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:
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:
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:
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:
- attenuate the input waveform to a low voltage signal
- convert the signal to a square wave keeping its original frequency
- convert the frequency to a voltage/current that is proportional to it
- 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?