recently I bought a DSO138 DIY oscilloscope
I have white screen problem just couple of minutes after turning it on
After detaching LCD I found U5 voltage regulator got overheated
Firstly I removed L3 but overheating happened again
Then I put back L3 and removed jp4 connection and overheating disappeared
Now I know the +3.3V branch has a problem like short
But I don't know what to do for solving it permanently
I tried test mode for possible short on MCU pins but LED was blinking slowly so there is no short on MCU pins and for blinking trigger LED twice it is clear the MCU is working
I checked +3.3V resistance with GND and there was no short, so there is no internal short in MCU
then I checked pin 32, 33 and 34 of LCD header for possible short but it was OK
Then I checked C16, 17, 18, 26 and 15 but they were OK too
Finally I checked pin 5 of CPL, SEN1 and 2 for short with GND but there was no short also there


Any Idea will be appreciated deeply
Thanks

 

BTW I am using 12V 1amp PS FYI

 

Did you purchase the DSO138 already assembled or as a kit?
Was the DSO138 working at any time?

Do you have a PSU that is lower than 12V, say about 7.5VDC?

 

I have purchased the digital scope from Banggood last year as a kits. Thought the mcu was pre-installed on the PCB, I think it was not an easy kits to assemble. There were 19 smd resistors as well as TL084.

I used my regulated adjustable PSU for testing after finished assembling. It was working fine luckily without any mistakes.

Though it warned that the input voltage shouldnt be more than 12V but did you confirm that your power supply is not >12V with a meter? I used exactly the voltage stated on the PCB.

As the regulator is a 78L05 and the LM1117 for 3.3V. I have bad experiences with the LM1117 from China. I burned a couple of them on my breadboard as regulators for my 3.3V supply.

1. Did you check id the -5V is available from the 79L05? (mcu must be running first).
2. Is the 1KHz test signal available on J2?
3. Would you show us a picture of your finished PCB (without the GLCD) so I can compare?

Allen

 

Did you purchase the DSO138 already assembled or as a kit?
Was the DSO138 working at any time?

Do you have a PSU that is lower than 12V, say about 7.5VDC?

PSU is exactly 12.5V, 1amp while it's stated 9V 300 miliamps on output, crazy, however I measured total current consumption of board and it's around 130 miliamps
Does it mean the overheating is coming from PSU? Why does it consum 130 miliamps while 78L05 is just 100 miliamp at max? Does it dictate by PSU?

 

I have purchased the digital scope from Banggood last year as a kits. Thought the mcu was pre-installed on the PCB, I think it was not an easy kits to assemble. There were 19 smd resistors as well as TL084.

I used my regulated adjustable PSU for testing after finished assembling. It was working fine luckily without any mistakes.

Though it warned that the input voltage shouldnt be more than 12V but did you confirm that your power supply is not >12V with a meter? I used exactly the voltage stated on the PCB.

As the regulator is a 78L05 and the LM1117 for 3.3V. I have bad experiences with the LM1117 from China. I burned a couple of them on my breadboard as regulators for my 3.3V supply.

1. Did you check id the -5V is available from the 79L05? (mcu must be running first).
2. Is the 1KHz test signal available on J2?
3. Would you show us a picture of your finished PCB (without the GLCD) so I can compare?

Allen

Actually my board has been soled all SMD parts pre heated like all resistors and MCU so I just soldered Dip components and it was difficult yet
My PSU is exactly 12.5V 1amp however it was stated 9V 300 miliamp, Chinese again, actually in that couple of minutes I tested all you said
1- so -5V was -4.92
2- 1kHz test signal worked flawlessly
3- and also it's the picture

 

Actually my board has been soled all SMD parts pre heated like all resistors and MCU so I just soldered Dip components and it was difficult yet
My PSU is exactly 12.5V 1amp however it was stated 9V 300 miliamp, Chinese again, actually in that couple of minutes I tested all you said
1- so -5V was -4.92
2- 1kHz test signal worked flawlessly
3- and also it's the picture
View attachment 166703
View attachment 166702

Your board looks exactly like mine and I couldnt find any components that is in different positions as mine.

So, I guess you have to do some serious trouble-shooting on your board.

You said if you remove JP4 and the overheating on 78L05 disappears. So the fault is on the 3.3V lines...

But first, do you have a 9V or 7.5V power supply? If not try to step down your 12V DC with a 7809 1A regulator just in case the 12.5V is the real murder of your circuit.

Make a list of all the components that use 3.3V. I only notice the mcu and GLCD are using 3.3V dc.

I'll look again tomorrow morning as it's getting late now and I have other important things I want to attend to first.

cheers

Allen

 

Your board looks exactly like mine and I couldnt find any components that is in different positions as mine.

So, I guess you have to do some serious trouble-shooting on your board.

You said if you remove JP4 and the overheating on 78L05 disappears. So the fault is on the 3.3V lines...

But first, do you have a 9V or 7.5V power supply? If not try to step down your 12V DC with a 7809 1A regulator just in case the 12.5V is the real murder of your circuit.

Make a list of all the components that use 3.3V. I only notice the mcu and GLCD are using 3.3V dc.

I'll look again tomorrow morning as it's getting late now and I have other important things I want to attend to first.

cheers

Allen

It is really your kind my friend
I replaced the PSU with another one and I measured it, it's exactly 8.7V 0.5 amp in unloaded state
Tonight I am going to test it again
Also I purchased 78L05 and highre current capable of 7805
Maybe a new voltage regulator works
If you have any idea about changing 78L05 with 7805 please let me know it
Thanks many times

 

Before you assemble your next electronics kit, come back to AAC and ask for lessons on soldering.
You need to improve your soldering techniques. You are using too much solder and/or your soldering iron is not hot enough.

