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transistor only switching with oscilloscope probe

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ToBeFrank:
I'm trying to diagnose why my transistor is not switching. I've used this circuit in several other boards with no problems. I'm using it the same way on this new board, which is to invert an input signal and level translate it. The input signal is high at ~7.2V and low at 0V. The circuit is below. If I probe at the R4 pad I see 1.29V for high. I also see 1.29V when I probe the R5 pad. This is as I expect from the voltage divider. When I probe the base of the transistor (pin 1), I see 690mV. The odd thing is when I probe the base of the transistor, it starts switching as it is supposed to and the circuit works. As soon as I remove the probe from the base it stops working. What does this indicate?

Psi:
First thing i'd check is that R5 is grounded properly.



--- Quote from: ToBeFrank on August 30, 2011, 09:14:15 am --- If I probe at the R4 pad I see 1.29V for high. I also see 1.29V when I probe the R5 pad.

--- End quote ---
Where exactly are these two pads in relation to the circuit?


Also, when its not working, does the transistor stay on or off?

Gall:
Did I understood correctly that you measured 1.29 V at the OUTPUT of the divider (which goes to point 1) and different voltage at point 1? Most likely, cold joint (bad soldering) at transistor's base. A npn transostor parallel to the lower voltage divider's resistor (like this circuit) would limit the divider's output to 0.6 V if working properly. Broken transistors with bad internal joints do exist, too.

FreeThinker:

--- Quote from: Psi on August 30, 2011, 09:28:12 am ---First thing i'd check is that R5 is grounded properly.



--- Quote from: ToBeFrank on August 30, 2011, 09:14:15 am --- If I probe at the R4 pad I see 1.29V for high. I also see 1.29V when I probe the R5 pad.

--- End quote ---
Where exactly are these two pads in relation to the circuit?


Also, when its not working, does the transistor stay on or off?

--- End quote ---
Yep my first though was bad grounding and  the probe is providing the ground connection. Measure the resistance from base to ground should give you some clues.

ToBeFrank:
I checked the ground at R5 and it is ok. I then took out the soldering iron and touched all three joints of the transistor. It's now working fine. As Gall pointed out, it was a cold joint. Thanks!

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