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MSO 3012 Shows only weak AC signals

DPO/MSO/MDO4000 Series, DPO/MSO/MDO/TDS/THS3000 Series, DPO/MSO/TDS/TPS2000 Series, TBS/TBA1000 Series
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Proxxima
Posts: 4
Joined: November 21st, 2018, 2:26 pm
Country: Netherlands

MSO 3012 Shows only weak AC signals

Post by Proxxima » November 21st, 2018, 3:11 pm

Hello everyone,

I have a weird problem on both channels of my MSO 3012. It seems to have only a
very weak capacity between the input and the measuring circuit. It does only display the
AC part of the probe calibration. And as you can see it is a very low level signal as well...
(Unfortunately it is not a broken probe wire. That could give about the same behaviour.)

Also external signal sources show the same problem. And on an external DC signal the line doesn't move.

Both channels have the same problem. So it might be a common problem. I've updated the
firmware to 2.31 hoping it would reset settings. But it didn't solve the problem.

Is there any documentation or schematic available for these scopes? And/or does anyone
recognise this problem and knows a solution? It looks to me like a fault in the attenuator Board.

Thanks in advance!

Here is a photo of channel 1, DC coupling, 20mV/div 1:1 on the cal output.
Only ac.jpg
Only ac.jpg (334.86 KiB) Viewed 528 times

Proxxima
Posts: 4
Joined: November 21st, 2018, 2:26 pm
Country: Netherlands

Re: MSO 3012 Shows only weak AC signals

Post by Proxxima » November 22nd, 2018, 1:07 pm

Because I was pretty sure the fault was in the attenuator board,
I've removed the metal shields from the input circuits.

Immediately I spotted two fried resistors.
input resistors.jpg
input resistors.jpg (460.52 KiB) Viewed 488 times
To test I've put two 47R resistors in parallel with the broken ones.
47R resistors.jpg
47R resistors.jpg (564.81 KiB) Viewed 488 times
The result was a normal measurement again...
DC again.jpg
DC again.jpg (255.68 KiB) Viewed 488 times

It seems to me that the resistors are a part of an over voltage protection circuit...
But I'm not sure about the exact values of R0501 and R0201.
I hope anyone here can help me to get the schematic of at least the attenuator board.


Thanks in advance!

Proxxima
Posts: 4
Joined: November 21st, 2018, 2:26 pm
Country: Netherlands

Re: MSO 3012 Shows only weak AC signals

Post by Proxxima » November 28th, 2018, 12:44 pm

In the meanwhile I've checked and confirmed that these resistors need to be 50.0 Ohm.
(At my work was a 4 channel version, and I've checked it there) I've replaced the
resistors with thin film 50.0 Ohm resistors. All ranges are working normal.

But I have some questions left...

What happened here? I think it had a huge over voltage/HF power. Probably
some crowbar construction with the 50 Ohm resistors protected the scope?

If there is a protection circuit, I would like to know if that is still working well.

Are there any schematics available on these MSO 3012 scopes?

Proxxima
Posts: 4
Joined: November 21st, 2018, 2:26 pm
Country: Netherlands

Re: MSO 3012 Shows only weak AC signals

Post by Proxxima » December 2nd, 2018, 9:36 am

Pretty sad that I'm the only one answering my posts here.
No reply on my email to Tek...
No reply on the forum...

The schematic of the input circuit seems to be like this in a simplified way....
So if there is any info about that clamp circuit... Please speak up :D
input.jpg
input.jpg (37.18 KiB) Viewed 206 times

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