Transistors

[Refugee]

I can remember the funny story about old Mullard transistors my father told me.
They had built one of the very first plastic bag heat sealing machines with a new safety feature to stop anyone putting there hand in.
When they put all the covers on to show it to a customer the new fangled magic eye did not work any more as it had all been set up in the light.
The sales rep ended up with his hand sealed into a plastic bag.

[Mark Hennessy]

There's also an alternative, and rather more plausible version of the story:

From http://www.markhennessy.co.uk/articles/v...lass_types

Also, should the paint flake off, the transistor will behave to some extent as a phototransistor - plenty of unsuspecting engineers wasted hours searching for the source of mysterious humming before realising that their bench lights were the cause! Incidentally, there was an official phototransistor available - the OCP71 - which was filled with a clear silicon grease to allow maximum light transfer. The regular transistors were treated to a more opaque filling (as seen above) - apparently, if you scrape off the paint from a transistor to find a clear filling, it was probably a rejected phototransistor that was tested to see if it could be sold as a regular OC-seried transistor (reference).

Forgive the self-quoting, but the link referenced is a good one

[Terry]

Mark, the plastic sleeved version you show in this picture:



only appeared very late on in the life of the OCxx series.

Strangely, I don't ever recall us stocking replacements other than the original painted ones!

I can't imagine the plastic sleeved ones only being supplied to manufacturers, though.

[Nowhere-Man]

I read the reviews on Amazon for Art Of Electronics and reviews were good. When I started, the first book I read was Chas Miller's Valve Radio and Audio book and then some Owen Bishop books.
Speaking generally on application vs theory, I've encountered cases of people who are highly theoretical but at a big disadvantage in the environment. Where I live, all the money sadly is in welding for men and maybe banking for women. I know someone personally who did a degree in TV engineering and he says he's struggled to profit from it in this area. I realised gradually I needed to learn more pure survival skills so I learned basic woodwork, engine wiring, mechanica,l AC boat installation, painting and fibreglass moulding. Even then, the main advantage has been self-sufficiency. I may knock up a shed for my radio workshop as mentioned.
The plan remains to phase back to practical radio work as the weather turns chronic. I hope by then my fibreglass boat will be finished. Radio wise, I still have repairs to complete and I will see if I can construct something. The typical sixties vintage design is the wooden box with screwed in chassis (which is +). I have one 1965 tranny I repaired ages ago and it has an earth and aerial wire and no tuning indicator.

[Craig]

Interesting page Mark. Regarding the lockfit noise problem, I have had similar problems including total failure of TO106 encapsulated silicon transistors used in 1960's Tektronix gear. I put it down to problems with early epoxy encapsulation, which over time creeps sufficiently to put strain on the device and bonds between the device and legs. Similar to the epoxy potting of HT transformers. Tek used to wax pot them, which worked perfectly. Then they went to epoxy for just a few years (in the late 60's/ear;y 70's), until they found it went lossy over a period of time and the 30kHz oscillator no longer oscillated. Then they went to silicone potting in about '73, which again works perfectly.

Craig

[Mark Hennessy]

Yes, it took them a while to sort out plastic encapsulations. I'm looking at a HP334A at the moment (thanks, Andy ) that uses a mix of metal and plastic, and most of the plastic types have hairy legs! I'm not sure what exactly I'm seeing - I'll get the USB microscope out when I have more time - but so far, they all seem to be working OK, so it's hopefully something benign that hasn't made its way into the plastic yet.

Also, I'm looking at a HP1740A with numerous faults - all caused by 'jellybean' 2N3904/6 types. No visible signs of distress, and no obvious electrical reasons for their demise. This 'scope is mid/late '70s, so you'd think we'd be past the teething troubles by now. Similarly, the MPS6518 - as used by Telequipment in the '70s - seems to fail for no reason too.

The Lockfit types did seem to get more reliable as time went by. Ignoring the high-powered (relatively) ones used for audio outputs, I don't recall failures in them post the mid-'70s. But it seems that as mass-production geared up to deal with TO92s on a bandoleer, there was no need for the Lockfit package any more.

Overall, "assume nothing" has to be the mantra... Luckily, it's pretty easy to test most transistors in-situ by observing what is going on around them - or at least it is with experience and understanding. There's no better transistor tester than the circuit it's installed in

[Nowhere-Man]

Just to clarify this "holes" business. Electrons in semiconductors form part of an atom. They orbit a nucleus. I visualised a row of small groups of marbles, each with a nucleus. Imagine only the green marbles are electrons and form a line of the bunches with + one end and - the other end. You can say group A has a central marble with some normal and a green marble placed around in orbit. Next to it group B and C and so on. When voltage is supplied the first green marble of bunch A jumps into the + wire and leaves a hole or empty space in the group. No problem as the green marble in group B hops into the empty space in group A. That leaves a hole in group B so a green marble from group C fills that vacancy. So, the holes move in one direction as the electrons move in the other. The holes don't really exist but they do set up a + polarity opposite to -. Hence bipolar transistors.
The point I ought to make is transition from valve theory to transistors isn't a time consuming step. Based on my own experience to date I recommend:
(1) Learn briefly how electrons jump in atoms and how the holes appear to flow.
(2) Learn about PNP sandwiches or layers. Google potential hills - Owen Bishop explained this in 3 pages.
(3) Beta is pretty much similar to amplification factor in valves.
The holes thing is important to understand and visualisation for me was the only way. If the TV engineers of past years struggled then the use of marbles or bunches of apples and oranges is helpful.
The good news is valve people only really need to tackle the BJT.

[Terry]

Just to clarify this "holes" business ...

You seem to overlook the fact that electron flow in electrical and electronic circuits had been well understood for a very long time before  the invention and development of the transistor but nobody ever mentioned anything about 'holes' ...

Yet you seem to be saying that you can't understand how an electric current flows through any circuit without expressing it in terms of holes ...

Forget all about your multi-coloured marbles and just concentrate on the electrons that are free to move. The simplest way is with a row of 2p pieces - it was old ha'pennies when I saw this done!
Get two rulers and place them either side of the row of coins, arranged at right angles to the edge of a desk and with the first coin 2" or a little less from the edge.

Place another coin on the edge so that it is overhanging - like shove ha'penny - then hit it sharply with the heel of the hand. The coin will hit the first one in the row and a coin at the other end pops out.

Voilà! A practical demonstration of electron flow and not a hole in sight!

[Mark Hennessy]

OK, so after that summary, no more posts about holes and electrons, please. We're done with semiconductor theory. Application and use of transistors only from now on.

[Kalee20]

(3) Beta is pretty much similar to amplification factor in valves.

Sorry, no it's not! Amplification factor in valves is a well-defined characteristic relating to the effectiveness of the anode and the grid at controlling anode current. Beta is a highly variable characteristic comparing collector current to base current.

It would be possible to define amplification factor for a transistor (Early effect would play a part), but not beta for a valve.