Vertical scaling defect on FNIRSI-5012H portable oscilloscope  - Page 1
Forum section: electronics / repair

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 12:54:07 am
content: I have recently bought this portable oscilloscope because i am learning electronics and it would be very useful, sadly mine started this strange behavior while I was testing a psu PWM. I couldn't diagnose the defect because I am a beginner with basically no experience and no one knows how to repair it where I live, so I made a video of the problem (link below) and I hope you guys can help me, any help would be appreciated.BTW, calibration only works in 10V/5V (1x) or 100V/50V (10x), in other scales I get a "Failed!" message.https://youtu.be/pKvbly4wNl0

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 01:20:40 am
content: This is a really strange failure mode. Horizontal scale affects the ADC sampling rate, and it is strange that this introduces a vertical offset.Does this happen on different (all?) vertical scale settings?I would at least disassemble it and see if everything looks fine visually. Take a few high res pictures.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 03:31:19 am
content: I couldn't upload the images to this topic so I did it on GDrive:https://drive.google.com/drive/folders/1v6h4areFM4AiSdD4VdOs90GqRPwvrhSh?usp=sharing

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 03:35:50 am
content: That ADC rework looks pretty bad. I'd at least use some alcohol and clean up the area around ADC.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 03:38:47 am
content: Quote from: ataradov on June 25, 2021, 03:35:50 amDid it came like that from factory? I didn't touched it.

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 04:00:10 am
content: Well, it is China. They do crazy stuff. They overclock the ADC like crazy, so it is possible that they assembled the device and it did not work. It is also possible that it had this issue and they detected it, and replaced the ADC, but never properly retested, and the ADC is not the problem.This scope is not great in general.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 04:14:38 am
content: It was working normally while I was measuring the psu output voltage, so I don't understand how measuring 12V (max) could do any harm to this. Maybe changing the MCU solves the problem? Then I'd have to flash the new one, right?

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 04:32:19 am
content: Was it plugged in into the power supply at the time of measurement?It is not the MCU problem. There is also no official firmware binary available.Here is a big topic on that scopehttps://www.eevblog.com/forum/projects/reverse-engineering-fnirsi-5012h/There are schematics of the front-end, you can try to do some diagnosis based on that. Especially pay attention to the reference voltage level. It should remain constant when you switch time base.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 06:26:32 pm
content: 50s - 6ns = 1,648V measured from ground to probe connector;1,8mV difference between OPA356 Out and +In;6,3MΩ forward polarized "red" diodes / +200MΩ (couldn't measure) reverse polarized diodes resistance;I couldn't find the two optocouplers LEDs in series, they all have one transistor to them;156,5nF (brown) capacitor in parallel with the solid state relay and 0,370nF (white) capacitor in parallel with the 470kΩ resistor.

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 06:50:40 pm
content: So with no probe connected input voltage (==reference voltage) remains stable. Is OPA356 output (ADC input) also stable when you switch time scale? If it is stable, then it just looks like some ADC issue where its input is sampled differently depending on the clock.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 07:38:15 pm
content: Measuring OPA356 Out pin to ground and changing Voltage scale:10V = 1,55V5V = 1,48V2V = 1,31V1V = 1,16V500mV = 0,98V200mV = 0,80V100mV = 0,80V50mV = 0,73V

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 07:40:40 pm
content: In your original video you changed time scale, not voltage scale. Or have I missed something?

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 07:42:33 pm
content: I changed both, first time scale to 500ms, then I changed V scale to all values getting strange readings (like -30V) only by changing V scale

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 07:56:18 pm
content: Ok, but why did you measure Vref over different time scales, but not voltage scales.Your goal here is to figure out what changes on the outside when you make changes to whatever setting that is broken. In all cases, opamp output should remain about the same. And the difference may come wrong the wrong Vref, or damaged signal path to the opamp.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 08:06:50 pm
content: You said it should remain constant while changing time base, so I changed Time base, but these are the values while changing V scale:V scale x Vref:10V = 1,645V5V = 1,648V2V = 1,601V1V = 1,605V500mV = 1,629V200mV = 1,619V100mV = 1,551V50mV = 1,351V

