This bit of the website has a few items you might want to build.  You'll need basic electronics construction tools and a bit of practice to build them. 

First of all here is a useful piece of test equipment that I designed many years ago.  You can use almost any audio amplifier IC with this circuit (with appropriate pin changes).

AF and RF Signal Tracer

This is a simple piece of test equipment,  which will make fault finding much easier.  It allows you to trace AF and RF signals through the circuit that you are working on,  and with a bit of luck the point where they disappear  - there by finding the fault ! 


No special techniques are required  to build this.  I put it together on a piece of Veroboard,  which for me was the cheapest and simplest method.  If you have the time and the tools you could make a PCB to build it on.  Tag strips and other "lash up" construction methods should do quite well too.  All the components are readily available from Maplin Electronics in the UK,  or Farnell.  You should have no problems obtaining these components from anywhere in Europe.  More than likely there will be enough bits in the junk box to make this - that's what I did.  None of the components are critical,  you can substitute any audio amplifier IC you've got handy.  I chose the LM380N because it's reliable, cheap and simple it runs happily at 9V with low current consumption.  Other amps such as the TBA820M will work equally as well.  The detector or demodulator diode should be a good quality germanium type,  OA79 or similar.  Don't be tempted to use a silicon diode  - it won't work ! 

I housed my final version inside a small die cast aluminium box,  mainly because I had one available. This should work well in a plastic container too,  as interference is not really an issue with this circuit.  Use your multimeter test lead as a probe.

Components List

Resistors (0.25W Carbon)
R1 22K
R2 4.7K
R3 3R3  (3.3 Ohms)
VR1 22K Linear with switch

C1 1nF Ceramic Disc  (300V)
C2 0.1uF  Ceramic Disc (300V)
C3 10uF 25V Electrolytic
C4 68nF  Polyester / Milar
C5 0.1 uF Polyester
C6 470 uF 25V Electrolytic

D1 OA79 germanium signal diode
IC1 LM380N

Miniature L.S. 8 Ohm 300 mW
Veroboard,  connecting wire,
9V PP3 battery, probe,
suitable enclosure.

I suggest that you follow the circuit diagram layout to build this  on Veroboard.  The layout is not critical,  but do make sure you get the diode and electrolytic cap's in the correct way around.  Also remember to cut the tracks in-between the IC pins (unless they are supposed to be shorted). 

C1 and C2 are specified as high voltage components so that you can use this to test  valve set's.  If you are only using this to test transistor radio's, then low voltage components may be used.

Operation is straight forward.  With the switch set to RF,  the signal is 'detected' or demodulated by diode D1.  DC is 'stopped' getting into the diode by coupling capacitor C1.  The detector 'extracts' the audio information from the RF signal,  and passes the audio frequency information to the amplifier,  which of cause amplifies the signal to a level to drive the speaker.  Use this setting for checking out RF and IF stages in your radio's

When AF is selected,  the detector stage is bypassed,  and the audio signal is fed directly to the amplifier by capacitor C2.  This again removes any DC from the input signal.  Use this to check out audio amplifiers - you should be able to tell with this where the signal disappears,  which usually tells you where the fault is.

No claims for originality are made with this circuit,  it's just made from
common building blocks,  but it does work well,  and will save lots of time fault finding.  Any comments or suggestions are most welcome.

Many thanks to John Panton for the picture of his signal tracer and for providing the pcb layouts.

A Simple 3 Valve EF91 radio
A long time ago in a shed far far away I built my first ever valve radio.   I was 12 years old at the time and to my parents dismay I decided to move on from OC44 transistors to those high voltage warm things called valves.   This was published in the December 1962 edition of Practical Wireless.  After my Dad had sorted out the spaghetti wiring system (apparently there were no mistakes,  just too much wire!) it worked well and picked up plenty of stations.  At the time I experimented with different tuning coils and by decreasing the number of turns on the coil you could make it work on short wave frequencies. The design uses the EF91 valve for all stages,  this being a miniature B7G type.  You can use them for almost any stage in a TRF or Superhet receiver,  being one of the most flexible valves ever made.   The other good thing about them is that they are cheap!

So if I could do it at 12 I'm sure you stand a good chance.   We can supply any bits you might need although most of them should be easy to rob from scrap radios.     Click here to download a copy of the original article.  You'll need adobe acrobat reader V5 or above to open this file.  Good luck,  and do let me know how you get on. 

In the future when I locate the article there is a 5 valve EF91 Superhet to build.

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