I'm debugging/repairing a broken Logitech Z623 2.1 speaker system that I received in a broken condition (doesn't power on at all.)

I have pulled apart the subwoofer/amplifier unit, and after removing a heat sink, found an exploded component inside the 240V (mains) side of the power supply circuitry.

Magic smoke came out here

Both of the 2A 250V fuses were blown. I have managed to de-solder the component, which looks a little like a capacitor to me, but its PCB footprint has a symbol I'm not familiar with - please see picture. It looks like two diodes with tails facing each other.

Mystery circuit symbol

Its PCB silkscreen label is either Z1 or TH2 (not sure which label belongs to it.) Physically speaking, it's quite close to the 240V mains input, next to a small transformer and some inductors and large diodes and the main capacitor.

Can you help me identify this component?

Mystery component

The component body is partially destroyed, but I can see a backward RU manufacturer logo that I've seen before on other components, as well as the code V743 and the number 2.

  • 7
    \$\begingroup\$ Now because this was blown, along with the fuse(s), expect more damage elsewhere. There are probably one or more MOSFET(s), ensure none of those are shorted. There also should be rectifier diode(s) (perhaps ultra-fast variety) that may also have failed. When ready to test, wire an incandescent light bulb in series to the power plug. If it lights and goes out, it should work. If it stays on, something else is still wrong. \$\endgroup\$
    – rdtsc
    Aug 9, 2022 at 12:36
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    \$\begingroup\$ The idea of the incandescent light is to use it as a powerful resistor between power outlet and device. If the device has a short circuit, the bulb will receive all the power and light up, if the device is at least not dangerously damaged, the bulb will quick fade out when the capacitor(s) are charged. You can connect it instead of a fuse. This is the non spectacular version of a first test with nothing exploding. \$\endgroup\$
    – Jens
    Aug 10, 2022 at 1:05
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    \$\begingroup\$ Just wanted to share that I got a replacement varistor in place and 2 new fuses and now the speakers work like a charm :) No other components were damaged. Thanks all for the assistance! \$\endgroup\$ Aug 10, 2022 at 6:37
  • 2
    \$\begingroup\$ BTW: The mentioned UR (written backwards) is not a manufacturer logo. It is an Underwriter Laboratories (UL) quality mark called "UL Recognized". UL is an independent safety certification organisation. Generally safety relevant components may have such a mark. You can find a good article about it here: forum.digikey.com/t/understanding-ul-when-is-it-applicable/7223 \$\endgroup\$ Aug 11, 2022 at 8:03
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    \$\begingroup\$ Hi @SashiKumara B M - Yep. After replacing this component and both fuses, the unit works perfectly. It is sitting on my desk as my main speaker system for my keyboard to this day. \$\endgroup\$ Apr 2, 2023 at 1:23

3 Answers 3


The manufacturer logo is "Matsushita". It is one of their "V" series varistors. Looking trough their product catalogue, only the "ERZV07D431" can be a match, with marking "V7431U".


You are looking at a varistor. It's an overvoltage protection for your circuit. https://en.wikipedia.org/wiki/Varistor

To identify a replacement, you should choose one that is around 20V above your peak input voltage.


It was a varistor. Choose one rated for your voltage and similar in diameter to the blown component.

  • 13
    \$\begingroup\$ Above the rated voltage! \$\endgroup\$
    – winny
    Aug 9, 2022 at 10:56

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