I built the esr meter in qst April 2014 magazine and having a problem with the led for leaky cap. The problem is there is not enough voltage differential to "trip" the Transistor thereby letting LED to illuminate.
Without a schematic, it's unlikely whether anyone here will be able to help. The contents of QST magazine is copyrighted by ARRL, and so it's not available on the Internet to non-members of ARRL.
Not sure about this circuit, but I'm always leery of any project in the April QST.
This is the issue where they pull April Fool Jokes on the members.
I have been a member for over 50 years, and remember many April issues that had projects that appeared interesting, but were totally not what would work.
Lol well if it is a fool joke then I'm a fool. No its really a project. Everything works with the exception of LED. And it tries to work but just not enough voltage to trip the Transistor "on" ..... The Transistor is not reaching saturation. Sounds simple but the solution is eluding me in this particular schematic . I think the 15k (r19) may be a 150k. But my math could be wrong. Getting 2.24volts at Transistor. But only . 61volts differential. So if any one can help i would seriously appreciate the help.
Lol well if it is a fool joke then I'm a fool. No its really a project. Everything works with the exception of LED. And it tries to work but just not enough voltage to trip the Transistor "on" ..... The Transistor is not reaching saturation. Sounds simple but the solution is eluding me in this particular schematic . I think the 15k (r19) may be a 150k. But my math could be wrong. Getting 2.24volts at Transistor. But only . 61volts differential. So if any one can help i would seriously appreciate the help.
Practical joke projects in magazines I find pathetic.
Assuming you have not fallen foul to such a thing try doubling or tripling the value of r14 and r15 (they need to be the same) to increase the gain of the circuit.
I built an esr checker a while back, it uses a transformer from a dead pc power supply, the o/p is around 0.2v so it doesnt force semi's into conduction, so you can reliably check caps in circuit.
I found the original circuit sluggish in turning on the led. I changed the transistor to a comparator and it snaps right on now. I also added a piezo-buzzer so I can made sightless esr measurements. See mine at https://www.qsl.net/kh6grt/page4/esrmeter/esrmeter.htm
The datasheet for the 2N3904 and for almost every other little transistor shows that it needs a base current of 1/10th the collector current to completely turn on.
The LED current is about 36mA (enough to burn out the red LED) if the transistor turns on completely so the base current should be 3.6mA but the actual base current is only about 0.1mA so the actual LED current is maybe 1mA (who knows since the resistor values are wrong?). R20 should be 120 ohms for an LED current of 21mA and R19 should be 680 ohms. Then the value of C6 should be much higher.
I did not calculate how well the R19 and C6 filter smooths the squarewave.