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I've been trying to repair my lecroy 9314l for the past couple of days and i've sort of run into a wall. out of ideas. The power supply started to fail a while ago. It wont turn on when i connect a regular 3-pin (L-N-E) power pplug to the wall. But a non-earth referenced 2-pin plug works. Obviously this is unusable, since the probe ground leads leak heavily and the scope turns off immediately as soon as i connect the probe to any circuits ground. I initially thought it might be a faulty x/y class cap on the primary side. But after initial troubleshooting i realised that the primary side is working fine. No shorts anywhere. I have attached the circuit schematic here for reference.

circuit schematic pdf

Then i started with the secondary side and straight away discovered a short on the DC output lines. -5.2v,+5v lines are all shorted to the ground. Hence the power supply wont turn on when earth is connected. I have so far individually removed all the rectifier diodes to check if they have failed. All the diodes are fine and within their stated tolerance levels. I have replaced all the capacitors which were fine anyways. Both the optocouplers U1 and U2 used for feedback for voltage and current regulation are working fine.

The problem i am running into is that my knowledge of a/c and smps circuits is basic and i cant figure out how to proceed or what further steps to take in order to isolate the problem further. Could the transformer be at fault here? But then the scope works when earth is removed. If the transformer has shorted any of its secondary coils then the failure should be persistent isn't it?

Any advice or suggestions would help.

Regards

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  • \$\begingroup\$ Start by measuring the floating ground voltage , then reduce that with resistance to measure current and reduce more to threshold of failure to suspect a leaky part. \$\endgroup\$ – Sunnyskyguy EE75 Jul 2 '18 at 2:18
  • \$\begingroup\$ i dont have access to an isolation transformer. Hence havent tried active testing yet. Any safety precautions needed before starting?. Sorry but this is my first a/c repair :D \$\endgroup\$ – Asusrog Jul 2 '18 at 3:07
  • \$\begingroup\$ Use same precautions as 2 pin plug test and expect 0.5mA AC filter noise current if you touch frame, but should not cause over-voltage protection shutdown when connected from chassis ground to earth ground or PC frame (plugged in) > Test if working, starting with a wire, resistor or capacitor to join frame earth grounds.. Wear latex gloves , for precaution. ( Maybe you have a bad cordset? \$\endgroup\$ – Sunnyskyguy EE75 Jul 2 '18 at 12:19

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