

Joined Feb 28, 2020
11
I apologize but I have never reverse engineered a PCB to schematic, nor would I know how or where to start. I find PCB and circuitry fascinating and love to learn about it. So I guess you could say my experience level is somewhat amateur. But we've all got to start somewhere.
 

dl324

Joined Mar 30, 2015
16,845
Wouldn't that mean that the fuse is working properly?
It depends on how things are wired.

I just checked the fuse and got 0 resistance in ohms.
A logical next step would be to apply power to the board and start measuring voltages to see if they made sense. If the input to the component we suspect is a bridge rectifier, the output would be filtered DC.

If the input to that bridge rectifier is 120VAC, the DC voltage would be close to 170VDC. Are you comfortable working with voltages that high?
 

Thread Starter

jb361800

Joined Feb 28, 2020
11
It depends on how things are wired.

A logical next step would be to apply power to the board and start measuring voltages to see if they made sense. If the input to the component we suspect is a bridge rectifier, the output would be filtered DC.

If the input to that bridge rectifier is 120VAC, the DC voltage would be close to 170VDC. Are you comfortable working with voltages that high?
Yes I am comfortable working with the voltage. So you recommend me checking the input bridge rectifier (labeled MB105)? It appears to have 4 pins (2 on each side).
 
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