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Old 26th Sep 2020, 8:00 pm   #1
Bazz4CQJ
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Default Homebrew Wobbulators-Current Thoughts?

In another thread on signal generators https://www.vintage-radio.net/forum/...d.php?t=171268, the use of wobbulators to align IF amps has come up.

The discussion on wobbulators has centred around the design by Ronald Haigh, published almost 20 years ago in BVWS. The design has good "heritage" and has been built by a number of people on this forum, but the commercial coils used are now quite obsolete. My guess however is that they weren't special in any way, and could be substituted without much effort. Does anyone know if those Toyo coils can still be found anywhere?

But just to open up the discussion, does anyone have views on whether or not the Haigh design is still the way to go, or are there other (more recent) designs that offer as good or better way forward, and perhaps be easier to build.

B
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Old 26th Sep 2020, 9:24 pm   #2
Ambientnoise
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Default Re: Homebrew Wobbulators-Current Thoughts?

This is the link which triggered my use of DDS chips. This would be my way to go for frequency generation but then I am happy to cobble something to do the job and not end up with an lab instrument.

http://m0xpd.blogspot.com/2014/04/rf...generator.html

A google search of “wobbulator DDS” will give lots of leads. Earlier contributors have correctly pointed out that screening and decent amplitude control is likely to be the most difficult aspect of a home built item.

This is the DDS module though you can halve the price if you wait for a delivery from China.

https://smile.amazon.co.uk/ARCELI-AD...s%2C182&sr=8-7

Last edited by Ambientnoise; 26th Sep 2020 at 9:28 pm. Reason: Add link
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Old 26th Sep 2020, 9:48 pm   #3
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Default Re: Homebrew Wobbulators-Current Thoughts?

The use of a DDS gets around the problem of trying to get a varactor tuned oscillator reasonably linear.

If your wobbulator isn't linear then all the bandpass and discriminator tuning you do with it will be distorted in the opposite sense.

If using a trad type analogue wobbulator or sweeper, you need a marker generator to be able to have any confidence in frequencies.

I've got three options here...

A Marconi TF2008 sig gen which is reasonably clean, has crystal markers and an output for a counter. It can do narrow through to wide sweeps. It's a full blooded lab sig gen so it does AM and FM and has the attenuator and screening to go well below a microvolt for sensitivity tests. For sweeps etc, just add a scope.

HP 8657 sig gen. millihertz steps and I can programme sweep limits and let it go. Add a scope.

HP8591E spectrum analyser with tracking generator. This qualifies as cheating! If I want to watch DC or demod outputs across a sweep, I sync a scope up to it.

For just aligning radios and discriminators, these are overkill and apart from the Marconi TF2008 are too expensive to seek out, unless you need some of their other capabilities.

As far as the homebrew ones are concerned, go for it if you want. You'll learn a lot building one and using it. A design intended for home construction may be best. One circuit posted on here recently for a commercial one uses a saturable reactor type tuning coil which would be a major undertaking to duplicate. Rather than having one of those per band, it has one and then mixes it with other oscillators to cover a wider centre frequency range.

The varactor diode based hobbyist designs will work, but watch out for non-linearity. Use another sig gen to create a marker and stick a counter on it.

Just keep your eyes open at future radio rallies and club sales (whenever such can be held again...) because sometimes lab grade instruments and sometimes the prices are remarkably low. THere's no substitute for being in the right place at the right time. You can't predict it, it's pure luck. The more you keep your eyes open and know what's what, the luckier you get.

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Old 26th Sep 2020, 10:36 pm   #4
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Default Re: Homebrew Wobbulators-Current Thoughts?

