Signal Generators
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Written by Bryce Ringwood   

Signal generators produce an audio or radio frequency signal that can be used to trace faults in a radio receiver, and also may be used to adjust a set that requires re-alignment.

These instruments used to come in two distinct flavours:

  1. Service type
  2. Laboratory type

Nowadays, you will be hard-pressed to find a new signal generator at an affordable price. There are some Chinese instruments available, but they are not easy to find in South Africa. You will find a variety of function generators, but they are usually limited in frequency range.

Simple Hobby Type
Instrument

The simplest signal generator is the "Service Type" - I guess the one I use most does't really qualify even for this title. I think mine would be a "Hobby Type". It has two valves. One is a radio-frequency oscillator covering 120kHz to 240MHz in 4 ranges. The other is an audio oscillator operating at 1kHz. Coverage is from 120kHz to 480MHz, because it uses harmonics of the oscillator stage to provide a 5th range. The output is fed into a potentiometer to give some control of the output level.

In practice it fulfils my requirements most of the time. For example, you can set the instrument to 455kHz and proceed to check whether an IF amplifier is working by feeding a low level signal in and listening to the resulting tone in a loudspeaker attached to the set. If there is no tone - you can see at what stage the signal disappears using an oscilloscope, or just an audio amplifier.

This is an example of "signal tracing" . There is a much simpler instrument called a "Signal Tracer" that consists of a square-wave generator and an audio amplifier. I don't have one, but many people prefer this simpler instrument and have a great deal of success with it. A signal tracer can't be used to align radio receivers.

 IF Alignment

Signal generators are used for aligning superhet radio receivers. Very often, specific instructions are provided with the set, telling you exactly how to go about the process. Usually, you need to disconnect the AVC, then connect an output meter to the audio. (Maybe someone reading this will describe the mythical output meter to me. I don't think I have ever seen one. I have always used an oscilloscope. - next article)  Next inject a signal at a specified level into a specified point in the receiver circuit and peak each IF transformer tuned circuit in turn. Often you have to remove the local oscillator valve and inject the signal at the grid of the mixer. Some instructions require you to "rock" the signal generator about a central frequency and peak the tuned circuits either side in order to provide a wider bandwidth (and better audio quality.) All the while, using the minimum signal generator output.

Bear in mind that many of these instructions were writtem in the 1930's and '40s before there were digital frequency meters and so on that we have today. As I write, I'm busy with a 1934 set that has an IF of 472.5 kHz.

Another 1960's radio (Eddystone 940) suggests that very specific levels of IF  voltage must be applied to each stage and very specific levels of gain should be achieved.

FE 342 Frequency Synthesizer

A Valve-Type Frequency Synthesizer.
Intended as a transmitter master oscillator

None of these things are possible with a simple signal generator by itself.  I normally set a modern-ish digital radio (SONY - SW-100) to the required frequency with the BFO on and then tune the signal generator to zero-beat on the radio.

Setting the output level can be done with an oscilloscope. Bear in mind that the 1mV signal required is RMS and you will need 2.8mV peak-to-peak on the scope.

The problem with "hobby-type" signal generators becomes apparent with really low level signals. The shielding of the oscillator stage is often very poor, and even a theoretical zero output level signal blasts a strong signal into the radio being serviced.

Laboratory standard signal generators are better. They have lots of shielding and ways and means of setting the output level to a specific value. You can modulate the signal to any depth. Finally, they may have a quartz crystal so you can set the output fequency reasonably accurately. (Not to 472.5 kHz though.) Of course, if you have an HP synthesized signal generator - then none of this applies.

Standard IFs for single superhet receivers are 455,465 and 470kHz. Double superhets use 1600kHz and 85kHz.

