[This article first appeared in 'Radio Age,' the monthly newsletter of the Mid-Atlantic Antique Radio Club.
It is reproduced here by permission of the author.  It has been edited for space.   -- F.W.]

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When you first begin servicing or restoring radios, the first and most essential piece of test equipment is a volt-ohmmeter, or VOM--to read voltages and check resistances.  A vacuum-tube voltmeter or its modem solid-state equivalent can be purchased for relatively little money.  Once you have a VOM, the next two most helpful instruments are a tube checker and a signal generator.   The purpose of the tube checker is pretty obvious. The signal generator provides three distinct useful functions:

• It can provide a test signal that can be inserted at various points in the radio circuit to determine whether a particular stage is working.
• It can provide a stable signal to align the intermediate frequency amplifier of a superheterodyne receiver and the radio frequency section of any receiver.

• (In some cases) the signal generator can serve as a wireless broadcaster. (More on this later.)

Usually when people talk about a "signal generator" they are referring to a device that generates signals above the audio frequency range.   I find it helpful to have a separate audio generator whose frequency can be adjusted over the entire audible frequency range in order to test the frequency response of speakers and audio amplifiers.

This article focuses on RF generators, most of which will start at perhaps 100 kHz and go up to, say, 50 to 100 MHz in the case of very inexpensive types or up to a couple of hundred MHz in the case of better ones.  Today it is possible to obtain signal generators that go way into the GHz (1,000 MHz) range, but these are not of interest to broadcast radio restorers.

This is a Heathkit Model SG-8 from the mid-1950s.   It is typical of signal generators of that era that would have been purchased by someone servicing radios.  It provides RF output from 160 kHz to 110 MHz, but has harmonics that can be used up to 220 MHz.  It provides an audio signal output at about 400 Hertz.

RF signal generators began to be available for radio servicing about the time that more complex ac radios appeared and superheterodynes became more common.  Battery radios of the early- to mid-1920s were sufficiently simple that to service them about all you needed was a voltmeter to check for proper voltages and a continuity checker to find short circuits and open circuits.  By the early- to mid-1930s, most serious service shops would have had a signal generator on the workbench.

Old tube-type signal generators from the late 1930s through the 1960s that were used in radio service shops turn up at MAARC auctions pretty often.   Brands like Heathkit and Eico are very common, and can usually be purchased for $10-15 or less.  Other brands like RCA, Conar, Precision, Jackson, Hickok, and Knight also show up fairly often.  Like the Heathkit, these are typically in utilitarian metal boxes containing a circuit with just a couple of tubes.  (Although prior to WW II, companies like Supreme and Weston put their instruments in beautiful dovetailed wood cases.  And Philco made some signal generators with a machined metal front panel that were sufficiently attractive to make nice display items today.)

During the past few decades, an entirely different class of signal generator has become available from companies like Hewlett-Packard and Fluke.   These are precision instruments used in laboratories for more sophisticated tasks.   Jammed with integrated circuits, some cost thousands of dollars when new.   They feature precision digital signal synthesis, digital readout, and computer bus interfaces.  Such instruments are excellent and work great for servicing radios if you happen to have one, but they are usually much more expensive and are very complicated to repair if anything goes wrong.  The older simpler signal generators like the Eicos and Heathkits are easy to repair.

What are the differences between, say, a cheap Eico signal generator from 1955 and a precision Hewlett-Packard generator from the 1980s?

• Precision and accuracy in setting and maintaining the frequency. If you are aligning the IF section of a radios with a 455-kHz IF, you can set your HP generator to 455,000 Hz and it will put out precisely that frequency, and the frequency will not drift with time.  With the old Eico, it is difficult to set the frequency.  You may have to estimate where on the dial 455 is.  As the generator warms up, the frequency will certainly drift to some extent. 
• Purity of the signal. The output of HP or other modem expensive generators is a pure sine wave of a single frequency.  Older generators had oscillators that intentionally put out a signal rich in harmonic content. They were designed so that on the higher bands the generator output stage could be tuned to the second, third, or even higher harmonic of the oscillator.  If you look at the output signal of these generators on an oscilloscope, you will likely see a very distorted sine wave.
• Other nice features.  Modern generators may be able to produce waveforms other than just a sine wave; for example, triangular waves or square waves.  Sometimes these waveforms can be useful.  Modern generators usually have digital readout of the frequency and they usually have a computer interface for data logging and control.

