Need troubleshooting help: U5 overheating, white screen

[Oszillor]

Hi there,
I recently put together my DSO138 kit but I stumbled upon a few problems that I could not solve yet.
U5 is overheating and producing between 2.2 and 1.8V (as it heats up, the voltage goes down). Since this is the main voltage for the board (from what I got), the rest of the voltages is off.
(TP22: 0.99V, AV-: 0.59V, AV+: 1.29V, +3.3: 0.23V, +5: 1.78V, V1: 0.51, V2: 0.61, V3: 1.18, V4: 0.55). V+ and Voltage at D2 is ok (9.3V), everything from U5 down is off. I first used 9.3V from a 12V battery with step down regulator and then 9.3V from a regulatable and very stable bench top supply, same results, also the LED does not light up at all.
I've already replaced U4 and U5 but it did not help. Have ordered new Q1 and 2 as I read in another thread that they could be a problem too.
Also I am getting a white screen but I guess that stems from the voltage problems.
I reheated / resoldered close to every solder joint to avoid broken solder points.
The second question I have: I inadvertently destroyed JP3 because my iron was still too hot from soldering the BNC connector (forgot to dial it down from 450°C to 300°C). I connected the end of R11 to the end of C9 to remedy this which could cause more problems? Is there a better way to 'recreate' JP3? I did not close JP4 yet because 3.3V was not ok yet.
Cheers,

Oszillor

[jye1]

hot U5 with low output voltage usually indicates that there is short somewhere at its output which causes over current. Since JP4 is not closed the short is most likely between AV+ and GND. You can verify this by disconnecting L3. Please check the soldering of pin 5 of all three slide switches. These are spots where beginners are easy to make mistake. Pin 5 of slide switches is connected to AV+. The switch shell is connected to GND. Please make sure there is no bridge between pin 5 and switch shell.

[Oszillor]

Thx for the feedback. I will check Pin 5 on the switches and disconnect L3 for a test soon and let you know if it worked.

[Oszillor]

Ok, switches are ok, no bridge between Pin 5 and the shells of the switches. Disconnecting L3 now gives correct positive voltages (5V, 3.3V and V+ at 9.33V). Where would be the next best point to look for a short? And do I need to replace L3?
Checked Ohm values from another thread:
DGND to ... (with L3 removed)
AV-: 2.67k
AV+: 4.38k
+3.3: rising to 12.6k
+5: 4.84k

Also Vc2 and Vb1 one are basically at V+ so I will replace Q1 and Q2 (polarity checked ok - still have to get the parts).

[jye1]

From the residtance you got there is no short beteeen AV+ and GND. You can try put L3 back to if short is still there. I don't think L3 needs to be replaced.

[mistahyee]

From the residtance you got there is no short beteeen AV+ and GND. You can try put L3 back to if short is still there. I don't think L3 needs to be replaced.

I am having the same problem. I have 3.3V on TP22, so I shorted JP4 and I connected the LCD screen. I had a white screen, and the LED was always on. I took the screen out and found that U5 gets really hot. After desoldering JP4 and L3, the heating problem goes away. Is there a short somewhere?

[jye1]

I am having the same problem. I have 3.3V on TP22, so I shorted JP4 and I connected the LCD screen. I had a white screen, and the LED was always on. I took the screen out and found that U5 gets really hot. After desoldering JP4 and L3, the heating problem goes away. Is there a short somewhere?

You can follow the similar way to uncover the cause. Disconnect L3 to find out the short is in the 3.3V branch or the AV+ branch.

[mistahyee]

I am having the same problem. I have 3.3V on TP22, so I shorted JP4 and I connected the LCD screen. I had a white screen, and the LED was always on. I took the screen out and found that U5 gets really hot. After desoldering JP4 and L3, the heating problem goes away. Is there a short somewhere?

You can follow the similar way to uncover the cause. Disconnect L3 to find out the short is in the 3.3V branch or the AV+ branch.

Connecting L3 -> U5 is normal

After connecting L3, then connecting J4 -> U5 gets hot

This means the short is on the 3.3V branch, right? Is there anywhere I should look in particular for the short? Just on a simple inspection of the board and solder joints, nothing seems to be wrong.

[jye1]

Did you check the resistance between +3.3V and GND? One possibility is the MCU short inside. Check resistance can tell you something.

[mistahyee]

Did you check the resistance between +3.3V and GND? One possibility is the MCU short inside. Check resistance can tell you something.

I measure about 63 ohms, so I'm guessing a short inside the MCU? Unfortunately I think this is a fake kit (no S/N), so I will just buy another one from a trusted seller.