The finished signal tracer

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Disclaimer

These designs are offered in good faith. It is the responsibility of the constructor to ensure his personal safety, and that of others, when building and using the devices.  Bear in mind that mains-powered items must be constructed to a high standard of safety. If you are in doubt about your knowledge and ability to construct and use any of these devices safely, you are advised not to attempt the work.

PLEASE NOTE: the PCB layouts are included for reference only and I CANNOT supply print-outs of these. The designs have been on the web site for some time now and the original data was unfortunately lost.

Safety Lamp

This simple design is extremely useful to the restorer. It allows power to be fed to an 'unknown' radio in order to test for potential problems with mains transformers, smoothing capacitors or rectifier short-circuits. The device works like this:

The power is applied to the set under test in series with a 60W lamp*. Once satisfied that no danger of burn-out or catastrophic failure is likely, the switch is thrown to short out the lamp and apply power directly. It's that simple - and that useful.

Do NOT attempt to build this device unless you are sure of your ability to do so safely.

*Never apply power through lamp limiter or direct until you have ensured no obvious short-circuit or other power supply fault exists. 

LIST OF COMPONENTS AND MATERIALS

1 X 60W mains lamp

1 X batten holder to suit above lamp

1 X one-way mains-type surface mounting light switch and pattress box, plastic

1 X surface mounting switched 13A socket and pattress box, plastic

1 X choc-bloc, at least 3 way @ 13A, for facilitating lamp wiring

Quantity of timber. Original uses a 9mm ply top panel with 15mm MDF side rails

Woodscrews, fixing clips for cables, PVA adhesive, Danish Oil or varnish

Dummy Aerial for signal generator

This simple little device is built into a felt-tip pen and uses a short length of plain brass rod as a probe. It is suitable for use in the alignment of most valve radio sets.

Why a dummy aerial? Well, radio sets are intended to work with an aerial of some kind: valve portables usually have frame aerials, which are, in effect, the aerial coils would onto a large former. This is the reason why the cases often need turning, as best reception occurs when the axis of the frame aerial is in line with the transmitting station. The same sort of process gives the principle behind radio direction-finding. Portables should always, therefore, have their frame aerials in place when alignment is carried out. Such sets need the generator to be linked to the aerial by means of a inductively-coupled coil when RF stages are being aligned. Inductively means, in practice, that no electrical connection is made - ideally, a radiating coil, shielded and constructed to the Radio Manufacturers Association standard, should be used, connected to the generator AF output by means of a shielded cable and placed about a foot from the radio set's frame aerial. If you would like details of the construction of such a device, I have them and you are welcome to e-mail for a free e-mail picture/text download about the subject. However, here's an alternative  for battery portables. Construct a loop consisting of about four turns of - preferably - stiff insulated wire on a former about the size of the set's frame aerial. Connect the loop ends to the generator output and set the aerial about 2 feet from the receiver. Initial setting-up might require the generator leads to be placed near the frequency-changer grid for inductive coupling, to roughly align, before proceeding with the loop.

With mains radios, it is easier and quicker to have a suitable dummy aerial built up as shown. This design is very basic, using no inductances at all, but is fine for general use, though not suitable for alignment of short waveband RF stages.

Construction is simple. The components should be mounted on the veroboard. The cable should be fed through a hole drilled in the base of the empty pen case and soldered into place on the board. A plastic strap should be fitted to prevent strain on the cable. This should fit inside the end of the assembled case. A hole to suit the brass rod should be drilled in the nozzle end and the rod bonded with Araldite resin adhesive so that about an inch sticks into the case body when the cap is replaced. 

The rod should be notched with a file to help the Araldite resin glue hold it firmly in place. If the cap is too loose, a turn of adhesive tape around the body top should help. The probe may be insulated by sliding some plastic sleeving over it, leaving just a small tip exposed. The sleeving can be obtained by stripping heavy duty electric wiring cable.

LIST OF COMPONENTS AND MATERIALS

One felt-tip marker pen, about 0.75" diameter by 4" overall length.

One brass rod, about 3mm diameter by 3" in length

One 300pF capacitor

One 390 ohm resistor (ideally a non-inductive type)

One length of flexible screened or co-ax cable

One plug to suit RF output socket of signal generator

One short section of matrix board or veroboard

Signal Injector/Tracer

Please read the disclaimer at the head of this page before deciding on construction.

A 555 timer chip is connected in astable mode, the frequency being determined by the time constant network R1/VR2/C2. The output is to coax socket. VR2 allows adjustment of the time constant and therefore the pitch of the audible sound when fed into the amplifier section either via the radio under test or direct into the amplifier input socket to set the frequency to a suitable pitch.

The amplifier is an LM380N-14, a low cost and efficient little chip. It is a DIL-14 pin package. The 555 standard is DIL-8 so there is no chance of confusion. To minimize possible damage and make life easier if a change of chip becomes necessary I recommend the use of chip holders rather than soldering the chips themselves.

The optional detector probe shown above should allow the detection of signals in the IF stages up to the detector diode itself (usually one diode of a multiple valve). With IF stages in order, it should be possible to hear radio programmes over the tracer internal LS. Don't expect high fidelity, however!

One very desirable addition not shown in the diagrams but fitted to the original is an 'on' indicator. This is a standard red LED with a series 1k resistor limiter, wired across the power input on the 'dead' side of the switch.

Notes

The circuit shown uses a single 9V battery but the PCB design has provision for a mains power supply. NOTE THAT THE AC SUPPLY IS VIA THE SECONDARY OF A 9V ISOLATING TRANSFORMER, NOT CONNECTED DIRECTLY TO THE MAINS. The one I used provided about 12V well rectified and smoothed by a bridge diode block and two capacitors, with a linking wire-wound resistor. The two chips seem to work very well at this voltage.

It is wise to place a capacitor of around 10nF in series with the oscillator output, with a high working voltage rating, say 400VW. This is to protect the oscillator against inadvertent high DC input.

IMPORTANT NOTICE. If you are in doubt about your knowledge and ability to construct the device safely, you are advised NOT to construct a mains-powered version.

COMPONENT LIST

C1,2,3: 10µF electrolytic

C4: 100nF

C5: 470µF electrolytic

R1: 1k

R2: 2R7

VR1: 10k

VR2: 47k preset

Loudspeaker: 8 ohms

Battery decals

Batteries for valve portable radios are sadly no longer available. In fact, not since about 1973! However, it is possible to build your own HT or combined HT/LT battery by placing sufficient PP£ batteries in series (10 required for 90 volts). If you want LT supplies too, best to use large 'D' cells, perhaps two in parallel, held in suitable holders. Holders and PP3 clips are freely available from component suppliers. The battery should be assembled in a lightweight wooden box. Thin MDF or 6mm plywood should suit. To add the finishing touch, battery decals can be added. Click on the link below to find good quality scans of a standard Ever Ready B136, as used in 'Sky Queen' and other larger portables. Ensure that the scans are printed to the correct size - see instructions on the page.