Wobbulator nonsense

[Argus25]

I'm starting this thread, not to pick a fight, but more like a case of The Emperor's New Clothes, to see if the boy calling it out has got it wrong.

It has been stated (just like only a fool couldn't see that the fabric is woven with beautiful golden thread) that a wobbulator is a useful device to align superhet radios (take the raspberry Pi wobbulator as one example where a similar statement was made).

I'll state the case:

A frequency sweep of a radio's intermediate frequency amplifier is a required process to design a superhet's IF circuitry and transformers, but NOT to repair or align them.

On the other hand a frequency sweep of a TV video IF amplififier is a requirement to both design and align/ repair the stages.

In superhet radios, the IF amplifier stages are designed to be peaked on a single frequency, say 455kHz in the alignment process. The actual bandwidth you end up with is set by the design of the IF transformers.

With a single amplitude modulated RF source, you can easily determine the bandwidth for any radio IF , if you wanted, after they are peaked, by merely adjusting the frequency on either side of centre until the output level falls to 70%. No wobbulator required. But you don't even have to do that during alignment, because the bandwidth is all looked after by the design of the IF transformers. These transformers don't "re-design" themselves after they leave the factory. If they go open or get shorted turns is plainly obvious that they are faulty and if they don't produce a peak on tuning then it is also obvious that they are off resonance.

On the other hand, if you were in a factory in the business of designing and winding 455kHz or similar IF transformers, the wobbulator might be very helpful. Or say, if you want to align TV set IF amplifiers the wobbulator is helpful, if not essential.

But the wobbulator in my view holds zero utility value for repairing or aligning existing superhet radios. So I remain baffled why it has been suggested that this is a useful or even required tool for aligning superhet radios.

To check this notion I have trolled through dozens of superhet radio alignment instructions to see it there were any common valve or transistor superhet radios, where it was suggested by the manufacturer, that the available IF adjustments, be staggered tuned in any way for a specific band pass response curve as seen from a wobbulator test. I cannot find one.

Perhaps there are some. Obviously there are Comms receivers with variable bandwidth IF's, but here I'm talking about common garden domestic LW/MW and SW radios.

But now what I'm interested in is this:

Can anybody explain to me why a wobbulator has any utility value for the alignment or testing or repairing of the common garden valve or transistor superhet radio, over and above a simple amplitude modulated RF carrier from a generator of a known variable frequency ?

[Radio Wrangler]

The reasoning behind the above seems to be that in a conventional broadcast superhet, the IF transformers constitute a coupled-resonator class of filter. Coupling between some pairs of resonators is set by the proximity of the coils in an IFT, and coupling to the resonator in the next IFT is via the IF amplifier valve.

The filter response shape is designed by choosing those coupling factors, and is monotonic. No square corner attempts, and a rise towards the centre. The coupling factors aren't adjustable (except for one sort of exception). This sort of filter is easily adjusted by peaking each stage in turn once you know that the test signal is on the right frequency down the IF. The shape you then get is what the fixed coupling factors were designed to give, and the overall IF gain is the best you can get.

IFTs are adjusted by moving a core slug in the former of each coil. Having the slugs in the centre gives maximum L, move either way and the inductance drops. To have allowance for adjustment either way, the number of turns is chosen so that on the correct IF frequency, the slug needs to be part way out. There are thus two positions of the slug where each coil peaks the IF. In an IFT this could be with the slugs in a total of 4 combinations of positions. Although all 4 have got the resonances spot on, the coupling factors are different. Usually the designer picks the coil spacing to be right with the slugs in the resonant positions with them furthest away from each other. It keeps the IFTs smaller.

Plotting the IF shape whether you do it with a sweeper, wobbulator or by hand on graph paper will show up if you've got the slug positions wrong, but it's a lot easier to just make sure each slug is on the resonance which has it furthest from its colleague.

Comms receiver IFs were done by peaking for the narrow bandwidths, but tuning for a controlled bandpass shape is needed for the wider bandwidths. Ols sets with a single crystal C filter were best tuned in the reverse process. Do the narrow bandwidth first, to find where the crystal peaks, then align the other bandwidths to match it, and align the front end to suit whatever your set's individual IF happens to be.

