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For the purpose of understanding how it works, I'm attempting to reverse-engineer the schematic of an analog gauge. For some context, it's an analog tachometer from the 70s, with a scale from 400 to 7000 rpm for a 4 cylinder engine.

I've come across this component on the picture below, with a package and markings I cannot identify. There are two of them on the board (you can see them next to each other), each with 3 terminals. I'm thinking it might be a transistor, but the strange (to me) package confuses me a bit. The markings on the package are:

V15 S10

Unknown component closeup

I'm thinking the circuit might be similar to this one from an earlier ('66) unit, but without the thermistors.

Update: I traced the schematic, and I believe I could identify all parts except for the Zener (the 6.7V measured across it make me think it's a 6V8 one) and obviously the two components from this question (which I now believe to be transistors, though).

I can't make much sense of it yet and I need to double check it, but here it is:

VDO analog tachometer schematic

The input signal is from the (mechanical) distributor points of a classic vehicle without electronic ignition. It is a periodic waveform coming from output of the primary side of the ignition coil (low voltage side), and each cycle looks like the blue trace on this image:

Ignition circuit waveform

The maximum frequency of the signal is 233 Hz (7000 rpm), but in practice it would be less, as the vehicle's ECU would cut off the injection at a max of 5400 rpm (180 Hz).

Essentially, if I understand it correctly, the tachometer circuit should:

  1. attenuate the input waveform to a low voltage signal
  2. convert the signal to a square wave keeping its original frequency
  3. convert the frequency to a voltage/current that is proportional to it
  4. drive the tachometer needle's coil with that current

Would someone have an idea what component it could be? Or perhaps which type of package it is?

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  • \$\begingroup\$ For a while that kind of package was not uncommon for a transistor. You could try partially reverse engineering a schematic or plug that thing into one of the eBay transistor testers? \$\endgroup\$ – PlasmaHH Nov 19 '17 at 17:36
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    \$\begingroup\$ If it has four terminals I'd go for "bridge rectifier". If three then it's probably a transistor. Extract and test. \$\endgroup\$ – Transistor Nov 19 '17 at 17:40
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    \$\begingroup\$ Can you get us a different angle closeup on the part? \$\endgroup\$ – ThreePhaseEel Nov 19 '17 at 17:59
  • \$\begingroup\$ There is another one just to the left of it, looks three pin. Probably a transistor, maybe a double diode. Or something else! \$\endgroup\$ – Trevor_G Nov 19 '17 at 18:08
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    \$\begingroup\$ Transistor then, most likely. A diode tester will let you identify the base and whether it's NPN or PNP, Si or Ge. Then context (if it works, power it up and measure voltages; otherwise, what it's connected to) will let you distinguish collector and emitter. Unusual package ... what part of the world is it from? \$\endgroup\$ – Brian Drummond Nov 19 '17 at 18:29

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