Faint/weak trace

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sinewave
Posts: 3
Joined: Wed Mar 14, 2018 9:41 am

Faint/weak trace

Post by sinewave »

NOTE: I've learned from visiting these forums that my DSO138 is a clone, and I will be making enquiries with the seller and taking appropriate measures to purchase a genuine or fully open-source unit next time.

Greetings!

My recently built DSO138 exhibits a very weak trace during calibration (please see the images for what I mean). Problem exists when measuring an external signal.

Voltages are:
Battery = 8.9v
D2 edge = 8.9v
V1 = 0v
V2 = 1.65v
AV- = -5v
AV+ = 5.04v
+3.3V = 3.3v
+5V = 5.09v

I've been over quite a few threads on this forum and found this thread, where MNTech mentions:
* "You can check the operation of the switch by measuring from U2 pin 10 to ground. It should be 600, 300 ,120 ohms as you go from X1 to X5"
* "V1 test point to the nearby DGND should measure 600 ohms"

However, I don't get that when measuring from U2 pin 10 to DGND on my unit:
* As I switch X1 through X5, resistance goes from 0.2 -> 149 -> 96 ohms.
* Resistance from V1 to DGND is 0.2 ohms.

I have checked all solder very carefully and beginning to think his might be a manufacturing defect with the board itself, especially as it's a clone.

Do you think there could be a "leak" (unintended connection) between V1 and DGND? And if so, where would you suspect it, and how can I best test to find the point?

The V1 hole itself looks quite suspect in that the solder pad seems to get very close to the ground plane. But I'd feel silly if I drilled out the V1 hole only to find it didn't solve the problem.

Any ideas appreciated. Despite being a clone, I do want to see it working.

Attached are picures of:

1) the faint trace problem
2) testing resistance from V1 to DGND
3) testing U2 pin 10 to DGND as I switch from X1 to X5
Attachments
faint/weak trace
faint/weak trace
resistance to DGND at V1
resistance to DGND at V1
Resistance from U2 pin 10 to DGND at X1
Resistance from U2 pin 10 to DGND at X1
Resistance from U2 pin 10 to DGND at X2
Resistance from U2 pin 10 to DGND at X2
Resistance from U2 pin 10 to DGND at X5
Resistance from U2 pin 10 to DGND at X5
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Faint/weak trace

Post by MNTech »

Your resistance readings are consistent with a short to ground on the traces connected to V1 test point.

I would first suspect a short at the switch. The trace to the switch passes under the corner of the metal frame of the switch. If the solder resist coating is damaged there that could be the site of the short.

The short could be solder under the switch too. If you are getting desperate you could cut the trace to the switch and see which side the short is on. The switch would be very hard to remove, maybe with ChipQuik it could be done.

On the bottom of the board there is a short trace from R6 to a via which leads to V1. You could cut that trace too.
Small cuts can be bridged with solder once you find the problem.
sinewave
Posts: 3
Joined: Wed Mar 14, 2018 9:41 am

Re: Faint/weak trace

Post by sinewave »

Indeed it was a short being created by excess solder between the switch and the board. I was able to remove the switch with a hot air gun and clean up the board. The problem with the weak signal is gone!

Many thanks to you.
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