And of course, there are the two bandpass filters in the front end of the RA17 family of receivers....

You can do a lot without a sweeper, but sometimes you do need one. Also if you're teaching someone, being able to see the effects of adjustments can help them visualise what's going on.

David

[ukcol]

An interesting point Hugo and I agree that for most instances in domestic radio a sweep generator of some sort is not required. There are a few domestic radios that have variable bandwidth IFs however, like the Murphy A40C but they are the exception.

One scenario that does occur to me though. Suppose the capacitor in the IF tuned circuit has changed value or the coil has changed inductance due to damp or some other influence? Perhaps the change in resonance will be small enough for the transformer to still "peak" but its bandwidth at the new inductance/capacitance ratio will have changed. A wobbulator would be useful in that circumstance perhaps?

I think you can justify using a wobbulator in aligning FM detectors.

A last point to make in our hobby is not a technical one and that is that using a wobulator can be fun.

[Sideband]

I have seen the mention of a wobbulator for FM alignment..more so in the early days when FM was still 'new'. Not so much in later circuits. As ukcol says, using one can be fun and educational.

[Neutrino]

I recently checked the IF response of two Murphy A72 receivers the hard way with a signal generator and a meter connected to the external loudspeaker socket. The first two photographs show the plots. The A72 has a 3-position tone switch that in position 1, for the best quality of reproduction near a powerful station, adds a coil that increases the coupling between an I.F. transformer primary and secondary.

The second photo shows something wrong with the IF alignment. This could have been found faster and more easily with a wobbulator. The third photograph shows the IF response after peaking the IF transformers at 465 kHz in accordance with the Murphy Service Instructions. I feel that even with the tone control switch in position 1 the I.F. response might be a little too narrow and suspect that the maker might have originally stagger tuned the I.F. transformers for best reproduction, although there is no mention of this in the Murphy Service Instructions.

If I had a wobbulator I could try stagger tuning the cores in an attempt to get a near perfect I.F. response with steep sides and flat top that will give good selectivity and a suitably wide acceptance of side bands in all positions of the selectivity switch. Without a wobbulator it is far too much trouble.

Radios that are subjects for restoration may have drifted or have been got at and a wobbulator is a quick and easy way to see if the I.F. alignment needs attention.

Regards
David

[Nuvistor]

The alignment needs to be done as per the manual, attached AM alignment instructions for a Hacker RP38A, wobbulator required, or plot it by hand. These sets were designed for a bandpass response not peaked like many radios, some of the Hacker manuals warn against peaking as it can cause instability.

[Radio Wrangler]

Interest in swept measurement for aligning FM receivers fell a lot because receiver design soon became dominated by purchased ceramic block filters. Any IFTs were very broad and could just be peaked.

Sweeping is good for setting up discriminators.

David

[turretslug]

I can see that someone using a "stock", bandwidth-decided-at-design domestic radio might be using a wide-band AM pantry transmitter and want to offset-tune the 2 or more IFTs to suit. It could be done by swinging a signal-generator around the IF with an output meter and progressively tweaking, but a sweeper and display would be quicker and easier. ISTR both the AR88 and CR100 manuals have something about "emergency" or "workshop" alignments by swinging a conventional signal generator in conjunction with output level meter, but this sounds painstaking and prone to progressive frustration over several bandwidth settings and lots of tuned circuit adjustments. No doubt that, in the day, some got good at it.

[mhennessy]

Originally Posted by Nuvistor

The alignment needs to be done as per the manual, attached AM alignment instructions for a Hacker RP38A, wobbulator required, or plot it by hand. These sets were designed for a bandpass response not peaked like many radios, some of the Hacker manuals warn against peaking as it can cause instability.

Yes, Hacker explicitly state that AM IFs shouldn't be peaked in some of their sets.