You may think I am trying to convince you to avoid the older, cheaper generators.  Not really.  If someone gives you a modern HP or Fluke precision signal generator free, or sufficiently cheap, for goodness sakes take it.   It will do a splendid job.  But for normal radio repairs an inexpensive Heath or Eico can do the job just fine.  Let's assume that you have one of these typical old tube-type generators.  How do you cope with the drift problem?

• To minimize drift, the more expensive generators of years gone by used components that were less temperature sensitive than typical components, and a number of ingenious circuit approaches to minimizing drift were tried.  It is a fact that the inexpensive old signal generators will tend to drift quite a bit as they are warming up.  The secret is to turn on your signal generator an hour before you intend to use it.  Once it comes to equilibrium, the drift rate will be small, and during the short time you align your radio, the drift in most signal generators, even the cheap ones, will not compromise the alignment.

What should you look for when buying a signal generator?

• Get one with a manual. Having the detailed instructions and a schematic can make using and maintaining it much easier.  If yours does not have a manual, advertise in Radio Age or Antique Radio Classified for a photocopy.   Also, there are dealers who advertise manuals and reprints of manuals for sale. Check the ads in Radio Age and ARC.
• Bigger is better. Get one with a large dial that has fine increments of dial calibration.  With a large dial it is much easier to set the frequency you want.  Chances are the bigger and heavier the unit, the better the quality.
• Look for one in good cosmetic condition.  If the case is not rusty, chances are the inside will be clean and not badly corroded and the switches will still work.
• Look for a sweep-type signal generator. A sweep generator is one that has a provision for sweeping the frequency back and forth over some restricted range at a constant rate.  Later when you get an oscilloscope, this feature comes in very handy, especially for servicing f the more complicated radios that have features such as variable IF bandwidth or stagger-tuned IF transformers to broaden the bandwidth for higher fidelity reproduction.  With a generator that can sweep the frequency back and forth across the IF bandwidth, you can see the response pattern of the IF section on your scope, and do a much better job of aligning the radio.
• Get one that has an external modulation jack.  This feature allows you to use the generator as a wireless broadcaster as noted later in this article.  If you tinker only with AM radios, then having a generator that features only variable AM modulation is sufficient.  When you work on FM radios, you may not need to use any modulation--check your manual for details.
• Check the frequency range.  At the low frequency end it should go down in frequency below 200 kHz, preferably to 100 kHz.  Some old superheterodyne radios used IFs between 150 and 200 kHz; ~ 175 kHz was a popular frequency in early sets.  If you work only on AM radios, you can get by with a signal generator whose upper frequency limit is about 30 MHz, but if you like FM radios, you would want a generator that covers the FM band, hence at least 110 MHz.

Restoring that old generator that you just bought at an auction.

• Restoring a signal generator is pretty much the same as restoring a radio.  The usual things that go wrong are weak or dead tubes, leaky capacitors, oxidized or corroded switch contacts and tube sockets, resistors that have changed in value, and frayed wires.
• Start by giving the generator case a good cleaning with soap and water, being careful to check for water soluble ink on the dial before you plunge in.   Go-Jo will take off layers of yellow cigarette smoke deposits.  Remove the chassis, checking the line cord and replace it if needed.  Check the tubes.  (If they are a little weak, they may still work OK, so don't throw them out.)  Mica capacitors are usually still good, but I would replace the electrolytics and any wax/paper capacitors, just on general principles.  Check resistors for changed values.   Spray all switch contacts and squirt the inside of pots with contact cleaner and work them back and forth repeatedly to remove oxide layers.  Replace any frayed wiring.  If it has a selenium rectifier, replace it with a modem silicon diode.   Usually these steps will restore the generator to like-new operation.
• If the manual has instructions for checking the calibration, follow those instructions.  It is handy to have a frequency meter to check the accuracy of the output.  You can purchase military surplus frequency meters for various bands for just a few dollars at antique radio auctions, and these are usually pretty rugged instruments.

If you don't have a frequency meter, a generic calibration method can be used.  Take a radio in good working condition and position it so that you have access to the antenna terminal or the loop antenna.  If it is an ac-dc radio, BE SURE to use an isolation transformer.  Don't try this without an isolation transformer or you may electrocute yourself and possibly damage your signal generator.  Connect the "hot" output of the signal generator to the antenna terminal of the radio through a .01 µF capacitor and connect the ground lead from the generator to the chassis of the radio or the ground lead.  If the radio has a loop antenna with no antenna terminal, remove an old loop from a junker set, prop it up close to the loop in the radio, and connect the signal generator leads to the main winding on the junker set loop.  Let the signal generator warm up for about an hour before beginning the calibration check.

