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Old 31st Mar 2004, 7:55 pm   #1
PaulR
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Default Buying a signal generator

Having caught the bug of old radios I think I now need a signal generator to use on AM and FM sets. There always seem to be some on Ebay although they don't all cover the frequencies required for FM.

These usually have a rider saying that they work but are uncalibrated. I guess that using one which produces the wrong frequencies is worse than not doing anything! My question is are they likely to be out of calibration or are they pretty stable? I realise that there can be no guarantees, but in general are they usually OK or should I steer clear?

Thanks

Paul

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Old 31st Mar 2004, 8:12 pm   #2
jim_beacon
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Default Re: Buying a signal generator

Paul,

most older signal generators have fairly poor calibration by modern standards, but are normally accurate to within a few KHz. In this case, you can find the IF, for example, by adjusting the generator for max ouput, and then align the IF to the generator. When adjusting the tracking of tuned stages, repeatability of setting is more important than absolute frequency accuracy. The accuracy of the output level is of no consequence in general domestic radio work, but a good attenuator and a well screened generator that prevent you overloading the circuit under tes are more important.

The frequency accuracy can be improved by the use of a cheap frequency counter, or the more traditional heterodyne wavemeter (or crystal calibrator), but these are things you may want to borrow just to check your new purchase, remember that a lot of modern DVM's will measure frequency up to 1MHz or so.

You can also check the generators frequency calibration by beating the output against known radio stations on a good receiver.

Finally, Paul has a nice design for a homebuilt transistorised generator elsewhere on this site.

http://www.vintage-radio.com/project...enerator.shtml

Jim.
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Old 31st Mar 2004, 11:35 pm   #3
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Default Re: Buying a signal generator

Hi PaulR.

I think this is a bit of a grey area. If you are carefull where the signal generators come from, you will probably be ok. I managed to get two very good original Advance generators from Philips when they modernised their workshop back in the 70's. One is an AM FM type which goes from 6Mhz up to 250 Mhz and the other is AM only and goes from 100Khz up to 70Mhz. The latter one is used most as it covers all of the AM bands, the former one only being used on FM sets if they need a complete re-alignment. The AM/FM generator is completely original and reasonably accurate....probably not used as much as the AM only one. I know its fairly accurate as one of my other interests is listening to shortwaves, mainly the amateur bands and for that I use a (relatively) modern synthesizer tuned communications receiver accurate to within 0.5kHz. If I 'beat' the AM/FM genny against the comms receiver, it's accurate to within a couple of Khz right up to 30Mhz....better at the lower frequencies. If I tune it (on FM) to my Hi Fi tuner, it is only about 0.5 Khz out at 100Mhz so really it's not enough to worry about.

The AM only genny is near perfect right through it's range although it does tend to drop off a bit at around 20 Mhz.

Generally I have no problems and knowing both generators well, I can decide which one to use at what frequency.

I think you must select generators that are complete and don't appear to have been messed around with. Both of mine date from the 50's but are built like tanks and appart from changing the rectifier valveholder and smoothing in the AM only generator, are completely original and untouched.

Rich.
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Old 1st Apr 2004, 5:25 pm   #4
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Default Re: Buying a signal generator

Thanks.

I have seen a couple of references to Advance E2 generators that go up to 100Mhz. Would one of those do for AM and FM alignment?

Thanks

Paul
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Old 1st Apr 2004, 6:21 pm   #5
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Default Re: Buying a signal generator

Paul, The E2 is a very good general purpose AM generator which is simple to fix, has a good performance and very good calibration, provided the " phantom " has not been there first. Watch the brown moulded mica capacitors used for decoupling the HT and as mains bypass capacitors; these go short at the drop of a hat.

Generally, unless you need to set the discriminator " S " curve accurately on an FM set, an FM generator is not necessary; most alignment can be done on CW. For the finer points of calibration (really only needed to set receiver oscillator trimmers), use broadcast stations. Move the receiver pointer slightly to the trimming position after setting the oscillator on the nearest broadcast signal, then " find " this with the generator to set the RF coils. This yields a near perfect result.

Make sure the Max RF output has its cover if you want low attenuated levels. These are often missing.

Regards, Leon.
 
Old 2nd Apr 2004, 5:52 pm   #6
PaulR
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Default Re: Buying a signal generator

Thanks Leon,

Could you just explain a bit more about FM alignment, please. I am quite new to this and am not quite with the jargon yet!

Thank you

Paul
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Old 2nd Apr 2004, 7:17 pm   #7
Leon_Crampin
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Default Re: Buying a signal generator

Paul, I’m pretty sure there is a section on FM alignment on Paul Stenning’s website, but here is my potted version for a general case.

An FM receiver is usually an ordinary single superhet, operating at higher frequencies than most AM receivers and with more IF stages. The essential difference for alignment is that the detector is designed to give an AF output which is proportional to the amount by which the IF frequency is shifted in frequency away from the nominal centre IF (usually 10.7 MHz), hence the term frequency modulation.

The most common arrangement on a valve radio is the ratio detector, where 2 diodes (the big ones in the EABC80) are arranged to give an AF voltage output of zero when the IF frequency is centred. This is done by utilising the principle of the phase shift within a resonant transformer due to its mutual inductance, varying with frequency. The rectified carrier also produces a DC output across the stabilising capacitor (about 5 uF) which is proportional to signal strength and hence useful for alignment. I’ll stop there as far as the theory goes, but there are plenty of good books. A knowledge of vector diagrams is useful for a proper understanding of this topic.

To align an FM set, apply 10.7 MHz to the grid of the final IF amplifier. You can usefully use the AM modulation to “find” the signal at this stage because the set will be sensitive to AM, especially if it is out of alignment. Peak the primary of the final IF transformer, using the voltage across the stabilising capacitor as a guide. Aim to have 7 to 10V across this capacitor, adjusting generator output as you proceed. Move back through the set until all the IF transformers are peaked, the exception being the secondary of the final transformer. Be very careful now, not to shift the generator frequency.

Switch the generator to CW and connect two matched resistors in series of about 100k across the stabilising capacitor. Connect a DVM between the junction of these resistors, and the tertiary winding of the transformer. With the conditions outlined above, carefully set the secondary core for zero output on the DVM, ensuring that a true null is found, ie. meter goes positive in one core direction, then negative in the other. Ensure before you start, that the diodes within the EABC80 are matched to +/- 10% Vf at about 5mA or you will have a hard time. Many old EABC80s fail this test.

Repeat everything, setting the secondary for a null as your final IF adjustment. RF alignment is straightforward; I use broadcast signals to set the oscillator, and the generator on CW to peak the RF circuits. As arrangements are diverse, I would strongly recommend doing this with the receiver’s service manual to hand.

Phew! Leon.
 
Old 2nd Apr 2004, 10:12 pm   #8
high_vacuum_house
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Default Re: Buying a signal generator

Hello there,
At the last Radiophile event I bought a big Airmec signal generator model 204 that goes between 1MHz and 320MHz. It has facilities for AM modulation and FM modulation, It has many valves such as E88CC's and is extremely heavy (can only just hump it onto the bench! ) and the screening is very complex. There are many aluminium castings inside and the dial drive has lots of large machine cut brass gears. This must be a very good and useful piece of equipment when working I think (When all of the Hunts mouldseal capacitors have been replaced )
Christopher Capener
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