Such are the breadth of functions delivered by integrated circuits, it’s now rare to see a simple small-signal transistor project on these pages. But if you delve back into the roots of solid state electronics you’ll find a host of clever ways to get the most from the most basic of active parts.\
Everyone was familiar with their part numbers and characteristics, and if you were an electronics enthusiast in Europe it’s likely there was one part above all others that made its way onto your bench. [ElectronicsNotes] takes a look at the OC71, probably the most common PNP germanium transistor on the side of the Atlantic this is being written on.
When this device was launched in 1953 the transistor itself had only been invented a few years earlier, so while its relatively modest specs look pedestrian by today’s standards they represented a leap ahead in performance at the time. He touches on the thermal runaway which could affect germanium devices, and talks about the use of black silicone filling to reduce light sensitivity.
The OC71 was old hat by the 1970s, but electronics books of the era hadn’t caught up. Thus many engineers born long after the device’s heyday retain a soft spot for it. We recently even featured a teardown of a dead one.
I remember removing paint from this transistor to use it as a photo-diode.
And I just learned that this is a myth :D
I was just a kid… what do I know.
It’s not a myth, I did it too and there were more than a couple of magazine projects that used ‘scraped’ OC71s.
I even remember seeing construction projects that suggested you could alter the sensitivity by scraping more or less of the paint off or make the response more selective by using different colour gels/paints/nail varnish (usually with an ‘of the era’ sexist comment along the lines of ‘don’t let the little lady know’)
in my experience the little lady is always eager to give up a little enamel for the workbench :)
Not so strange, you reminded me that my first digital camera used a delidded DRAM chip as sensor :)
and I did it as a kid, so not a myth..
And me. According to the article only later ones were filled with black stuff, supposedly to avoid problems with the paint flaking off in service. But the fact that they sold a special OCP71 optical one at a higher price might have something to do with it!
They filled the inside of the case with silicone grease for better thermal transfer from the die to the case because beyond 70°C they exhibited runaway leakage, gain loss, or thermal runaway.
What’s inside a OC71 transistor from 1957 (PWJ277)
https://www.youtube.com/watch?v=L7GaFaqFGR8
Internal diagram – a slice of n-type germanium with the indium collector and emitter lead bonding “solder” serving to p-dope their area of contact:
https://www.thevalvepage.com/trans/manufac/p1_fig1.gif
The first transistors I bought were ‘Red Spot’ reject AF devices from Henry’s Radio in Edgware Road, London. 10 shillings each, IIRC. I think they were by AEI.
10 shillings !!
You were robbed, I got them way cheaper. That was about 1960.
Henry’s Radio, now there’s a blast from the past, I believe they’re still in business as Henry’s Electronics and much changed.
So many electronics shops lost to history (and it was happening pre WWW as well), so many memories,
Henry’s was but a dream to me. I was too young to go there on my own. But, I was euphoric to find that an electronic components shop was hidden away behind a newsagents shop really near my school. It was called Chromasonic Electronics, and was accessed by walking through the newsagents, which I think was called ‘Raymond’, in Muswell Hill Broadway. It had previously been in Fortis Green Road, but the newsagent moved, and the Aladdin’s cave component emporium moved with it! It was about 50 years ago, so might have been the other way around! Wasn’t a problem – I could still walk there after school.
I went there at least once a week. And bought many OC71s and LEDs to make flashers. I still recall the feeling of profound joy when I had saved up enough money to buy myself a ’17” Veroboard’. I probably still have some remnants of it in my boxes.
I was introduced to the OC71 by the Ladybird ‘Making a Transistor Radio’ book. I still have the book. And some of the OC71s. Sadly not the Jackson Dilecon variable capacitor!
Such fabulous memories.
Edgware Road, an electronic hobbyist’s paradise when I was young!
Wasn’t this just posted already? https://hackaday.com/2025/10/10/inside-a-germanium-transistor/
It was …
In the 1960s, when I was in elementary school, I had a few OC70s and an OC71. When I made my first flip-flop with them, I thought I had invented it! Funny, but the excitement I felt has not left me yet!
The most valuable piece of equipment was a 0-6 V voltmeter. I still couldn’t afford a soldering iron, so I connected the components by wrapping wires.