Just to note, I already have one of the Haigh wobs, bought in a Silent Key sale, but I've never got around to using it on the project which I was doing, and I'm not sure whether it is complete or even if it works. What's there, seems to be quite well built but the constructor must have been only interested in one frequency range (I suspect 10.7MHz); there is no range switch and only one coil (a Toyo I think). I'll see if I can check it out further soon, because I would like to have it available. Should it prove to be non-working, and not easily sortable, then I will have to consider the options. Sorry to advise, but it is not for sale (unless I cannot get it going!). I have two vintage "Workshop" signal generators, but neither has a sweep function

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Old 27th Sep 2020, 2:09 pm   #5
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Default Re: Homebrew Wobbulators-Current Thoughts?

Years before I got my Siglent spectrum analyser I needed to tune a 2 Metre 4 pole band pass filter.. I built a VFO swept with a nice linear sawtooth.. and added two marker generators... with identical inductirs.. leaving out the "wobbly bit".. unfortunately, the schematics were lost in a computer accident which lost all my early pcb designs and schematics..... but it was fairly easy to build.
Tuned up the filter with ease..... the monitor I was using was a Marconi 8" or there abouts sweep display. The monitor sweep was driven from the sawtooth generator.
I scrapped the generator, recently.
My circuit could be easily scaled down for other frequencies.
Incidentally Toko coils lend themselves to rewinding very easily.
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Old 27th Sep 2020, 8:10 pm   #6
David G4EBT
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Default Re: Homebrew Wobbulators-Current Thoughts?

Quote:
Originally Posted by Bazz4CQJ View Post
Just to note, I already have one of the Haigh wobs, bought in a Silent Key sale, but I've never got around to using it on the project which I was doing, and I'm not sure whether it is complete or even if it works.

B
Just a little note of caution to highlight a wiring error in the wiring of the range 3 & 4 coils, though in your case Bazz, it might not be relevant as you only have one coil in situ. The circuit diagram in the article is correct, but the wiring diagram has an error. On the PCB overlay in the article, the range 3 & 4 coils need to be swapped over, as a glance at the circuit in comparison with the layout will show. I didn't discover that until I checked the coverage of the ranges on my frequency counter.

I built mine back in 2013 ten years after the article was published and even then, obtaining the coils and the varicap diode was difficult.

The original spec Toko coils for the Raymond Haigh RB Wobby have long since been unavailable. They were:

L1 - RW06A7752EK (green core)
L2 - 154FN8A6438EK (violet core)
L3 - 154FN8A6439EK (yellow core)
L4 - KXNK3767EK (pink core)

Jabdog did have some equivalents, but I doubt they still have them in stock, albeit he does stock a range of TOKO coils which could perhaps be repurposed and formers on which to wind your own. (Only one winding on each coil is used, which is tapped).

http://www.ebaystores.co.uk/JabDogEl...4.l1581&_pgn=2

Circuits abound on internet both solid state and valved for simple and complex woobulators. There was a Television Mag article which ran from Sept to Nov 1961. It employed six valves, and only covered 465 kHz, 10.7 MHz and 24 - 38 MHz. Far more hours of effort would need to go into building that than the hours that would ever be spent using it. That's fine for anyone whose hobby is building test gear rather than in radio/TV restoration.
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Old 27th Sep 2020, 11:18 pm   #7
Bazz4CQJ
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Default Re: Homebrew Wobbulators-Current Thoughts?

Thanks for that info Dave. I've just taken another look at this one with a magnifying glass. It's got a Toko L132 PC651 coil, the core is pink. I wonder where that fitted in to the scheme of things? A quick look on Google suggested that a company in Athens is/was selling them, but there's no tech data

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Old 1st Oct 2020, 10:55 am   #8
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Default Re: Homebrew Wobbulators-Current Thoughts?

If you are (or anyone else is) interested in the DDS route then do give me a shout. I built one years ago - there's a little info here http://dds-60-siggen.blogspot.com/ - I notice my link to the schematics etc is broken but if you want them I could dig them out.

It works fairly well and can be used for FM too at a push. Since I moved I've not been able to find the actual device I built and was musing the other day on maybe building another!
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