X7 - LDO or CPU broken

Hardware help and support for the FrSky Taranis
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Radek
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X7 - LDO or CPU broken

Post by Radek »

Dear friends,
I have Taranis qx7 for half a year. It's a radio with a 0.5 revision board.
Some time ago after flashing it to 2.2.2, it began to draw a lot of electricity and consume batteries quickly. USB works only in boot mode. Sometimes it lost Data and Time and LCD and power LED are very dim. Anything else works very well -binding, displaying info an LCD etc.
I think, that I found the broken part, but I'm not sure.
It's U501 located on left side of mainboard (power section?).
When touching it's very hot, but at this same moment LCD backlight is a bit stronger. Measured temperature is over 60 Celsius degree.
At he the same moment CPU is about 50 Celsius degree, but working solid.
I have checked battery case, cables, re-solder battery's socket pins and no effects.
So the question is: broken LDO, CPU or any other passive elements?
If someone could help, I will be grateful.
Best
Radek
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

If the CPU is that hot you could have short(s) between two or more of its pins, or somewhere else on the board..

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Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

Thanks jhsa. I'll check CPU legs just after back from work. But how it be possible? The CPU has never been touched.
I bought it used. One hitch from the haptic motor was broken and this motor has probably moved over the circuits. In this version of board, motor is on left side next to U501 element. Now motor is secured. I do not know where to look for a short circuit :(
Also European Frsky Service in Czech Republic told me that if LDO (U501?) is hot then processor is probably died. But processor is working.
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

well, something could be pulling a lot of current. It could be the processor, or it could be something else.. and if this is the case, replacing the regulator will probably not fix the problem. I could be wrong of course.. :)

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Kilrah
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Re: X7 - LDO or CPU broken

Post by Kilrah »

You should measure the voltage at the output of the regulator e.g. across C518. Guess is the LDO is faulty and probably passing 5V straight through instead of regulating down to 3.3.

It probably does so with some internal resistance which has prevented the CPU from blowing up but it's certainly not liking that if it's hot. Who knows what damage or accelerated wear it could be sustaining.
Almost everything is on the 3.3V supply so other stuff is likely suffering.

If voltage is correct then the CPU probably received a spike at some point because of a short and part of it is damaged, even if some functions still work.

Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

Thx Kilrah.
I will measure the voltage today. Can I disconnect all other boards / modules from the motherboard for the duration of the measurement and leave the motherboard under voltage only?
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Kilrah
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Re: X7 - LDO or CPU broken

Post by Kilrah »

Well you need the switch PCB to turn it on, you can disconnect the RF module but it shouldn't make a difference.
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

Yeah, I know at least one regulator that lets pass the input voltage to the output if the output was shorted. No short circuit protection. It happened to me. I think there is a reversed diode between the output and input, and if it goes bad, normally it shorts. This regulator could also suffer from the same syndrome :) Yep, measuring the voltage is important.. :)

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Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

The STM legs were inspected - no visible shorts. Whole board was cleaned, using soft toothbrush and a bit of spirit, from potential dust and to making legs more visible.
Voltage across C518 has been measured. All external modules, excluding power switch, were disconnected. Result = 2.66V.
Measured temperature of LDO - 68°C and CPU - 57°C.
I will inspect whole motherboard tomorrow in a daylight.
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

2.66V might mean that something is using too much current and overloading the regulator. It could be the CPU itself as Kilrah also mentioned :(
But yes, a close inspection could reveal some problem. try disconnecting stuff like the gimbals, etc.

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Kilrah
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Re: X7 - LDO or CPU broken

Post by Kilrah »

If something else than the CPU is drawing too much then it would be getting warm as well. Check for anything warmer than normal, including all the pots.
Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

Guys,
I took about 60 zoomed photos and no shorts at all.
Only CPU and LDO are so hot. Some components next to LDO are also warm arround 40-44°C, but probably from LDO heat disipation.
So what I think at least one of this elements should be replaced.
The cheapest CPU I found on eBay in the VET6 version is $ 5.60. Normally, they are around $ 10. At Farnell I also found for about $ 6 but in the VET7 version. The data sheet show that they are the same but I'm not 100% sure. Maybe I should order an LDO also just in case, because it suffered a lot, but I do not know what it would be like. Colleagues from Digi-Key and also from Mouser did not recognize this symbol.
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

Did you disconnect the gimbals and tried? Also measure the voltage with them removed. Or are you already sure that the problem is the CPU?? :) ;)

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Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

Yes. Yesterday's measurement was w/o gimbals, switches and other boards. Only power switch board was connected.
I'm don't know what else could making these issues.
Only CPU and LDO are so hot. Maybe they should be hot? I don't have access to any other x7 to do comparison :)

When I turn on the radio, the first is the failsafe warning and if I do not go further, the temperature is a few stops less c.a. 50 ° C on the processor and 60 ° on the LDO. After passing the warning and let system to "load itself completely", the temperature grows a few degrees.
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

2.66V on the 3.3V rail is way too low.. :(
Something is pulling that voltage down.

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Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

No idea what. Maybe CPU because is very hot too.
When the processor is getting so hot? If is damaged, but still working? And when has shorts on legs (didn't find any)?
If I could have a schematic then could measure some voltages, but at this moment its only guessing what ;)
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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

I'm out of ideas :(

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Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

That's okay. Thanks for help.
On USB only w/o battery the CPU is also hot. The idea is to remove regulator an provide 3.3 at this place. If CPU will be hot after that, then probably is fried up. At this same moment I'll be able to check regulator itself in separate environment.

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jhsa
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Re: X7 - LDO or CPU broken

Post by jhsa »

I don't think it is the regulator, even if I believe that at this point it should probably be replaced as well.. I think something is just loading it :(

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Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

If all other modules are unplugged, the energy consumed must be separated in the form of heat only on the one that consumes the most. All other regulators are cold. Only the LDO that serves the voltage and the CPU are hot. It seems to me that the CPU is probably damaged.
Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

I upgraded to ErSky firmware. Everything working very well, CPU & LDO are still hot. But I found that the maximum junction temperature for this CPU is 125 ° C, so 58 ° C is in the middle.
I also found a formula for the temperature and current for STM in LFPQ case: http://www.stm32duino.com/viewtopic.php?t=2248#p30394
In my case it looks that the CPU is sucking 0.198mA and the maximum in data sheet is 120mA. But there are other components on the board. probably not only CPU is sucking this energy.
I did a test on professional laboratory power supply with everything connected. On 7V was 700mA and radio was showing 6.7V. On 14V was 300mA and voltage on radio was 13.3V. Maximum in documentation is 210mA 6-15V, so 210 is for 15V I think.
During test LDO were starting heat-up and after that LCD backlight and power LED started dimming down.
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Kilrah
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Re: X7 - LDO or CPU broken

Post by Kilrah »

The normal total current consumption at 7V of an X7 with internal RF on in D16 mode and no external module is 150mA with backlight off and 175mA with backlight on at 100% brightness, aka 1W-1.2W.

RF off is 100mA less, so 50mA with backlight off and 75mA with backlight on, aka that's all the CPU and accessories are supposed to draw.

Yours is clearly doing a partial short.
Radek
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Re: X7 - LDO or CPU broken

Post by Radek »

OK, so it's clearly showing, that something on mainboard. I believe that the problem is caused by CPU. We have Flir thermal imaging camera in my work, but unfortunately last week something happen to it focus and must be serviced. It would be nice to see what else is hot. But I checked almost everything with Omega meter and thermocouple and nothing more was so hot.

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