[HELP] Checking transistor gain with Geofex method

[Urso]

I'm trying to check the gain of some transistors I have with the methode shown at Geofex: http://www.geofex.com/Article_Folders/ffselect.htm

My problem is that the voltage displayed on my DMM is always increasing, and it takes a while to get stable values. The problem occurs with both the switch on or off.

To measure the gain, I made this PCB:


and here is the layout:

R1 = 2.4k
R2 = 75 Ohms
R3 = 2.2M
I found a 2.4k resistor for R1 and a 75 Ohms resistor for R2 so that their sum (R1+R2) is equal to 2.472k.

I use a Visual Sound One Spot power supply instead of a 9V battery.

Here is how I check the gain and leakage:
I measure the voltage accross the 2.472 resistor (R1+R2). First, I put the switch off, and I check the voltage in Volts. I call this value Voff. Then, I put the switch on, and I check the voltage in Volts. I call this value Von. So the formulas are:
- gain = (Von - Voff) * 100
- leakage (µA) = (Voff / 2472) * 1000000

Thanks

[Urso]

bump

[Urso]

No one?

[R.G.]

If you're testing germanium, the germanium increases due to heating. One way this happens is if you touch the case of the transistor with your fingers while you put it into the socket. For some devices, even breathing your warm breath on them will cause the leakage reading to increase. It can even happen due to self heating from the leakage current and the voltage across the device.

If it never stabilizes, it's a bad transistor. If it takes a while, it's fairly normal, perhaps high leakage.

[Urso]

Thanks

Yeah, heating problems, that's what I was thinking of. I've tried to put them into the socket using pliers and not my fingers, and to breath away (), and I don't have the problem anymore with NPN silicon (I've tested on 2N5088s). But the problem is still here with PNP germaniums.

The problem occurs with 2 OC44 from Small Bear that I tested on a treble booster and that sound good, and with 1 AC122 that I also tested and that sounds good. The values are always increasing.

What about using a diode connecting the emitter to the base to stabilize the temperature? like in those articles:
http://fuzzcentral.tripod.com/rangemaster.html
http://www.geocities.com/SunsetStrip/Studio/2987/britface.html

[jlullo]

i was searching in the archives for a little more visual information on checking Ge trannys for hfe and leakage, and i must say that this is 100% what i was looking for.  If i want to replace R1 and R2 with just a 2.49k resistor, could i just leave R1 empty and put it in R2?

[GibsonGM]

In my 3 second look at the board you've set up, yes, but you'd have to jumper from the bottom of your resistor across where R1 is to preserve continuity.  In that spot, I use something like a 2.2K resistor that measures to like 2.14K, and a small pot in series with it to dial in the exact 2.47K.  It comes close. 

I give the Ge's about 5 minutes to settle down from temperature when I test them.  You don't have to be insanely accurate with your components, as close as you can tho.  This will get you in the ballpark ;o)

[rasco22862]

I did it with two AC188. The Voff value is greater than the Von value, so i cant do the operation.

Anyway one got 6, and the other has 200, is this possible??