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 08:50:26 pm
content: And the output of the opamp at the same time?The idea here is that in order for the trace to remain stable, ADC input must be stable. Varying Vref would translate to the ADC input, unless this variability due to calibration that accounts for inaccuracies in the resistor dividers.If ADC input changes proportionally to Vref, then it appears that calibration parameters are incorrect. And I have no idea how to fix that with the stock firmware.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 09:21:58 pm
content: Quote from: ataradov on June 25, 2021, 08:50:26 pmV scale x Out Pin to ground V10V = 1,55V5V = 1,49V2V = 1,33V1V = 1,16V500mV = 1V200mV = 0,80V100mV = 0,82V50mV = 0,73VQuote from: ataradov on June 25, 2021, 08:50:26 pmIt was working, but after the psu test, all went wrong. It is possible that the firmware has corrupted?

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 09:35:10 pm
content: Changing the Time scale, the Voltage changes too.

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 10:36:55 pm
content: Quote from: Tetris_BR2 on June 25, 2021, 09:21:58 pmNo idea, I don't know what stock firmware is doing or how it works. I ran it for 10 minutes to realize it was borderline unusable.

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 10:37:41 pm
content: Quote from: Tetris_BR2 on June 25, 2021, 09:35:10 pmDoes Vref change too? If it does change with time scale, then something is definitely screwed up.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 10:43:28 pm
content: Quote from: ataradov on June 25, 2021, 10:37:41 pmNo, 1,648V at 5V scale

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 10:55:42 pm
content: Ok, so Vref does not change with time scale? Does opamp output change with time scale?If Vref stays the same and opamp output stays the same, then it is some sort of ADC issue.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 11:08:50 pm
content: Quote from: ataradov on June 25, 2021, 10:55:42 pmOpAmp Out pin and ground Voltage is 1,475V to 1,485V while changing time scale. It isn't keeping a steady voltage even at the same scale.

author: ataradov
author_rank: Super Contributor
timestamp: June 25, 2021, 11:51:12 pm
content: Minor variations are probably fine.I don't understand though. How is it possible that Vref is changing, but ADC input is not. What happens to that changing voltage? I'd trace it out and look at it at various stages of the signal path.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 25, 2021, 11:56:39 pm
content: Quote from: ataradov on June 25, 2021, 11:51:12 pmQuote from: ataradov on June 25, 2021, 10:55:42 pmThey don't change in Time scale, only in V scale. The trace on display only changes with the time scale in 500nS-200nS(7V), 200nS-100nS(4,6V) and remains the same in 100nS-6nS(4,6V), "calibrated" at 5V.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 26, 2021, 12:08:25 am
content: Some optocouplers seem to have the LED reverse polarized while one (optocoupler) is active.

author: ataradov
author_rank: Super Contributor
timestamp: June 26, 2021, 12:18:33 am
content: Quote from: Tetris_BR2 on June 25, 2021, 11:56:39 pmIf the trace changes with time scale while ADC input remains the same, then it means either ADC is broken and its result depends on the sampling rate, or MCU does some weird adjustment in the software, which would be strange. But I have no idea what the default firmware is doing.Only one optocoupler is active at a time, that's how they select the range. Except for a couple settings where two of them are paired up.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 26, 2021, 12:27:16 am
content: Quote from: ataradov on June 26, 2021, 12:18:33 amSo changing the ADC would be the best option? If so, I can order it now (AD9288, right?) but it will only arrive from China by the next 6 months (mostly because of mycustoms)

author: ataradov
author_rank: Super Contributor
timestamp: June 26, 2021, 12:59:54 am
content: I don't know for sure. Because changing of the Vref with vertical scale this much seems strange to me too.

author: Tetris_BR2
author_rank: Contributor
timestamp: June 26, 2021, 09:01:34 pm
content: I've seen many other scopes like this with OPA356 problems on Google, so I will try this first. If it makes no difference, I'll try to change the ADC.

author: Tetris_BR2
author_rank: Contributor
timestamp: August 15, 2021, 06:24:25 pm
content: Changing OPA356 solved the problem! Many thanks to you all!