[ SOLVED!! ] Hantek DSO4072C Oscilloscope - Both Channels BAD

[X-Blade]

  Hello guys!

I bought an Hantek DSO4072C Oscilloscope but shortly it burned out.
I connected the probes to the front calibration port (5v@1khz) and the scope behaved like this:





Then, I found the channel input schematics here in this forum that I want to thank to the author (DSO 5000 series).
Looking in another forum from Hantek support I saw someone with similar problem, then I replace the transistors/diodes of each channel:



BAV99 (marked A7), MMBTH10 (marked 3EM), MMBF4393 (Marked 6G) and the 2 x BC846B (marked 1B)



Then the signal has stabilized but came another headache:




Peak to Peak voltage and the waveform amplitude decreases as I move the trace down (Vertical position), and I cant move down from the center of the screen, both channels behave the same way.

I have a second oscilloscope (analog) and I can perform some measurements, but unfortunately my knowledge is limited.

Anyone can help me, please?

Thank you all.

[X-Blade]

UPDATE:

I found that -5V rail from 79M05 is putting out -7,4V (-5V Testpoint) . It looks like the regulator is gone, I will try to get a new one, replace and retest.
Cheers.

[gongwei@hantek.com]

Can the instrument work properly after replacing （U904） 79M05?

[X-Blade]

Hello.
I am still waiting for parts from China, it is very dificult to find those parts in my country.

[X-Blade]

Hello people.

NEW UPDATE:

I replaced the 79M05 then the -5v testpoint is giving -5.01v that is OK.
I turn On the oscilloscope and did Autoset on CH1.

It looks good, and I am able to move the waveform without any problem.



Channel 2 is not working good anyway:



When I press Autoset is shows a line without waveform:


as I turn vertical position up, the waveform appears and grows as I turn it up, I am still unable to bring the waveform down.
Maybe some defective component I replaced on channel input, or something.

Any suggestion?
Thank you all for your help.

[X-Blade]

I started to make some comparisons from the testpoint signals on both channels:



At tp100/200 the signal was equal, Sq. wave at 5V amplitude:


At TP101/201 same thing but with an amplitude about 0.110V (110mv).

But things gone different as I testing the TP103/203:



At center position of the waveforms, signal looks equal on both channels, about 20mv amplitude sq. wave:


As I move the good channel (CH1) up and down from minimum to maximum, signal is unaltered, BUT on CH2 does not behave the same way.
Up from the center there is no change, below the center, the amplitude of the waveform decreases and the offset goes up until there is no signal but a voltage:




I am still trying to undestand what is happening.

[X-Blade]

HELLO GUYS!

It is time to celebrate :-)




After measure and compare the signals, it was all right except at output of the AD8510, then only left the transistor circuit where the BC846B is.
I started to test each resistor and I found the guilty!




It was cracked at one end, I put some solder and I was able to fix it.
Thank you all of this fantastic forum, this will be a good reference for those who had blown it in the same way I did.

**************** Cheers  ****************

[MasterT]

Could you post schematics?
My 4072C works so far, but noisy at 20/50 mV scale. I also see  unknown wide band spikes on the fft plot. Have to fix some day.

[X-Blade]

Hello MasterT!
The schematics I found here on eevblog.com.
Thanks to tinhead for all the effort to bring us this schematics.

The main schematic is this one:
https://www.eevblog.com/forum/testgear/hantek-tekway-dso-hack-get-200mhz-bw-for-free/?action=dlattach;attach=55294

BUT the channel input schematic is a bit different and is this one :
https://www.eevblog.com/forum/testgear/hantek-tekway-dso-hack-get-200mhz-bw-for-free/?action=dlattach;attach=12233


This is the thread where I found that:
https://www.eevblog.com/forum/testgear/hantek-tekway-dso-hack-get-200mhz-bw-for-free/

These scopes are very noisy at millivolt levels, so that is why I have my analog one.
You can remove all the cables connected and run the self calibration and see if it gets better.

[MasterT]

Thanks.
Those threads a huge to read , >100 pages. And all references I find are for Hantek DSO-50XX.
Calibration 4-5 times every year, seasonally with temperature changes. Doesn't help. 
2 software upgrades, from official Hantek web-site (last dso4kc_func_hantek_dso_dso4072c_20180305.0) doesn't make any difference.
 Symptoms similar to slowly drying electrolytic caps, noise is raising up with time.

[X-Blade]

Maybe it is time to check the power supply or capacitors near to the voltage regulators.
This very common on modern electronics