RF Alignment

Once you have set your IF to 455 kHz or somewhere near to what the instructions say and everything is working well, you can tackle the RF stages. This usually involves setting the local oscillator first. I use another radio for this. First I set the low frequency on the dial by setting the inductance - the high end frequency will then be set using the trimmer capacitor. Then the low frequency is re-set and so on - repeating until the frequency range is correct. Do be very careful with this - because if you make a mistake, the set will be very disappointing. You should never ever have to do this unless someone has fiddled. The same goes for the RF circuits in the RF and Mixer stages., but this time you will use the signal generator to provide a signal that you can peak the coils and trimmers on.

Wobbuloscope for aligning
FM TVs and Radios

Philips Wobbulator for AM radios.
It needs a scope and sig gen.

I don't think you need an accurate signal generator for this - unless you plan to use it to align the oscillator stage. Even so, an accurately calibrated signal generator will be acceptable for broadcast receivers.

The service instructions for most sets will provide a table of frequencies that should be adhered to for RF alignment. In a conventional superhet radio, there are normally just three places on any waveband where the local oscillator and amplifier/mixer tuned circuits can be in exact alignment. The instructions may choose these three points in each wave band.

Just a word of caution - be very careful with the alignment of the highest frequency range of shortwave and communication receivers. It is very easy to get the local oscillator running on the "wrong side" of the tuned frequency, or even to get it "crossing over" - running on the low side of the tuned frequency at one end of the tuning range and on the high side at the other, leaving  everything a bit odd in the middle of the range. I've done it and wondered why the set wasn't quite as lively as when I bought it.crying

Audio Generators

I made one of these to test home made audio and DC amplifiers. I suppose that if I were presented with a radio having a complicated audio amp emitting a distorted sound, I would find the insrument very useful. Up to now, I haven't had very many HiFi or guitar amplifiers for servicing and repair. Those that I have attended to, have had painfully obvious faults. The exception being the Crosley - but the audio generator didn't help much. Nevertheless, they can be used for signal tracing.

But - write in - I can learn from you here.

Note that some communications receivers have a second IF of 85kHz. Many signal generators don't cover such a low frequency - but audio and function generators do.

FM Signal Generators

Now for something else I don't really understand. Two of my "Lab Type" signal generators feature FM modulation as well as AM. You can set the deviation of the output signal and so on. I don't understand how this feature aids with the alignment of an FM radio. The standard method of aligning FM radios having a two-diode discriminator is outlined in the article on superhets. As far as I can see an FM signal can tell you that the FM discriminator is working - but not much else.

Marconi TF1066b UHF Signal Generator

There is a gadget called a "wobbuloscope" that does the job very elegantly by displaying the discriminator response curve on a CRT screen. The one illustrated has an annoying fault which I can't get my head round. Its something you might use once or twice - and qualifies as "clutter". Basically, it is a signal generator that sweeps about a central frequency. As it does so, the response is plotted on a CRT.

There is a similar instrument for plotting the response of AM radio receivers, which you also don't need.

Getting a Signal Generator

Because signal generators are not easy to find and are too big when you succeed, there is a case to be made for making your own basic "hobby type" instrument. Please look at the design on the UK Vintage Radio Repair site.

I keep toying with the idea of making one - especially since there are some suitable and unusual valves to be had locally. Watch this space.

From time to time used signal generators appear on eBay. Look out for the AN-URM 25d military signal generator and the HP 8640B. They could be expensive and they may need quite a bit of attention to get them to work. See the case study on the Krohn-Hite audio generator.

Do you really need one ?

If you are faced with aligning a pre-'80s radio receiver, you do need something. If its a simple communications receiver, you can sometimes use the set's own BFO (Beat Frequency Oscillator to align the IF.

Another trick is to use another radio's local oscillator to align the radio being serviced. You have to do some mental arithmetic to work out what frequency is being generated.

Nearly all of the sets I have had to re-align have had some trauma. Either the cores have come loose in the IF transformers, or the said transformers were removed and I had to wind my own (urgh) or I have just fiddled and not improved anything.

BUT - if you are making your own radios from scratch, you probably do need one, if only to test things are working as you go along. Maybe its the second most important thing in the workshop - but its a long way behind a decent multimeter.

 

 

 

 

 
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