Identify SMD component?

[JimRemington]

Hello, All:

I'm working on reverse engineering the remote, floating sender (433 MHz xmit) for a fish finder found on eBay, and have stumbled across what could be a critical component that is not in any data base I can find.

It is a three pin package the end of the 125 kHz receive amplifier chain, identified with the marking "RRN" and Q4 on the PCB (far left in photo below). Using the diode check function on a multimeter, it could be a bipolar NPN transistor, but  I measure 0.8V from pin 1 to 2 and 1 to 3, as opposed to the 0.71V for the other three NPN transistors on the board. 

I'm puzzled by what appears to be a grounded collector, but it seems possible that the "transistor" is being used as a diode switch.

The function of the device appears to be switched on via pin 10 from the MCU, which provides a 60 ms(*) 3V pulse, presumably to gate the output of the amplifier to pin 6 of the MCU.

Comments appreciated!

(*) 60 ms is the expected return time of an echo from an object in water, 45 m distant, which is consistent with the stated maximum range of the sonar unit.

EDIT: schematic is missing a 100K resistor from MCU pin 6 to GND (see post below).

[JimRemington]

If I go with just the "RR" Q4 is a Siemens BFR93P NPN RF transistor.

I overlooked a resistor from output to GND in the previous schematic, and after adding that in, together with some guesses as to the cap values, simulated the circuit using LTSpice . It looks to be a level shifter circuit, which makes sense.  Seems like an odd choice, though.

[gamalot]

The component with "RRN" mark in your picture is obviously smaller than the SOT23 package, it should be SOT323 package, so it is unlikely to be BFR93P. You might try 2SA1579 in your simulation and see what happens.

https://alltransistors.com/adv/pdfview.php?doc=2sa1579_2sa1514k_2sa1038s.pdf&dire=_rohm

[JimRemington]

Thanks, good point about the package size, but why 2SA1579?

It is a PNP transistor, which does not seem consistent with my diode-check tests.

Also, at least from this manufacturer: https://www.mouser.com/datasheet/2/348/2sa1579 the ID of pins 1 and 2 are swapped versus SOT23. Is that always the case with this package?

[gamalot]

Thanks, good point about the package size, but why 2SA1579?

It is a PNP transistor, which does not seem consistent with my diode-check tests.

Also, at least from this manufacturer: https://www.mouser.com/datasheet/2/348/2sa1579 the ID of pins 1 and 2 are swapped versus SOT23. Is that always the case with this package?

Just because 2SA1579 is the only SOT323 component with "RR" Mark I have found so far.

There are 2SA1579 data sheets from several different manufacturers, you can check if they have the same pinout or not.

https://alltransistors.com/transistor.php?transistor=52107

[JimRemington]

Thanks for the link. The offerings all seem to have the same pinout, and a PNP transistor with that pinout would not make sense in the circuit.

I injected a 120 kHz sine wave into the amplifier circuit on the board, and monitored the output on the 100K resistor using the scope. It agrees completely with the LTspice simulation result, so it is indeed a simple level shifter.

Problem solved! Thanks to everyone who took the time to look.