Check your board very carefully and look for poor solder joints or solder bridges.

 

Before you assemble your next electronics kit, come back to AAC and ask for lessons on soldering.
You need to improve your soldering techniques. You are using too much solder and/or your soldering iron is not hot enough.

Check your board very carefully and look for poor solder joints or solder bridges.

Was going to be my comment. Every single solder joint looks cold to me. Even if the over heating issue gets fixed, it it is a long shot if the scope actually works.

 

It is really your kind my friend
I replaced the PSU with another one and I measured it, it's exactly 8.7V 0.5 amp in unloaded state
Tonight I am going to test it again
Also I purchased 78L05 and highre current capable of 7805
Maybe a new voltage regulator works
If you have any idea about changing 78L05 with 7805 please let me know it
Thanks many times

Doubtful it is the 7805. They are built like tanks. I would try disconnecting the output of your regulator to see if that fixes the overheating issue. Do any other components get hot or even warm?

 

Sorry for hijacking your thread TS but I have a question.

What is going on here? What is going on in that part of the regulator?



Also why the need for all of the inductors? I have never seen that in a regulator involving a 7805

 

Before you assemble your next electronics kit, come back to AAC and ask for lessons on soldering.
You need to improve your soldering techniques. You are using too much solder and/or your soldering iron is not hot enough.

Check your board very carefully and look for poor solder joints or solder bridges.

Yes Iron is not hot enough
Actually I did it intentionally because I thought more heat will ruin components because my soldering skill is not good
I am going to take care of it and resolder them again
Thanks for advice

Doubtful it is the 7805. They are built like tanks. I would try disconnecting the output of your regulator to see if that fixes the overheating issue. Do any other components get hot or even warm?

I will test it and let you know the result then

Sorry for hijacking your thread TS but I have a question.

What is going on here? What is going on in that part of the regulator?

View attachment 166722

Also why the need for all of the inductors? I have never seen that in a regulator involving a 7805

View attachment 166725

As you and MrChips complained about my soldering skill it's obvious I am not an expert
I hope you can explain me but as I'm guessing it maybe is look like for sake of noiseless and stable output
IDK

 

Yes Iron is not hot enough
Actually I did it intentionally because I thought more heat will ruin components because my soldering skill is not good
I am going to take care of it and resolder them again
Thanks for advice

If it is melting the solder then it should be hot enough. I should be the last person to criticize and give advice on soldering. My soldering is not the best either but I think mainly do to shaky hands.

There are a couple of main things to know when soldering. Make sure you have a clean and properly tinned tip. The tip of the soldering iron should be the first to be applied to the joint and the last thing to be removed. The idea is to have the joint melt the solder not the iron.

If you just melt the solder on to the joint, this is what is known as a "cold solder joint". A solder joint should be nice and shiny and sort of conform to what you are soldering. A dull blob is a bad thing. It should be removed and try again,

There are many videos on soldering. Try to find a good one and watch it before you try your next project.

 

I will test it and let you know the result then

Is anything else on the board getting hot or even warm?

 

Hi spinnaker.
The regulator section in you first picture you have posted is an inverting switching regulator generating a negative voltage that is then regulated down to -5 volts by a 79L05 regulator (NOT a 78L05) L2 is for energy storage. L1 is for filtering out the ripple from the switching regulator.

Les.

 

@spinnaker
The circuit in question is a charge pump to create a negative supply from the positive supply.
The switching signal comes from VGEN (PB9) from the MCU.
Negative voltage (-8.1V) is regulated down to -5V with 79L05 negative voltage regulator.

The inductors is to keep the charge pump HF signals from getting into the analog +5V and -5V supply.

 

PSU is exactly 12.5V, 1amp while it's stated 9V 300 miliamps on output, crazy, however I measured total current consumption of board and it's around 130 miliamps
Does it mean the overheating is coming from PSU? Why does it consum 130 miliamps while 78L05 is just 100 miliamp at max? Does it dictate by PSU?

Don't fault the power supply. It is normal for a transformer to output higher voltage than its rated voltage.
The rated voltage is for full load.
A transformer rated at 9V will put out a higher voltage when no load is connected.
You really need a transformer rated at about 7.5V 500mA to 1000mA for the DSO138.

 

Is anything else on the board getting hot or even warm?

No it is just U5 as far as I know
I removed JP4 and overheating disappeared is it enough or I should remove U5 output to be sure

 

Don't fault the power supply. It is normal for a transformer to output higher voltage than its rated voltage.
The rated voltage is for full load.
A transformer rated at 9V will put out a higher voltage when no load is connected.
You really need a transformer rated at about 7.5V 500mA to 1000mA for the DSO138.

Right now I have 8.7 V 500 miliamp and in its users manual stated 8-12V and not exceed 12V otherwise U5 will get hot also overheating is still there however last night I tried 7.5V from Li-battery and LCD failure happend with overheating again I guess there's something I don't know
Any idea to replace 78L05 because I'm getting sure the problem lies behind its design, it seems the issue I have is a biggest challenge for everyone, however some people have no problem while some others have
The current consumption in normal is around 110 miliamps which is higher than U5 specification and my PSU is literally in range
What do you think so?