For FM, it's much more critical - especially the discriminator as it has a direct effect on the audio quality. In the past, I've suggested using a THD meter to align this - indeed I can do an entire FM lineup using nothing more than an FM generator (not swept, but FM modulated), a 'scope and a THD meter. I've never been asked to expand on my methods - presumably because it's seen as unconventional so is dismissed out of hand - but it works just as well as (and is quicker than) sweeping. A THD meter is no more specialised a bit of kit than a wobulator - indeed, I have several THD meters, but not a single wobulator. Given the level of distortion being examined, plus the fact that we're minimising it rather than trying to quantify it, a simple DIY notch filter feeding your 'scope is all you need.

Unless it's a synthesised model, most RF signal generators can be swept. You might need a separate oscillator to generate the sweeping stimulus (or use the ramp output of your 'scope, if your 'scope has such an output), but such a oscillator can be a simple audio-frequency thing which you probably already have if you're at least semi-serious about electronics and radio repair.

So in short, no, a wobulator per se isn't needed, but the ability to generate a frequency sweep is essential, and probably already present in most workshops, even if it's never been used in anger. If you have the means, you might as well do it properly - but if you don't already have an RF oscillator, then you ought to - they are so very useful for other tasks like RF alignment and checking sensitivity, and basic models can be found at sensible prices. Yes, with experience, you'll learn when you can take shortcuts like peak-tuning, but I would always encourage a beginner to do it properly until they've got their own feel for what's what.

My FM only Ekco set alignment instructions says to damp 'the other winding' of each IFT with a resistor and tune the other to peak, when the damping is removed the bandpass characteristic returns. I checked using my Siglent sooper dooper sig gen in sweep mode, a perfect double hump bandpass, don't sweep too fast it gets all lop sided.

[Nymrod121]

I've never had to use a wobbulator on any form of receiver IF system - however, there has been one occasion where with hindsight (20:20 vision), I rather wish I'd had access to one that covered 87.5 - 108 MHz:

The P.W. 'Sandown' FM tuner, back in the early '70's (J R Ames et al?) - which formed the basis of my first home-brew FM receiver (it had an RCA application-note-based CA3052 pre-amp plus 2xTBA810AS power amp chips to drive my headphones - so no, it wasn't just a tuner by the time I'd finished).

As I recall, the stated line-up procedure for the front-end was to peak the inductors (turns spacing) for best signal at 88MHz - then peak the varicap bias presets at the 108MHz end. This was duly done, and I was all set for a serious bit of DX'ing ... however, after a few hours trawling up and down the band searching for some of the more regularly obtainable Dutch/Belgian/French services (I lived in Essex back then), I discovered that although the selectivity was very good (Murata C/SFE10.7 ceramic IF filters), in RF sensitivity terms the thing seemed to be 'auditorially challenged' in the middle of the band. One weekend, I arranged to borrow an Advance Q1 VHF signal generator and over a seriously-focussed night-shift session I plotted the 'Sandown' signal strength meter reading vs. RF input frequency (this of course pre-supposed that the Q1's output was flat as well but at the time I had no way of verifying it).

Anyway, I found that there was indeed a large 'suck-out' between 94 and 98 MHz. Looking at the overall shape of the graph I'd plotted, I did a sort of "visual guesstimation" as to the frequencies at which I would need to optimise the L's and varicap bias in order to fill in the RF equivalent of a W**c*st**ershire C**nty Co**cil Hig*ways pot-hole. At 04:30 in the morning, I'd got it just about right - there were compromises at each end of the band but all in all, it was what it was i.e. a half-decent patch job.

I've often thought that had I had access to an RF wobbulator (sweeper), I could have seen the effect much earlier after construction had been completed and that subsequently, I could have optimised the RF front-end performance far more quickly.

I hope this post isn't too far OT ... and thank you for taking the time to read it.

