"When life gives you lemons, make lemonade" they say. This whole project is about "how did they manage to make lemonade back in the days ?"
To understand this, I explore the old technology and parts "from back then". In this page I try to evaluate the quality and variance of the 70+ OC139 of my stock.
They come from two sources and I was curious : do they come from the same batch ? Have they been already binned ? How much do they differ ? Is the leakage so bad ?
The OC139 seem to be really "vintage", looking at the oxidation of the leads. Date code seems to be around 1961.
Measuring the leakage is a pretty big deal because it badly affects the gain displayed by simple hFEmeters. Look at http://www.geofex.com/Article_Folders/ffselect.htm for the extended explanations.
I didn't use my multimeter this time, but a handy little Chinese-made tool that guesses the type of tripole and its characteristics, which I wrote below.
I bought this device at a local store with the usual markup (welcome to France) but I needed it to test my old capacitors and it has other useful features so I didn't care that much. And it was sold by an old, reputable store :-)
The gain itself is not a big issue, as long as it's about > 20 and there is no point if it is >40 for this specific application. It's also good to see which parts are real lemons and should not be used in a circuit. For example, too high a leakage will disrupt my circuits. We have totally forgotten to take leakage into account in this century !
Measuring these parameters is not an exact science since germanium is very sensitive to heat (even from the fingers) and even to light (the black paint can't stop all the light if it's intense enough) so sometimes I had to make several measurements.
|hFE||Vbe (mV)||leak (µA)|
Verdict : one totally rotten lemon, several in bad shape, most are bitter but usable, but some have interesting characteristics and demand more examinations. Is their high gain real or an artefact ? and if they really have such a high gain, can they be used for more noble purposes than digital switching ?
I also wonder about those parts with Vbe < 200mV.
I didn't/couldn't check for the gain-bandwidth product (or rise and fall time). That should be for another time and for the high frequency dividers :-D
I guess about half of them would be used for the intended circuit (the shift registers, I need gains between 20 and 40 and no significant leakage). I might have to get more of them to complete the circuit because there are quite a lot of stages... but they have become so expensive !
Next step would be to plot the data in a graph and re-bin the stock, according to the various groups I identify in the dot cloud.
And yes, germanium is fun but silicon is so much better :-D I measure so little relative variations in new parts, even cheap ones...