Geofex Transistor Tester

[carrejans]

Still have a few questions about the Ge Transistor Tester from RG Keen.

- Do I have to read directly when I put my transistors in the circuit? Or should I wait untill the measurement stabilizes?

- So to be safe, the leakage has to be below 300uA. Is it correct that then the first measurement with the switch off, needs to be under about 740mV?

Thank you!

[R.G.]

- Do I have to read directly when I put my transistors in the circuit? Or should I wait untill the measurement stabilizes?

It's far better if it does not have to stabilize. By far the biggest contributor to drift is heat from your fingers when you put the device into the socket. Use gloves, or tweezers or pliers or something so you don't heat them. But yes, the reading should not be drifting, at least not too fast.

- So to be safe, the leakage has to be below 300uA. Is it correct that then the first measurement with the switch off, needs to be under about 740mV?

The leakage current appears across the "2472 ohm" emitter resistor. So the voltage that appears there will be I * R, or 0.0003*2472, or 741mV. You did it correctly.

[carrejans]

An answer from the master himself. ;-)
Thank you!

[carrejans]

- How long do you have to wait? Is it normal that you first read something around 1200mV. You wait 5 minutes; and you read 700mV? (I sometimes have the impression that if you wait long enough it will become zero)

- And if you want a gain of 100, there need to be a difference of 1V between both measurements, I assume?

- A transistor with high leakage. What does it do electronically; and sound-wise?



AC127 TESTING

I just tested some AC127 transistors.
I bought these a while ago from different sellers. Some local old eclectronic hobbyists. (no audio guys)


I changed the polarity of the battery, because they are NPN transistors.


28: too big leakage. switch off => 1200-1400 mV (some of them around 7000mV)

2: switch off => 70mV. switch on => 70mV

4: leakage around 700 (after waiting 5mins) => gain between 50 and 60




AC187 TESTING


I also tested some AC187 transistors.

6: too big leakage (in the volts)

20: gain around 10




Are these results normal? Or just real bad luck? Or do you think I might done something wrong?



Thank you!!

[zombiwoof]

You wait until the reading stops going down and stabilizes.  Germaniums, as someone said previously, are very sensitive to heat from handling them with your fingers.  Put the transistor in there, and do something else until it stabilizes, if you want a correct reading.  I have had to wait 10 minutes sometimes.  I have tried using tweezers to handle them, but it is too difficult for me, I just put them in with my fingers as quickly as possible, and wait.

Al

[R.G.]

Insulate your fingers. Wear gloves.

[zombiwoof]

Insulate your fingers. Wear gloves.

Yeah, I saw that in your other post, good advice.  I just don't have any gloves, so I've had to go au naturel (in the case of my fingers, that is...).  I'll have to get some.

Al

[R.G.]

Insulate your fingers. Wear gloves.

Yeah, I saw that in your other post, good advice.  I just don't have any gloves, so I've had to go au naturel (in the case of my fingers, that is...).  I'll have to get some.

Band aids. Paper towel. Kitchen towels. Handkerchiefs. Potholders. Layers of newspaper... doesn't have to be bought gloves, just stuff that doesn't conduct heat.