Best wishes

Guy

[MotorBikeLes]

Many of the early '60s NordMende transistor radios refer to AM alignment at 460KHz using a wobbulator. I have a Transita Automatic for which my pal's nephew, still learning languages (dozens) at which he was brilliant translated all the alignment adjustments. They lead off with the wobbulator method, but then offer the alternate of "Alignment with "Messersender" (Sp ?).
FM is of course wobbulator only. I suspect the table radios etc were likewise, but only re-checked the one portable. The same basic circuitry and alignment was used for Tansita Spezial, Transita GT, various Globetrotters, Euro etc.. The instruction was to obtain a symetrical curve as I recall.
Whilst I had a wobbulator suitable for FM, I could only use my Advance E2 for AM at the time.

[Argus25]

So the answer for superhet radios is that:

The wobbulator is useful if the IF design is one where there are two core IF transformers where there could be 4 possible adjustment peak combinations with different band pass characteristics.

Useful for communications radios with special IF's.

The wobbulator is required where the manufacturer points out it is for some radios like Hackers where they recommend against tuning on peaks. (In that case always need a wobbulator)

The wobbulator is good to set up the discriminator or ratio detector in an FM receiver.

But for most radios like single peak'd IF transistor AM Broadcast style radios, typical of most Japanese types made from the mid 1950's till current day the wobbulator doesn't help, unless there was a freak fault affecting the bandwidth without altering the overall gain.

Would most agree with those remarks ?

[Argus25]

Also the next thing I'm trying to get a handle on is what % (roughly) of AM broadcast band transistor radios used double peaked IF's. I have two radios that do, a Nordmende Clipper and a Philips type that have the old flat twin permeability tuned IF transformer body that were used in valve sets. But what about Japanese and American AM broadcast band transistor radios ?

The very first transistor radio, The American regency TR-1 used single tuned permeability 262 KHz IF transformers, their TR4 two years later the same, but 455kHz. Every Japanese AM broadcast band radio after , the early Sony Gendis types were also single tuned, then the coloured tops standardised to the typical black, white and yellow topped coils IF (red for osc)

Also every American AM transistor radio I have seen since the TR1 have used single peaked IF's (but of course I have not seen all)

So it looks like the twin peaked IF's in British and European transistor radios were a hangover from twin peaked designs of valve radios before them?

[Skywave]

Originally Posted by MotorBikeLes

They lead off with the wobbulator method, but then offer the alternate of "Alignment with "Messersender" (Sp ?).

Messersender = Signal Generator.

Al.

[kalee20]

Originally Posted by ukcol

One scenario that does occur to me though. Suppose the capacitor in the IF tuned circuit has changed value...

...which is possible! If capacitance drops, you may never get enough inductance to resonate, in which case the peak will be (approximately) where the core is in the centre of the coil. This will give maximum inductance, but it won't be in tune.

There could then be another peak, where the core is midway between the two coils, giving maximum coupling hence maximum signal transfer. But again, it won't be in tune.

A wobbulator could identify both of these scenarios (assuming you knew what the passband response was supposed to look like).

I don't have a wobbulator - I have used a network analyser's swept oscillator and a storage scope - and although lining-up with a signal generator (or even by ear) is possible, a nice symmetric response is easier to aim for with a wobbulator.

I am sure most alignment instructions were written around the equipment which the average serviceman would possess. If wobbulators had been more commonplace, a slightly nicer sounding radio could have been achieved with a more sophisticated procedure.

[Argus25]

Kalee20,

The scenario you describe though is only for a type if IF transformer design typical of valve radios. For typical single tuned transistor radio transformers there can only ever be one peak if it is there. The primary or longer resonant winding is wound on a ferrite nub along with the shorter tightly coupled secondary. A ferrite cup rotates in threads over that. THe winding coupling is not affected by the position of the ferrite cup. From the 1960's onwards practically every American and Japanese transistor radio had IF coils like this, millions of transistor radios with a near identical coil pack, and if anything is wrong with those coils, or the capacitor that tunes them there is simply no peak, no false peaks.

So I think for all the transistor radios in the world that use these IF transformers,many millions, it seems to me the wobbulator has no utility value in either aligning them or fault finding them over a standard modulated RF test signal.

However, for the radios with other types of IF coil designs, twin tuned etc, the wobbulator could be a handy tool.