Fried analog board

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Jules
Posts: 16
Joined: Sun Mar 12, 2017 6:08 pm
Location: Rome Italy

Fried analog board

Post by Jules »

Hi,
yesterday I had used my DSO Shell to perform a measurement of a PSU whose output is 18.5 V DC
As soon as I have turned on the DSO I felt a kind of "burned smell" while the oscilloscope was not returning a valid output.
I disassembled my DSO and found that there is a burned trace on the analog board right after the BNC connector.
Fried.jpg
What do you suggest me to do? Buy a replacement analog board, or to buy a new DSO Shell as the main board could have been damaged too?
Best regards,
TonyW
Posts: 4
Joined: Tue Aug 15, 2017 6:29 pm

Re: Fried analog board

Post by TonyW »

Must be something in the air! Mine did exactly that (down to the burnt trace a couple of mm from the BNC plug) measuring a nominal 15v AC supply on Monday. I've just bridged the burnt section by soldering a single strand of copper wire over it - seems fine. I do wonder about the maximum rating though; that's two units with exactly the same failure with an input voltage well below the quoted 50v pk figure.
Jules
Posts: 16
Joined: Sun Mar 12, 2017 6:08 pm
Location: Rome Italy

Re: Fried analog board

Post by Jules »

Hi TonyW,
I did exactly the same: bridged the burnt trace.
My DSO is back at work but I have a lot of doubt about it.
TonyW
Posts: 4
Joined: Tue Aug 15, 2017 6:29 pm

Re: Fried analog board

Post by TonyW »

I'm not particularly worried. Looking at the schematic of the analog board, I think if anything suffered it would be one or more of C1-C6, R1-R4 or the TL084 op-amp. A dead op-amp would be immediately obvious (no response to an input signal at all) and dead or out-of-tolerance passive components would show up with a distorted and/or attenuated display of the 1kHz calibration signal. Mine looks fine. I discount any problem with the logic board - that's pretty much going to either work or not. I'll not be using my oscilloscope on anything over about 10v in future, though!
jye1
Posts: 1221
Joined: Wed Feb 20, 2013 4:58 pm

Re: Fried analog board

Post by jye1 »

We did some investigation on this issue. We believe the cause was the couple select switch. This switch looks does not have a nice "break-before-connect" feature. When it is shifted between DC and GND it briefly shorts the poles 7 and 8. That shorts input signal directly to ground. If signal has low impedance (can source large current) it could burn the trace. All other parts shouldn't be affected since this is like a 0V input to the scope. But this could damage signal sources if they are weak.

It is suggested to add a protection resistor (value about 470 ohm should be suitable) between J1 center pole and SW1 pin 7. This can protect the problem from happening to some extend. It still could happen if the signal source has high voltage and very low impedance (like powerful supplies). For such signal sources avoid shifting switch SW1 when input is connected.

Another better solution is swap the connections of SW1 pin 5 and 7, i.e. connect C1 to pin 7 and J1 center directly to pin 5. However, this solution requires firmware change to make couple display correct. The prints on the top plate need to be changed too.
Jules
Posts: 16
Joined: Sun Mar 12, 2017 6:08 pm
Location: Rome Italy

Re: Fried analog board

Post by Jules »

Thank you very much for your answer.
Could you please look at this post http://www.jyetech.com/forum/viewtopic.php?f=19&t=1250?
I need assistance for another problem.
Best regards,
Ric82
Posts: 18
Joined: Sun Jun 25, 2017 2:04 pm

Re: Fried analog board

Post by Ric82 »

Hello
attn to Jye1


when you said :
"It is suggested to add a protection resistor (value about 470 ohm should be suitable) between J1 center pole and SW1 pin 7"

do we have to cut the pcb trace between j1 and SW1 pin7 and solder a 470 ohms instead of ? to prevent new damages to this trace in the future?
thanks to confirm svp

regards Eric. ;)
TonyW
Posts: 4
Joined: Tue Aug 15, 2017 6:29 pm

Re: Fried analog board

Post by TonyW »

Makes sense. Thanks for the reply, jye1
jye1
Posts: 1221
Joined: Wed Feb 20, 2013 4:58 pm

Re: Fried analog board

Post by jye1 »

@Eric:

Trace cutting is required if the original trace have not been damaged. The protection resistor needs to be inserted between the two points mentioned.
Ric82
Posts: 18
Joined: Sun Jun 25, 2017 2:04 pm

Re: Fried analog board

Post by Ric82 »

@ Jye1

Thanks for your advice, i will take a look at that on next opening !
regards Eric. ;)
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