Startup test troubleshooting

Startup test troubleshooting

Postby SnusMurmik » Mon Apr 01, 2019 8:28 am

I have problems with three DSO shell units. Two of them did not boot up at all. One did boot up first time (with 9V power adapter), but next time I tried it did not boot up. One I tried with my lab power supply, and noticed that 1.5A went through the unit, until some component gave up a bit of smoke.

I have printed the schematic and did some testing yesterday.

Unit 1. Bought from Banggood. This did not boot when I first received it. This is the one that gave up some smoke after drawing 1.5A. When powering this on yesterday, using 3.3V (with the power supply, current limited) on the 3.3V point on the PCB, it draws about 700mA. The red diode lights up. The large IC (STM32 F103C8T6) gets burning hot within a few seconds. Too hot to touch. I only let this run for 5-10 seconds at a time. This PCB does _not_ have a short between 3.3V and GND. I suspect the IC has been destroyed.

Unit 2. I received this as a replacement directly from JYE Tech. This is the only one that booted up when I first got it (using the 9V power adapter). For some reason it didn't boot again. I noticed that there is a short between 3.3V and GND. I de-soldered the voltage regulator, as all legs seemed to be shorted. But after de-soldering this, there is no short between leg 1 and the others, though leg 2, 3 and 4 seem to be shorted (not sure if this is how it's supposed to be). After de-soldering the voltage regulator, I applied 3.3V to the 3.3V point on the PCB. I find no components that get hot. I turned up the current until 1.2A flows through the circuit. I still can't find any hot component. The large IC might get mildly warm, but not enough that I can be sure. I don't get burned by any components. Maybe there is a short through a small component somewhere, or on the trace? The red led does _not_ light up.

Unit 3 seems identical to unit 2. It did not boot up when I first got it, though. All legs of the voltage regulator are shorted (or 0.5 ohm).

How can I troubleshoot step 1? I saw that someone else just started a thread with a similar question. Unfortunately I don't own a IR camera, which I think would be usable for finding the short.

I'm thinking of desoldering some more components on unit 2 until I find the short. I hope I won't have to de-solder the large IC though.

I found this guide on the web, where 12V was used instead of 9V. The voltage regulator gave up smoke, as well as the STM32 IC. These were both replaced, and the unit started working again:
https://www.instructables.com/id/DIY-Os ... leshootin/
SnusMurmik
 
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Re: Startup test troubleshooting

Postby jye1 » Mon Apr 01, 2019 11:02 pm

Sorry to know you found three units not working.

For unit 1 it was pretty obvious that the main chip (the MCU) was burnt. When in production each board was put on a special testing fixture for shorts/opens checking to ensure soldering was done properly. After that each board was applied 9V power again to check for correct firmware programmed and booting. But occasionally we still receive reports of problem just like this unit 1. We don't have clear clue what the causes are. We thought ESD could be one of them. The power supply used and the way the power was applied could also cause chip damage if that was not done properly.

For unit 2 did you try use your lab power supply? If not I'd like to suggest to try it. Usually bench top power supply is much more reliable than power adapters. Some adapters are not regulated. They are labelled 9V but actually voltage could be strongly dependent on loading. The actual voltage of an adapter that has 9V open voltage measured could drop significantly when loading is connected. On the other hand, some 9V adapters could have much higher open voltage. So it is better to measure power supply voltage with load connected. One other thing to know about is that power adapter could have large ripples too even it is measured 9V with loading.

In step 1 testing there are two visual indications. One is the LCD. The other is the LED. If LCD display comes up and is correct that is enough to say the board is booting normal. If the LCD doesn't come up (being while or dark) then check the LED. In a normal boot the LED blinks three times. If the blinking is observed that means the MCU is running. Problem could be LCD connections or the LCD itself. If the LED doesn't blink correctly check the +3.3V voltage and V+. If these voltages are all normal the problem might be with the MCU itself.

Usually current limiting at booting is not required. At the moment of power is applied the board tends to draw more current than its normal running. If power supply can not provide enough current the voltage drops. This could fail the booting.

For Unit 3 it can be troubleshooted in similar way.

You mentioned shortage were measured between regulator legs. Please make sure the measurement is done after the board is powered off for a while. This is for the caps to completely discharge. This delay need to be longer if lab power supply is used because the caps there are larger.
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Re: Startup test troubleshooting

Postby SnusMurmik » Tue Apr 02, 2019 5:40 pm

On one of them, I de-soldered all capacitors and ICs, and finally found that it was the large IC (STM32 F103C8T6) that was shorting 3.3V to ground.

I removed the voltage regulators from both this, and the board without the short.

I did some testing, and found that the voltage regulators are outputting the input voltage, instead of 3.3V. That means they are both fried (probably the third one as well).

So, I guess that an over-voltage fried the voltage regulator, and, when it stopped regulating and delivered 9V+ to the STM32, it fried the STM32 as well.

One could hope that there was some protecting circuit, to protect the STM32 from over-voltage. For example a voltage regulator that doesn't short when fried.
SnusMurmik
 
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Re: Startup test troubleshooting

Postby jye1 » Thu Apr 04, 2019 7:53 pm

What you said is likely the case. If the regulator failed the unregulated voltage will kill the chip for sure. For new design the power supply will use 5V with USB style connector. This will significantly reduce the risk put by uncertainty of power sources.

We are happy to send you another replacement. Please contact us if you want.
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Re: Startup test troubleshooting

Postby SnusMurmik » Fri Apr 05, 2019 5:44 pm

Thanks!
I thought I was so careful this time. It even booted the first time, but I guess there was a voltage spike when I removed the connector, or something. I have measured the power adapter, and it does go a bit over 9V when unloaded (several volts, I've forgot the exact number).

I will try to fix it myself, but if that doesn't work, I'll take you up on your offer. I will receive the replacement components on Monday or Tuesday. I'll let you know how it goes.

I saw that I will need a code to unlock the newest firmware, after flashing the new CPU. I'll check the serial number and will email you about that.

Using a 5 V USB connector sounds like a good idea. From what I understand of the circuit, the 9V isn't used by the IC's, just the 3.3V. Using 5V will also give the voltage regulator a larger tolerance for over-voltage.
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Re: Startup test troubleshooting

Postby SnusMurmik » Fri Jan 24, 2020 6:22 am

I finally had the time to complete this project yesterday. I flashed it with an old firmware (.64), and all seems to work now!

I only replaced the CPU and the voltage regulator. I used my bench power supply to power it (set to just 8V just in case..).

The reason I used the old firmware is because the newest one needs a registration code. I'll email a request for it now that I know the unit works.
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Re: Startup test troubleshooting

Postby RayPooley » Sat Feb 01, 2020 11:29 am

Just scanned through this tale of woe. You have some problems there. Some sound a lot like short circuit issues perhaps from inaccurate soldering. The holes are very close together in some parts ike the switches. When I assembled mine I found that a PC based USB microscope was invaluable in checking the integrity of my soldering before connecting it to a power source. It showed up a couple of points where I had inadertently cross soldered two neighbouring holes. It's really impossible to see then clearly with the naked eye or even wearing spectacles.
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