RG Keen Transistor Test - an idiots' guide

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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johnnyg
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Post by johnnyg »

Notice I put idiots plural - not "idiot's"... as I hope I'm not alone :block: But I don't think there is a correct simplistic explanation of how to do this here or elsewhere? So I thought it might help others if I put one up - it would have helped me. Here's the infamous article kindly provided by RG Keen http://www.geofex.com/Article_Folders/ffselect.htm But it's a bit obfuscatory to the average homebrew tinkerer I think. Really I'm just spelling out what KindaFuzzy has already said viewtopic.php?f=10&t=8411&p=89198&hilit ... est#p89198 ...but I don't think that's a bad thing (if the mods do - feel free to flush this post down the FSB pan :D ).

The easiest way to do this, I believe, is to use a breadboard. You need a fresh 9v battery, a 2.2m resistor, and ideally a a 2.472k resistor... but I wouldn't drive yourself mad aiming for ideals (especially when dealing with germanium transistors). If you can get 2.47k that's good enough I think - I used a 2.2k and a 270r resistor in series to get 2.47k exact. You want to set things up on your breadboard along these lines (n.b. we are dealing with PNP transistors here - alter things to suit NPN if you need):
Transistor Test.jpg
You don't need to worry about adding a switch using a breadboard - you can just lift one end of the 2.2m resistor in and out of the board. So with your breadboard all set and your bag of ageing germaniums of mysterious worth you've paid too much money for (are they good or have you got the leftovers some scoundrel has already passed on! :hmmm: ), here we go.

1. Measure leakage: With one end of the 2.2m resistor disconnected and your multimeter set to 20v or 2v DC let the transistor stabilize* before noting down the reading you get. When you feel the transistor has stabilized and you've got a reading you think reasonably accurate compare it with the following guide:

0.137v = 50 uA / 0.05 mA
0.274v = 100 uA / 0.10 mA
0.548v = 200 uA / 0.20 mA
0.822v = 300 uA / 0.30 mA
1.096v = 400 uA / 0.40 mA
1.37v = 500 uA / 0.50 mA

N.B. uA = microamps and mA = milliamps (1000 uA = 1 mA).

E.g. A reading of 0.115v = a leakage of < 0.05mA

According to RG Keen transistors that are <0.30 mA are The Good; those that hit or exceed 0.50 mA are The Bad; those that greatly exceed 0.50 mA would be The Ugly I think :D .

2. Measure the nominal Hfe: connect the 2.2m resistor and, when you're happy the transistor has stabilized, note down the reading you now get and multiply this by 100. E.g. 1.89v = 189 Hfe.

3. Now the correct Hfe: subtract the reading for leakage from the reading for nominal hfe and multiply the result by 100. E.g. 1.89v - 0.115v = 1.775. 1.775 x 100 = 177.5 Hfe.

And that's it I believe. The example I've used above was a 2n404 - a 70s Germanium I believe when the technology involved in their manufacture was pretty advanced. Earlier transistors (the fabled OC range for instance) are likely to have greater leakage and often lower gain.

*Quite possibly you'll have to wait a while for the transistor to stabilize (especially if you've handled the transistor in question more than a few seconds). Some transistors seem to stabilize quicker than others - some I've had to leave for nearly 5 mins to settle down... meaning if you've got a lot of transistors to test, it'll take you a while. The differences between the early readings you get and the stabilized reading are great - leakage can initially appear to be very high and your transistor useless. 5 minutes later and it's dropped to a nice 0.40 mA or so. I hate to think of all the useful transistors that might have been binned by the hasty :D
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Post by JOHNO »

Many thank's for this info. Ive alway's had trouble with this too. Never really knew what DC setting to run the meter on either. This couldnt have come at a better time for me because on the week end my dad gave me an old australian made Ferris radio and i take the back off it and find a bunch of Mullard transistors in it :shock: Ac 128,127,172,125 and a AF117n but im not too sure how useful that one will be. Measuring them as i speak. Thank's alot Johnny :applause:
Johno

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Post by Greenmachine »

Thanks for taking the time to do this; I'll be using it.

