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I have an old PS-303D from Conrad in use at home. Recently it showed a problem.

If I measure with my multi from V- to V+, the voltage reading is correct, current and voltage regulation seems to work fine. But when I connect V- with GND the supply turns off. Connecting V+ with GND is no problem. So I opened the case and did a little check ups. Soldering looks so far ok, and no visible damage has occurred. The GND is only connected to the case and via 2 ceramic capacitors to V+ and V- which seem ok. I looked at the schematic and I understand what each section is doing, but after a day of measuring I can't seem to find the error.

Can anyone point me to the right direction on how to approach the problem? Help would be greatly appreciated.

Schematic:

Schematic

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    \$\begingroup\$ Are you sure the 2 ceramic capacitors are OK. Try disconnecting them and see if the problem goes away. From your description it appears that the capacitor from V+ to GND may be shorted. This would not cause a problem when V+ is connected to GND but would create a short between V+ and V- when the V- terminal is connected to GND. \$\endgroup\$ – Barry Jun 15 at 23:47
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    \$\begingroup\$ It's possible you already have a (partial) short from V+ to GND. C405 Maybe. \$\endgroup\$ – brhans Jun 15 at 23:47
  • \$\begingroup\$ Also check any power elements which are heatsinked to the chassis, one may have an electrical short to the case \$\endgroup\$ – sstobbe Jun 16 at 0:58
  • \$\begingroup\$ @Barry I completly desoldered C405, C406 and C402... The problem persists. \$\endgroup\$ – Joey Ssymank Jun 16 at 1:31
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I think sstobbe is on the right track here - something in the supply likely has a “sneak path” to the chassis. I would start troubleshooting this by connecting a high value resistor (100 k perhaps) between the minus output and the chassis ground. That shouldn’t make the supply turn off and you can measure the voltage across the resistance- check for both AC and DC voltage. Since there shouldn’t be any voltage the amount may give you a hint of where the problem may be.

You can also unplug the supply, make sure it is fully discharged and use an ohmmeter to check the resistance between various points of the board and chassis- they should all be high resistance (of course take the resistor mentioned above off first).

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  • \$\begingroup\$ This would make sense, since when I touch the cables with bare hands the current sense LED glows dim. I will try this and report back... \$\endgroup\$ – Joey Ssymank Jun 16 at 12:05

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