I have a HP 8568A (+ HP 85662A display module) spectrum analyzer.

In the last 2 days, I had 3 caps that released the magic smoke.

On the following schematics there is a capacitor C1 that I marked. The original, 40 years old ones exploded in about a minute, which is no surprise. On the other hand, the new one I bought also exploded. Capacitance is the same, 100nF.

enter image description here

What could be the reason for that? What kind of capacitor should I use here?

Old one: enter image description here

New one: enter image description here

  • 1
    \$\begingroup\$ What is your mains voltage? What type of capacitor (make, model, link to datasheet?) is the new one? Are you sure it is a Class X safety capacitor, meant for direct and safe connection between live and neutral? \$\endgroup\$
    – Justme
    Sep 13, 2020 at 20:26
  • \$\begingroup\$ 240 VAC is the nominal voltage from the mains. In real circuits, things like surges and switching noise will get you transients above the nominal voltage. You are probably getting a transient that is low enough in frequency that the emi filter coil isn't taking out. \$\endgroup\$
    – scorpdaddy
    Sep 13, 2020 at 22:11
  • \$\begingroup\$ It also depends on the waveform; e.g. if used with a square-wave inverter (from a photovoltaic supply), that puts more strain on the caps and harmonics might hit the resonant frequency of the L-C combo. Also check that the AC mains is balanced to ground - if you have a floating neutral, one side might be getting far more than half the voltage. \$\endgroup\$ Sep 13, 2020 at 23:18


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