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In this circuit, there is a wire between M5-1 and M5-3 (at the bottom). It's a little plastic connector with a wire in it that you can pull out.

I've been trying to debug a power issue after recapping this 1980's power supply. At first, I forgot to put that connector back. Some of the reading looked correct, but others were too low. When I added it back, the formerly-correct readings jumped to ~1.5x.

What purpose does this connector have in this circuit? Why would you want it connected? Why disconnected? Why isn't it just permanently there?

circuit

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  • \$\begingroup\$ The only reason I can think of for a re-storable wire connection would be to enable current measurements at that point in the circuit. \$\endgroup\$
    – Oldfart
    Commented Jul 8, 2018 at 17:36
  • \$\begingroup\$ This is a custom design option for soft start / fast stop on a high energy & voltage supply with PTC and NTC and situations may call for removing this. \$\endgroup\$
    – D.A.S.
    Commented Jul 8, 2018 at 17:38
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    \$\begingroup\$ 120/240 switch: repairfaq.org/sam/smpsfaq.htm#smpssb12i \$\endgroup\$
    – Jack Creasey
    Commented Jul 8, 2018 at 17:42

2 Answers 2

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It's just mains voltage selection, quite common those days. It turns the diodes and the two 220uF capacitors in either a bridge full wave rectier for 240 V use (wire removed) or a voltage doubler for 115 V usage (wire fitted).

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    \$\begingroup\$ "quite common those days" very common in the past, much less common today because of power factor regulations. \$\endgroup\$
    – Peter Green
    Commented Sep 30, 2018 at 22:35
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I suspect this offline DC high power supply uses M5 to enable cap charge current thru the PTC to degauss the CRT tube during startup. This can be disabled by removing jumper M5.

This only works with 120Vac polarized plugs with an earth grounded Neutral. It was never intended for 240Vac/50Hz

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