A list of helpful gadgets
User Rating: / 1
PoorBest 
Written by Bryce Ringwood   

If you are doing the odd repair on one of the sets in your collection, there are a few little gadgets that will help you determine faults easily, or may improve the quality of your repair. These are my favourites after the multimeter. Bear in mind that some multimeters have the functionality of some of the instruments described here.

Component tester

This sounds like a dramatic piece of equipment, but for around R500-00 you can get a device that measures inductance, capacitance and resistance. If you are well-heeled, you can buy a "Peak" microproessor based component tester. (I am ill-heeled and don't have one). The simple component tester also has a diode and  transistor tester built-in. At one time you could buy a gadget that lool like tweezers with an LCD. Called "smart tweezers" - about R3 500-00.

The drawback is that these testers do not give much guide on capacitor leakage (article on capacitors follows). You can use the Ohm-meter part of the test device to check if the capacitor has an undesirably low resistance.

The component tester also will give you an idea of inductance values for coils , such as medium-wave IF transformers, but alas not for short-wave and VHF frequencies. It will, however tell you if the coil is open or short-circuit. This is good because by far the most common problem I get with old radios is open-circuit coils.

If you can't run to a component tester, then a home made capacitance meter is a good thing to have. Your multimeter measures resistance, and whether a coil is open circuit, so here is a simple capacitance meter. This home made capacitance meter   was built from a circuit in the 1980 RSGB handbook (7th Edition). It is based on the 555 timer IC. It is funny how one trusts ones home-made instruments over commercial ones - especially when it comes to capacitors. The cabinet was reclaimed from something else and is almost empty.

Capacitance meter

Insulation Tester (Is it safe?)

Well, as Dr Szell would put it in the movie Bridge Megger"Marathon Man" - Is it ? The wiring in old radios is often suspect, but how often do we test it ? In South Africa, we are fortunate to have "Earth Leakage" detectors in our house wiring, and they are very good at preventing hurt to householders, but, I guess, you could be in another country where such gadgets are not required. You need to test the insulation between neutral and ground and line and ground. Use the 500 Volt range.

Insulation testers are also useful for testing small value capacitors. Obviously they won't give you the value, but they will tell you if the dielectric has broken down. I normally check the insulation resistance against a a known good capacitor, and if the value is somewhat less, I throw the old cap away. You don't need an insulation tester for power supply caps, for by their sign ye shall know them. (They will cause hum, the rectifier to glow blue or motorboating. If in doubt chuck 'em out.)

The snag is that insulation testers are quite expensive. I am fortunate to have an old hand-cranked "Evershed and Vignoles" bridge megger. I see new electronic ones cost about R 2000-00. Although it is "the responsible thing to do" to get one - try to get it second-hand.

Test Speakers (and amplifier)

And now for something affordable. If you are fixing up a set - do you know the speaker works? If you go down to RSE Electronis, you will find an assortment of speakers at a few rands each. Buy them in pairs . Other suppliers also have speaker baragain-bins - keep an eye open.

The snag is that some companies (i.e. Philips) often put high resistance speakers in their radios. These are unobtainable, if broken, and the only thing I can suggest is to try to get them repaired at Sonic Speakers. If this is impossible, try using small mains transformers to match the amplifier to the speaker.

Another plan might be to get Peter Souris to wind you an output transformer that matches the valve to the speaker you have got.

But hopefully none of these things will come to pass, and you can just use a pair of 4 Ohm speakers rated at about 10 Watts smiley.

An amplifier is often a useful adjunct. PC speakers have a suitable amplifier built-in and form a quite useful tool, especially if you're debugging home made radios, or want an output for a set that was intended for headphone listening. You might need a FET preamp for a crystal set, though.

Power Supplies

For repair work, you just need something that will supply HT for valve circuits, and something that will supply variable DC for transistor circuits. I don't find much use for either. I normally make sure the power supply is 100% correct before doing anything else on a set.

If I'm making a radio, then I do use a home-made variable HT supply, or low-voltage power supply. 

You should have a variac to provide a variable voltage AC supply. This should be driven from a double wound isolation transformer for safety reasons. Alas, this is a MUST HAVE. 

Remember that 9 volt zinc-carbon batteries in series can be an inexpensive way of  getting an HT supply for small receiver projects.

Crystal Calibrator

A crystal calibrator leads us to the next section on signal generators. A crystal calibrator consists of a crystal oscillator, usually operating at 1MHz, followed by a multivibrator circuit to ensure that there are plenty of harmonics to provide signals at an interval of 1MHz.  You can't really use it to replace a signal generator, because it is quiet easy to select the wrong harmonic, but once a set is correctly aligned, it may be used to accurately deterine the frequency you are tuned to.

Crystal Calibrator crystal oscillator
Home made crystal calibrator. Circa 1968 Inexpensive crystal oscillators are now available, which will do the same job

Crystal calibrators are often built-in to radio receivers, such as the R390a and Eddystone 770R/II receivers. The Eddystone uses a simple Pierce Oscillator - the same as the DL70 beacon transmitter. The National NC109 also has an optional crystal calibrator. The dial scales on the R390a and Eddystone can be adjusted to the read the correct frequency.

One of my Marconi signal generators also has a crystal calibrator built in. You fine tune the output frequency by listening for a zero-beat in a set of headphones plugged in to the instrument. (Remember that heterodyne thing - see the article on superhets. As the signal generator frequency nears a harmonic of the crystal frequency, you can hear an audible tone which becomes lower and lower in frequency until you can't hear anything because the two frequencies coincide. This is the zero-beat frequency. As the signal generator is tuned the other side of the calibrator frequency, the audible tone rises in pitch again). 

The output signal is fed into the antenna circuit - but beware! - It is all too easy to set the circuit up on an image. In this respect, the Murphy B40 is cunning, since the IF is 500kHz and the crystal calibrator is 1MHz - what can go wrong?

In Conclusion

With a multimeter and the above gadgets, you should have enough kit to do a safe repair of most valve and transistor radio sets. To be sure, I have an array of other test equipment - but, its mainly used for repairs to other electronic equipment, such as power inverters.

Occasionally, you get a tough nut to crack, and a signal generator or signal tracer as described next can be useful to find where the signal goes AWOL.

 

 
Joomla template by a4joomla