What is this part? Maybe transistor.

[Spoerle]

They are repairing an older lab instrument and I am stuck on one part.
In the device there are two boards fitted with 74HCxxx logic, 5V power supply.
On first PCB is output any 74HC logic output, symbolically represented by U10A. it goes to port 2 of that mysterious Q3 component in the SOT23 case . On port 1 is 5V and on port 3 is 200 ohm resistor and connector SMA for coax line.
On second board is SMA connector, 200ooh resistor a any 74HC logic input.
I would expect that Q3 is some kind of transistor that increases the output current of the 74HC in order to charge the parasitic capacitance of C28, i.e. the coaxial cable.
The problem is when I measure the terminals the only diode I can find is the diode between terminals 2 and 3 Anode on 3.
I'm not the wiser
NPN, PNP or JFET have two measurable PN junctions, MOSFT can have one but not between G and D.
Can't think of what it could be?

 

[barry]

Maybe it's a diode.

 

[betwixt]

It certainly looks like an NPN transistor wired as an emitter follower but it could also be a MOSFET wired as a source follower. Both of those would be easy to identify with diode/resistance checks though.

Are there any markings on the device?
Photo ?

Brian.

 

[andre_luis]

The context of the application often can give an indication of the function of the component, such as if you can identify the function of the input and output signal of that circuit.

 

[Spoerle]

Yes you are right.
Originally, we did not solve this part of the connection at all, it was quite clear to me. Coaxial represents a larger parasitic capacitance, therefore a higher output current is needed and there is a transistor as a current amplifier,

i.e.
input 1 will be Collector or may be Drain, what would it be different when it is connected to a +5V power supply
input 2 will be Base or maybe Gate, what would it be different when it is connected to the 74HCxx output
input 3 will be Emitter or maybe Source, what would it be different when it is connected to the SMA connector and input of the 74HCxx.
I would bet on an NPN transistor.

There is no marking on the component or on the PCB, to make it even more complicated, the entire fitted plate is painted with transparent epoxy varnish.
I was able to scrape my way to the pins, but I don't want to risk desoldering the part anymore, I'd probably destroy the PCB.

The component is still on the PCB, ie.
-input 3 is connected to ground via R22 200Ohm,
-input 2 is connected to 74HCxx output stage
-input 1 I managed to isolate it.
the diode I measured between 2 and 3 could theoretically be a protection diode in the 74HCxx output measured through a 200 ohm resistor. Measured voltage lost 734mV.

However, even then NPN, PNP JFET and even N or P MOSFET should also have measurable PN junctions