Jot down the exact frequencies of several local AM stations that you can readily identify.  Ideally, you should try to get at least five or six stations spaced out along the dial.  Switch the signal generator onto the frequency range that covers the broadcast band. Set the radio to the first station, making sure you have it tuned in to the center of the station frequency.  Tune the signal generator to the approximate frequency of the station.  As you tune the signal generator frequency through that of the station you will hear a whistle that changes in pitch.  As you approach the station frequency from one side, the pitch will decrease.   It will go through a point where the whistle disappears briefly, and then as you continue in the same direction through the station, the whistle will increase in pitch again until it gradually goes away.  The point where the frequency of the whistle lowers to the point where it disappears is called the "zero beat" point.

"Zero-beat" the generator with the station and note the exact frequency of the station (not the frequency on the dial of the radio, which may be a little off) and the reading on the dial of the signal generator at the zero beat point.  Plot these deviations on a piece of graph paper, connect them with a smooth curve, and save this calibration curve.  This information will allow you to set your generator more accurately in the future.  Here is an example of what a table of values and a calibration curve might look like from having done this exercise with a hypothetical radio and signal generator.  In the example, the generator reads a bit high at the low end of the broadcast band, a bit low in the middle of the band, and a bit high again at the high end of the band.  Let's say that in the future, you want to set your generator to precisely 1000 kHz.  According to the curve, you should actually set the dial at 998 to get an output of 1000.  (Your own generator may have a curve that looks quite different from this example, of course.)

This is what the data looks like plotted.

If you have a listing of powerful short-wave stations like the BBC in London and Radio Moscow, you can check the calibration of your generator in the Shortwave bands by using these known stations as per the process above.  At the very least, you can use signals from station operated by the National Institute of Standards and Technology, which broadcasts strong signals at 5, 10, and 15 MHz.

I will not try in this article to explain how to align or troubleshoot radios with a signal generator. Those topics are well explained in most radio service textbooks.

Using your signal generator as a wireless broadcaster.   It's great fun to listen to 1930s or '40s music or radio programs on your Philco cathedral.  The effect is much more nostalgic than listening to modern talk radio.  If you have cassette tapes of old programs or CDs of old music (which are readily available), all you need to do is take the audio output of your Walkman or CD player and use it to modulate the output of a tiny AM transmitter--a wireless broadcaster, or phono-oscillator as they were sometimes called.

• You can purchase wireless AM broadcasters from companies like Antique Electronic Supply or from companies that advertise deluxe devices in Antique Radio Classified. But, if you have a signal generator with an external modulation jack, and many of them do, you already have a wireless broadcaster!  Just take the audio output from the headphone jack of your cassette player or CD player and input it to the external modulation jack of the signal generator.  (If your audio source is stereo, in most cases, you can wire the two channels in parallel so you don't lose any sound.)
• I find that a wire about 10 feet long across the ceiling works fine as the antenna for the wireless broadcaster.  I just clip the "hot" lead from my Jackson signal generator to the antenna, and, with a little tinkering of the controls of the generator and the audio source PRESTO--an instant wireless broadcaster that you can set for whatever frequency you like.  Choose a quiet part of the broadcast band.  I find that where I live, setting the signal generator and the radio a little below 600 kHz is a good spot, since the RF noise there is less than it is higher up on the broadcast band, and there are no strong stations in that vicinity.
• Use the minimum generator output needed to give a readable signal in your radio so you don't interfere with your neighbor's radio reception or get in trouble with the FCC.  (If your neighbors hear Amos and Andy coming out of their modem radio, they will probably contact one of the tabloid newspapers claiming they have witnessed a miracle.)  Obviously the same approach enables you to listen to FM radio on your old AM radios  Most inexpensive signal generators do not have that strong a signal, so you will probably have to set the generator for the highest level output.

So, enjoy your old signal generator both for servicing radios and for listening to old music or programs!  Signal generators are cheap enough that you can afford to have one in every room of your house.

Biographical note.  The author is a member of the Mid-Atlantic Antique Radio Club and the Radio History Society.
A manager at the National Institute of Standards and Technology, he has been collecting antique radios for about 20 years.
Click here to send Brian e-mail.

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