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Post by dai h. »

nice post. I have a small tip. Don't be stupid like me :oops: and use a momentary switch (because as noted above, yes, it can take time for the measurement to settle). I ended up using the test described at Small Bear (but I think it's the same thing). Also, to conserve batteries and help provide a more consistent reading(batteries wear out and drop voltage), I made a compact -9VDC supply powered off the AC supply for my Marshall SE100 spk. emulator(something very ordinary and just a couple of parts, diode, regulator, couple of caps, connectors). I built it on a small ready made perf board with a power jack and 9V clip sticking out of it, and for the "housing" I used a piece of clear plastic from some packaging and stapled it together. You have to careful to not step on it, but it works okay. And also, I used cut down 8pin DIP sockets meant for wire wrapping for the Tr sockets which helped since they have long leads. Another thing I didn't try (that I thought might be useful in hindsight) was using SIP sockets(snipped) as "socket savers"(plugs for tubes meant to save wear on a tube tester tube socket) because the contacts can wear out after many repeated insertion/removals. It can take a while to measure a bunch but the GEO info is very solid and the measured/sorted ones I plugged into a FF circuit work great.

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Post by johnnyg »

I just realised an incredibly stupid mistake in my post :blackeye :block: - as I was typing in a rush without my notes (but with a beer :P ) I lifted my voltage/leakage guide from KindaFuzzy's post, and looking again there's a slight error! 274 mV was used instead of 247mV as representing 100 uA of leakage :slap:

The voltage to leakage table should be:

2.472v = 1mA
1.236v = 500 uA
0.989v = 400 uA
0.741v = 300 uA
0.494v = 200 uA
0.247v = 100 uA
0.124v = 50uA

Would it be possible for a mod (or rocker) to edit the original post please and take out the wrong figures??

If anyone's already used the wrong figures (Johno & Greenmachine perhaps!) sorry about that! But the mistake is small enough to mean that you don't really need to re-test your transistors. You'll still be in the right area for leakage - you'll get different figures on different days anyway due to the grouchy nature of germanium transistors!
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Post by JoxTraex »

Thank you for posting how to test transistors, its not exactly easy to test one.. Or people just can't explain it well. You did an excellent job! For those who need a clarification after looking at RG's post at http://www.geofex.com/Article_Folders/ffselect.htm, the 2.2m resistor is in fact 2.2M ohms.

Thanks,
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Post by bhill »

So I presume that for npn germaniums it is just reverse the battery and meter connections? Damn, the highest gain ones so far have stupid amounts of leakage :(

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Post by DrNomis »

bhill wrote:So I presume that for npn germaniums it is just reverse the battery and meter connections? Damn, the highest gain ones so far have stupid amounts of leakage :(


That's correct, in the case of NPN transistors, the Emitter should be pointing towards a negative circuit ground.... :)
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Post by exactopposite »

Thanks for taking the time to do this. Having that list of leakage #'s saved me a lot of calculating

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Post by Frostbite Slim »

Thanks SO much for this. I swear I nearly posted asking about the same thing!

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Post by johnnyg »

Nice to hear that post of mine from a couple of years back is still proving of use :D ... maybe it was verbose (too many of my posts are/have been I know :oops: )...

Perhaps worth adding I sold out and bought a Peal Atlas DCA last year - and I was pleasantly surprised to find that 9 times out of 10 the simplified RG Keen method I described was spot on. Leakage and Hfe measured near enough the same.

The odd miscreant in most cases was probably to be explained by the the time of year the measurements were taken.

For example, a Bell Labs AC125 I originally measured on a sunny, warm June day using the RG Keen method measured cira 90 Hfe and around 0.50ma leakage... at the time I thought that meant it wasn't too useful (but see my final point).

With the Peak measured on a winter's eve it measured a very nice 70 odd Hfe with less than 0.30ma leakage :? . (So great for a Tone Bender build for instance... that would prob sound best in the Winter :block: ).

