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| Peaktech 4250 AC/DC current clamp teardown and repair. |
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| MikePie:
Thank you to all for your most helpful inputs. First thing I did on my just-received PeakTech 4250 was to check its output ripple. Sure enough, there was this 550Hz ripple. Since the only cap I had on hand which would fit into the available space was a 1000uF/6V3, I did not want to add this across U5 input in order to not expose D4 to the resulting switch-on inrush current peak. Instead, I soldered it in parallel to D4, since I thought that this negative supply could well benefit from additional bypassing anyway. I soldered its (+) lead into the thru-hole of C7 (-) mounting position, and its (-) lead to D4 Vb- pad. After that, the ripple was gone. I also replaced the coiled output cable with its non-retracting banana plugs with a coaxial BNC cable of the same diameter (which I took from a defective pH probe). This made the broadband output noise amplitude a little less sensitive to handling and also reduced stray signal pickup. I added some red-marking to the schematic diagram which was posted by mqsaharan (Jul 07, 2020) to show differences I found on my probe, and the added capacitor. The PCB in my probe is Rev.E; there has been some relayout around the ZERO switch SW1, apparently to accommodate a switch with a different footprint, relative to the Rev.D PCB which is shown in above posts. I attach the scanned schematic drawing. It includes the measured current consumption (9.6mA) and the measured Vb- voltage. I also checked the squarewave response using a 50mA peak, 10kHz signal, and the zero stability of about +/-3mA during 10 minutes (Plots attached). Overall, I am quite happy with this current probe. |
| schmitt trigger:
I have modified my own probe, adding a 10 uF cap, and can confirm that IT WORKS!! The ripple disappeared. Thanks everyone for their contributions. Special thanks to Mike Pie for taking the time to draw a schematic. :D |
| MikePie:
I updated the schematic with some more differences I found. Thought I might as well share it. On the less sensitive range, I noticed large spikes at fast transitions as if the load was capacitive (which it was not, just simple carbon resistors). I decided to reduce the bandwidth by adding a 2n2 ceramic capacitor between the wipers of VR5 and VR6. This I did not show in the schematic. BW is now about 150kHz on the 100mA/mV range, closer to the about 50kHz of the 10mA/mV range. Before, it was about 500kHz, but apparently with artefacts. The four plots show this (with/without 2n2, 100mA and 10mA range). |
| pgodefroy:
There was indeed this 550Hz ripple. I soldered a 1000uF/6V3 capacitor parallel to D4: I soldered its (+) lead into the thru-hole of C7 (-) mounting position, and its (-) lead to D4 Vb- pad. After that, the ripple was gone. |
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