I thought I'd start this month with a little quiz to keep you on your toes. Answers should be written on a £5 note and sent to the editor.

Q1. What device can you use to check out a jack to jack lead for continuity and short circuits?

Q2. What would you use to check if a fuse is blown?

Q3. What would you use to checkout any cable, speaker, microphone or switch?

Q4. What is pocket sized, robust and cheap to build?

The answer to all these questions is, of course, this month's project. With my usual flair I have called it the CHECKA.

Now I accept that despite all its facilities it's not as flexible as a multimeter but in its favour are the points that it is very sturdy, unlike a fragile multimeter, it will carry out all the on-stage checks that a working band is likely to need and it's also a darn sight cheaper!

Parts List

One off DPDT switched stereo jack socket (eg Maplin BW80B)
One off skeleton mono jack socket (eg Maplin HF91Y)
One off PP3 battery clip
One off 0.2 red LED
Two off 0.2 green LED
One off 1.5Kohm ¼ watt resistor
Two off 1Kohm 2 watt resistor
One off plastic case, approx 80cm x 60cm x 40cm
One off jack plug (optional)
Insulated wire, foam

Total cost — slightly less than £3.

The Circuit

The circuit, whilst essentially fairly simple, uses a special switching jack socket to allow it to perform several separate functions without the need for complicated and expensive switches. The jack socket used is the type which is used in hi-fi setups to switch in a dummy load in the speaker lines when the stereo headphones are plugged in. It comprises of a stereo jack socket with a built in Double Pole Double Throw change over switch. The theoretical circuit is shown in Figure One. As the circuit is so simple there is no need for a circuit board; all the components can simply be hooked up with insulated wire. I've gone on about soldering techniques in previous articles so I'll take it as read.

Putting it all together

The first thing to do is to drill out the box (see Figure two). It is essential that a plastic, not metal box is used as the two jack socket mounting bushes must be insulated from each other. The two 9.5mm holes should be drilled with their centres exactly 3cm apart as this spacing will be critical for the 20mm fuse check facility. The interwiring is shown in Figure three. The LED and resistor wiring should be kept as low as possible in the case to allow sufficient room for the battery to sit on top of it, insulated by a piece of foam. There are a few other points to notice:-

1. The stereo jack socket 'ring' contact is not used.

2. The screen/earth tag on the side of the stereo socket is connected to the 'tip' contact, the red battery lead and also switch contact B.

3. The 'flat' side connections on all the LEDs are looped together and connected to the black battery lead.

Testing it out

Firstly carefully check the wiring, particularly that the LEDs are soldered the correct way round and that the connections on the back of the stereo jack socket are correct. If all seems well put a piece of plastic foam over the LEDs, connect the battery to its clip and sit the battery on the foam. Screw on the case lid and look at the LEDs — they should all be out. Using a metal object (eg screwdriver blade) bridge across the two jack socket mounting nuts. If the unit is operating correctly the red LED should light. Next plug a jack plug into the stereo socket (socket two) and repeat the previous test. This time one of the green LEDs (number three) should light and the red LED should stay out. Lastly plug a good guitar lead between the two sockets — both green LEDs (one and three) should light, the red LED should stay out.

If it passes all the tests you can pat yourself on the back 'cos it's working perfectly! If it does strange things then the most likely fault is in the wiring on the back of the stereo socket — it can get very confusing and the use of different coloured insulated wire helps a lot. Finally label LED one — inner, LED two — short circuit/fuse/probes, LED three — outer.

The CHECKA in use

As I said at the beginning the CHECKA can check out quite a few bits and pieces so let's go through a few of the most common testing procedures.

1. Checking a jack to jack lead.

a) Plug one end of the lead into socket one. The red LED lighting indicates a short circuit. In other words the two leads are electrically connected together. Likely causes are insulation breakdown in the cable or jack plug or a whisker of wire in one of the plugs bridging across the two terminals.

b) Plug the other end of the lead into socket two keeping the original plug in socket one. Both the green LEDs should light. LED one lights to confirm that the tip to tip continuity is okay (coaxial core wire is intact). LED three lights to confirm that the screen (outer) connection is intact. Failure of a LED to light shows a break in the appropriate wire.

Similar results will occur for any non coaxial lead (eg speaker lead). NB The CHECKA will not test a stereo jack lead but can only confirm the status of the inner and screen wires. At a pinch continuity of the 'ring' wire can be made using the fuse check below.

2. Checking a Fuse.

The CHECKA will test any fuse including HT, mains, car etc. In fact any fuse between 20mm and 1¼" long. With no jack plugs fitted simply bridge the fuse and caps across the two jack socket mounting nuts. A good fuse is indicated by the red LED lighting.

3. Checking a speaker cab.

Plug in a tested lead into the cab and then, whilst listening closely to the speaker, plug the other end in and out of socket one. A slight crackle from the speaker indicates it is okay. The red LED should also light.

4. Checking a microphone

Hold the capsule end of the microphone to your ear (it's not a joke — honest!) and plug it in and out of socket one. A scratchy, crackling noise indicates the microphone is working.

5. Cable, switch and general continuity testing.

For this you need to wire up the optional jack plug shown in the parts list. Solder two 18" lengths of insulated wire to the jack plug. These should be terminated with a couple of probes. These can either be bought from your local, friendly electronics shop or you can DIY. (I've been DIYing to get that in somewhere after all the DIYverse puns our DIYnamic, but DIYabolical editor keeps DIYving into on the contents page).

A set of cheapo probes (ie they cost nothing!) can be made up using a couple of old felt tip pen tubes (red and black) and 1" nails. (See Figure four) You will need a fairly hot soldering iron to get a good solder joint on the nails but with a bit of patience it can be done. The nails can be anchored in the end of the tubes with a spot of adhesive.

If you now plug your probes into socket one the red LED will indicate continuity allowing you to test switches and cables etc. It is worth pointing out that any tests done on equipment should be done with the gear unplugged and the smoothing capacitors discharged (bridge a 100 ohm resistor across the capacitor terminals for a few seconds).

6. Diode test

Connect a probe to either end of the diode and look at the red LED. Swop over the probes and look again. One way round the LED should light, the other it should stay out. (The LED lights when the red probe is on the diode so you can also check unmarked diodes for polarity)

7. Transistor test

NPN:- connect the red probe to the collector and the black probe to the emitter — the red LED staying out shows it is not leaking. Wet finger and thumb and use them to bridge across the base and collector. The brightness of the red LED gives a rough guide to the transistor's gain — the brighter the LED, the higher it is.

PNP: Exactly the same as for NPN except the probes are reversed, ie red to emitter, black to collector.

So there we are! As the CHECKA draws no current when not in use the battery should last at least a year provided you don't let anything metal short across the socket nuts in storage. One other hidden bonus is that you're always carrying a spare battery in case one of your pedal batteries goes flat.

First two items on parts list available from Maplin Electronics Supplies, (Contact Details).