And the final point worth mentioning, with all greatest respect to RG (a if not the doyen of stompboxes and DIY), I think his suggestions about good and bad leakage can be ignored for some effects at least. Sometimes transistors with 0.40-0.50 ma leakage can sound a lot better than lower leakage transistors in Fuzz Faces and Tone Benders :thumbsup
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Post by bhill »

I meant to post this over a year ago, but somehow other things kept pushing it back, but seeing this thread pop up again reminded me (actually kicked me in the ass to get it done :D )

This quicky tester came about one afternoon when I was about halfway through testing a bag of 100 AC128's. Looked down at the number left to do, looked at the breadboard and the amount of time I was spending inserting transistor, plugging in power, let stabilize, take reading, jumpering in the 2.2M resistor, take another reading, rinse and repeat etc. etc. Then looked at the other bag on the table of 100 AC176's and threw this together in about half an hour.

2.2M measured as close to 2.2M as my meter would show (Fluke 8060A), trim pot is a 5k set to 2.47k, pushbutton, socket, toggle and battery snap were from the parts stash and the mounting posts came from Rat Shack. It has been well worth the very small effort to draw the circuit (with sharpie) etch it and mount everything up.
trantest02.jpg
trantest03.jpg
Meter leads are reversed because I was switching things from PNP to NPN configuration when the pics were taken. Battery snap hasn't been reversed, it is in the PNP position. I found, and have carefully reserved for this, a 9v battery that measures 9.01 volts.

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Post by eastbayj »

I think I may need a super idiot guide. Actually, if I show my work, I'm hoping someone would be willing to let me know if I'm performing the test correctly. I am almost certain I have the circuit set up correctly for testing an NPN transistor, in this case, an AC187, but a shot of my tester is attached (how the heck do you insert a picture into your post?). The metal film is almost exactly 2.2M and I have two carbon film resistors in series (.981 and 1.491) to make a 2.472 k resistor.

With my meter set for 2V, my first reading (switch off) is .184 V. Based on the above (corrected) table I did the following ratio calculation:

1.236/500 = .184/74.43

This tells me my leakage is a little more than 74 uA, correct? No worries about this leakage, the device is good, correct?

My second reading, once I flip the switch, is .298 V. I subtract the leakage from the gain:

.298 - .184 = .114

.114 X 100 = 11.4

That indicates a gain of 11.4 Hfe. No good for a Fuzz Face, in spite of not leaking too much, but is that the correct procedure?
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my test setup
my test setup

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Post by johnnyg »

From what I can see, you have set up the circuit correct for npn. Just make sure you have the croc clips biting the correct transistor legs, I can't tell from your photo if you have. Normally a painted dot indicates the collector, occasionally for npn devices it can indicate the emitter... confusing. Try to find a data sheet to check if you can.... (this is where the peak is great, it recognises what each leg is automatically. Actually I had one or two transistors I thought junk, but the factory had painted the dot on the wrong side :shock: . Only with the peak did I find they were good to go).

Also make sure none of your croc clips or transistor legs are in contact with any others.

If all's good your calculations are correct:

.184v = <100ua of leakage (you don't need exact figures to decide if leakage is a problem or not, just an approx idea)

.298v for nominal gain suggests a junk transistor... so double check what I wrote above... If it is correct, then your calculation looks right. This transistor is of little use I'm afraid. (Maybe use as a ge diode in a tone bender mmk iii or buzzaround etc).
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Post by eastbayj »

Yes! It's so nice to finally have this figured out. I've been working on this for a while and your post and help were just what I needed.

I got my pinout info here:

http://english.electronica-pt.com/db/cr ... ?ref=AC187

I got fifteen of these off ebay a while back and I have a feeling none of them are any good, regardless (or perhaps because) of the fact the seller wrote "tested okay" on the packaging.

That Peak Atlas DCA55 looks awesome. Definitely takes the guess work out of testing these germanium rascals.

Thanks for the super fast response! Cheers!
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Tested okay?  Great!
Tested okay? Great!

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Post by eastbayj »

Turns out I did have the clips on the wrong pins! Doh! I double checked the pinout, hooked up the tester correctly and found more believable gains. I also found that all fifteen of these suckers have leakage ranging from 1000+ to 1700+. Now I'm wondering what exactly these things "tested okay" for. Landfill?

Fortunately, after testing a bunch of other transistors I have (MP35, 404B, AC141), I found I do have some that are in the correct gain range with low enough leakage to be usable.

Thanks again, johnnyg!

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