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Wednesday, 8 July 2015

0603 Resistor Codes

I was looking at a Printed Circuit Board populated with 0603 size surface mount resistors and puzzling over the markings. In particular R45 and R56 in the photo below. They are marked 18C, but the actual value is 15k.
 Now in the old days resistors were colour coded, and we all knew the code off by heart, Black = zero, brown = 1, red = 2 etc. There were usually three colour bands - 1st digit, 2nd digit and a multiplier which represented the number of noughts. So 4.7k was yellow, violet, red, for example. There might then be a gold band for 5% tolerance, or brown for 1%. Even when "E96" values came in, and many more "preferred" values were needed, it was just a case of adding another colour band to make three digits and a multiplier. This system was carried over onto surface-mount parts, but in numerical form. You can see some examples below - there is a 473 (47k) and a 223 (22k). But there is also 01C  and 30C which don't make much sense.
The answer to this is, of course printed in the data sheet for the resistors. It is is a way to address the problem of marking E96 values - presumably 0603 size parts are deemed too small to put four digits on. 

Perhaps an aside about "preferred values" might be appropriate here (although there is a very good Wikipedia Article about it). When a resistor is manufactured there is a tolerance in the value, perhaps +/- 10%, or 5% or 1%. So a 1000 Ohm resistor might be, say, 5% high (1050 Ohms). So there is no point in making a 1050 Ohm resistor because the value would overlap with 1000 Ohm. The next "preferred value" is 1100, because a 1100 resistor which was 5% low would measure 1050. For 5% tolerance resistors there are 24 non-overlapping values in each decade, and the sequence of values is called the E24 series. 10k, 11k, 12k, 13k, 15k, 16k, 18k, 20k, 22k, 24k, 27k, 30k, 33k, 36k, 39k, 43k, 47k, 51k, 56k, 62k, 68k, 75k, 82k, 91k, For 1% values there is an E96 sequence which requires three digits to represent each value. You can see this sequence in the table below which gives the manufacturer's 0603 resistor code on the left and the E96 value on the right. The Letter is the multiplier code.
So "18C" is indeed 150 x 100 = 15k Ohms. 30C is 20k Ohms, and 01C is 10k. All has become clear.

Now to print out this table and pin it up at my work-bench!

Monday, 6 July 2015

Fully Licensed Wireless Receiving Station

You will no doubt be greatly relieved to know that my two Great Aunts, Ethel and Nora, who lived in Norwood, South London, and who cached all this Crystal Set paraphernalia, were fully licensed. If you wanted to set up even a simple radio receiver to listen to the BBC in the 1920's you had to hold a valid licence. The annual charge for this was 10 Shillings (or 50p in new money). Although this sounds a trivial sum by today's standards, 10/- was quite a lot of money then, not just a coin, but a bank note.
 The paper these documents are printed on is very transparent - almost like tracing paper (or Izal toilet paper!). The conditions are printed on the back, and some of them read in much the same way as the UK Transmitting licence. This licence ran for a year from November 1923 to October 1924.
The design seems to have changed for subsequent years, here is one for 1928 - 9.

If you want to read more about the UK Radio Receiving licence, there is a very good website here: The History of the UK Radio Licence

Sunday, 5 July 2015

Crystal Set

The last of my cuttings gives details for building a crystal set. There were many different designs, mostly trying to improve the "Q" of the coil and so the selectivity. Any of you who have tried to build one of these things, hearing the radio stations is not usually the problem - it is hearing them one at a time. But here the listener is simply trying to get the most volume, and a very simple circuit is described - presumably sacrificing selectivity for output.

Saturday, 4 July 2015

Adding a Thermionic Valve to your Crystal Set

Today's radio cutting describes how to add a thermionic valve (tube) to a crystal set as a radio frequency amplifier.
It is very difficult to describe a circuit in words, and a shame the editor didn't allow a circuit to be printed. However, trying to understand the description as best I can, I have come up with the circuit below ...

I think ATI stands for Aerial Tuning Inductor, and together with the variable capacitor (condensor) which is mentioned, these provide the means of tuning the modified set. (Some values for these components might have been useful!) I assume that a triode valve, with a filament is envisaged (we are talking 1920's - so indirectly heated cathodes were not in use). Again no type number or Voltages for the HT (High Tension) or heater (Low Tension) batteries are given.

It also troubles me that the crystal set is connected into the anode, or plate circuit of the valve - directly connected to the high tension terminal of battery, which might be, say 120 Volts. This could have been an electrifying experience for the listener wearing the headphones if they had come into contact with the terminals or uninsulated wires attached to the 'phones.

An inventive person, with a good understanding of electronics might have made this work, but they probably wouldn't have needed the wordy description in the "Wireless Hints" column!

Friday, 3 July 2015

Choosing the right sort of wire

Today's snippet from "Better Radio Results" describes the different sorts of wire which may be employed with a wireless set ...
"Wireless wire" seems a bit of an oxymoron, but I'm sure we know what he means .

Thursday, 2 July 2015

Extending the tuning range of a radio receiver.

I'm sure we have all wished our radios would tune just a little bit further up the band. Well who would have thought it was so simple ... just put a small capacitor in series with the antenna lead!

Somehow I feel this might not work with modern synthesized radios. Frankly I am a little doubtful that it can even have worked with steam-powered radios, but a simple crystal set may have been detuned a little by the addition of more capacitance in the aerial circuit.


Wednesday, 1 July 2015

Rejuvenating your crystals

During the 1920's, the "Daily News and Westminster Gazette" had a regular column called "Better Radio Results". I have a few cuttings of this column, and since we are all looking for better radio results I thought I might share them.

The first is for the benefit of those employing galena crystals in their crystal set, it describes a way of rejuvenating these crystals. You will be aware that many people relied on entirely passive radio receivers which employed a point-contact semiconductor diode as a.m. detector. A thin wire, known as a "Cat's whisker" was used to probe the surface of a crystal, such as galena, while listening through headphones. Natural impurities on the surface of the crystal could form a semiconductor junction to rectify, and thus demodulate the radio signals. Evidently the galena became tarnished over time and it became harder to find the "sweet spot". Here is a possible solution ...

Hopefully it will make your "cat's whisker" the "bees knees".


P.S. Unfortunately  these cuttings are undated, however this one must be after Aug 1927, because that is when the BBC Wireless Military Band was formed.