DSO 138 oscilloscope - 3.3V shorted to ground

Thread Starter

henning2323

Joined Jan 20, 2021
5
Hello,
I bought a pre-soldered DSO138 and it worked when I started it for the first time. But soon after the screen went only white and the LED isn't doing anything, which means the MCU isn't working. Also U5 (78L05) gets really hot, which led me to thinking something is off with the 3.3V line. When taking measurements I noticed 3.3V and ground are shorted, so I desoldered some of the connections to 3.3V to see if this fixes anything. I desoldered L4, C16, C17, C18 and JP4 but the short still exists. Is one of the ICs fried? And is there a way to find out fif it is?

Thanks for your help.

P.s.: I know that there are allready some threads about this topic, but everything I found didn't help me.
 

MrChips

Joined Oct 2, 2009
23,515
Do you have a thermal imaging camera or a smart phone app to do thermal imagine?
See if you can find which component is sucking up all the current.

Another old tech trick is to use freeze mist spray and then apply power.
 

Thread Starter

henning2323

Joined Jan 20, 2021
5
Sadly I dont have access to thermal imaging. But what I can fell by hand is the U5 Voltage Regulator gets really hot. I cant touch it after only 5 - 10 seconds. I did not notice anything else getting hot.
 

dl324

Joined Mar 30, 2015
12,871
Welcome to AAC!

If the 3.3V regulator worked when you opened JP4, use a voltmeter on the lowest DC range setting and look for a point on the 3.3V supply that has the lowest voltage.
 

Thread Starter

henning2323

Joined Jan 20, 2021
5
Okay, I gave that a try. When JP4 was open, I got a nice voltage (3.2XV) at the voltage regulator (TP22).
After that I closed JP4 again and the voltage dropped to arround 1.58V when first connecting the power and it drops to arround 1.54V after 30 seconds when U5 heats up.
All the points where 3.3V connects to sadly have the same reading, probably because 1.54V is too high to notice small changes.
Do you have another Idea what I can check?

Also I followed the trace on the pcb and compared it to the schematic to find all the points where there should be 3.3V but i probably missed some spots. Do hav an idea where I can find resources about the pcb layout to check every spot?
 

dl324

Joined Mar 30, 2015
12,871
All the points where 3.3V connects to sadly have the same reading, probably because 1.54V is too high to notice small changes.
Do you have another Idea what I can check?
  1. Get a better meter. Preferably one that has a 200mV range. At the minimum, one that shows 3 decimal places for the 2V range.
  2. Get some freeze spray and cover all of the components and look for the area that melts first. Keep spraying in that area until you locate the offending component. Obviously it's going to be something across the 3.3V rail.
  3. Get a temperature probe (like HP10023A or a DVM accessory) or use your finger. If you use your finger, be careful because the component will be very hot to the touch.
  4. Get a current tracer like HP547A and probe around for the highest current on the 3.3V rail. If you also have an HP546A Logic Pulser, you might be able to trace with power off.
 

Thread Starter

henning2323

Joined Jan 20, 2021
5
Thanks for all your good tips!
By hand I couldn't notice anything getting hot (except U5). So i'll try to get my hands on some freeze spray and give you an update on how that went :)
 

Thread Starter

henning2323

Joined Jan 20, 2021
5
Is your DSO138 board identical to this one?
Yes, it's exactly that one.

So i'll try to get my hands on some freeze spray and give you an update on how that went :)
Okay, so this mornining I tryed to find a shortcircuit using freeze spray. The only part exept U5 that melted the ice faster than everything else was the MCU itself. It was not super fast, so I'm not sure if thats just the normall amount of heat it produces, but on the other hand it should not have any real load to handle. Does that mean it is probably fried? And is it worth trying to replace it?
 

MrChips

Joined Oct 2, 2009
23,515
You need to consider the obstacles ahead of you.

1) Removing the MCU chip is not an easy task. You need a PCB rework station or a hot air gun with a restricted nozzle. Even if you manage to replace the MCU there is no guarantee that that will fix the problem. Presumably, after removing the MCU and supply voltages return to normal one may assume that the short has been removed.

2) Soldering a small outline chip is not an easy task. You need a soldering station with a very fine tip and also very fine diameter solder.

3) You need to install the firmware on the MCU. Check with JYETech to see if the firmware is available. After that you will need a programmer board to program the chip.

When you consider the low cost of another DSO138 is it worth the trouble attempting to repair it?
 
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