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Old 1st Nov 2020, 7:28 pm   #1
Electronpusher0
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Default Haigh Wobbulator with Timebase

This post describes my construction of the Haigh Wobbulator with built in timebase and sourcing the components
It started for me with this thread:
https://www.vintage-radio.net/forum/...53#post1296453

This thread discussed marrying a Haigh wobbulator to an old scope.
David (G4EBT) suggested using an external timebase and suggested a circuit designed by Armstrong. I analysed this design in this post:
https://www.vintage-radio.net/forum/...6&postcount=15
and suggested using a current mirror for the constant current source, in that post I also said I was inspired to build a Haigh wobbulator.

The original article by Haigh was published in Electronics Today International Vol 27 Iss 13 Dec 1998 available for download at the Worldradiohistory website here:
https://worldradiohistory.com/UK/Ele...1998-No-13.pdf

The coils and Varicap diode for this project have become unobtanium but I found this document on the web:
https://www.robkalmeijer.nl/techniek...e44/index.html
That suggested that when selecting Toko 10K coils “In an application where only the primary winding of a coil is used (that is the pin 1-3 winding), there's not much to choose between coils with the same inductance”.
The site also tabulates the Toko coils in order of ascending inductance.

The original coils specified for the Haigh wobbulator are
7752 - Green
6438 - Violet
6439 - Yellow
3767 – Pink
Looking at the table on the web site I decided to try and obtain the next coil down in each case, except the green
These were
3333 – Violet
3334 – Yellow
3335 – Pink

The 3333 and 3335 were in stock at JAB Electronic Components e-bay store.
For the yellow I purchased an equivalent coil from spectrumcomms (again on e-bay) “Spectrum 5u3H coil is a direct replacement for the TOKO type 3334 or 3337”
For the Green coil I used a 10mm black IF transformer from an old transistor radio and disabled the internal capacitor by breaking it with a sharp point.

I am happy to report that all of these coils could be tuned to the desired range.

For the varicap I used a 1SV149 which are still available at reasonable cost.
I etched a pcb for the wobbulator using a paint pen as resist.
For the timebase I indeed modified the Armstrong circuit posted by David to use a current mirror. The schematic is attached.

T1 and T2 are general purpose PNP small signal transistors. These must be in close thermal contact so it is advantageous to use plastic devices and strap them flat face to flat face with heatsink compound.

I discovered a possible problem with the original Haigh design around the sweep width control. This is a simple potentiometer that sets the amplitude of the sweep waveform.
In the original design the ACW end of the pot is connected to 0V. Since the CW end is connected to a 6V p-p waveform with a mid sweep voltage of 3V it means that the mid point of the sweep will change as the sweep width is varied.
I decided it would be an advantage to keep the centre frequency fixed as the sweep width is varied. The op-amp used in the sweep generator is a quad device so I used a spare section to derive a 3V offset voltage to which I connected the ACW end of the sweep width pot (instead of 0V). Now when the sweep width pot is varied the mid sweep voltage remains unchanged.

David kindly sent me some front panel layouts but since I decided to use a different size of case I created a new design based on David’s using Front Design. (also attached).
This fits this case:
https://www.amazon.co.uk/Magiin-250x...s%2C200&sr=8-9
This is available from several vendors including on e-bay.

I decided to use batteries only to keep it simple.
It all works in as much as it generates the required frequencies and sweeps as required but I have not yet tried it on a radio IF. That’s the next step.

Peter
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Old 1st Nov 2020, 7:29 pm   #2
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Default Re: Haigh Wobbulator with Timebase

More pictures

Peter
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Old 1st Nov 2020, 7:40 pm   #3
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Default Re: Haigh Wobbulator with Timebase

Well, that is all very interesting, useful and impressive. I'm in the awkward position of having acquired a 90% completed Hague from a Silent Key sale and I'm now bolting on to that the Armstrong timebase, utilising David's (EBT) pcb.

I suspect the finished product maybe 'a bit less neat' than your version!

B
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Old 1st Nov 2020, 7:49 pm   #4
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Default Re: Haigh Wobbulator with Timebase

Thanks Bazz,
the only (slight) criticism I have of the Armstrong timebase is that using a series of diodes as a reference means that there will be a shift in sweep rate with temperature, probably not noticable however.
It might be worth bearing my observations about the centre frequency shift with sweep rate setting in mind however.

Peter
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Old 1st Nov 2020, 11:17 pm   #5
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Default Re: Haigh Wobbulator with Timebase

OK, that's noted.

B
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Old 2nd Nov 2020, 6:08 pm   #6
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Default Re: Haigh Wobbulator with Timebase

I have just noticed that R3 on the timebase schematic is wrong, it should be 47 ohms, not 1K!

The problem is that in sPlan 7, which is what I use for schemaitics, when you add a resistor it defaults to 1K, you have to go back and click "properties" and change it. I guess I missed R3.

Updated Schematic attached.

Peter
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Old 2nd Nov 2020, 8:31 pm   #7
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Default Re: Haigh Wobbulator with Timebase

You're doing a good job Peter - maybe you should offer a kit of parts, but I guess the market may be limited .

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Old 2nd Nov 2020, 11:10 pm   #8
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Default Re: Haigh Wobbulator with Timebase

Thanks Bazz, as you say, could be a limited market.

I just wanted to share how I sourced the components and the timebase circuit in case anyone else wants to build one.

Peter
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Old 3rd Nov 2020, 1:03 am   #9
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Default Re: Haigh Wobbulator with Timebase

Peter, could I politely suggest using the connection dot for where wires connect with each other.

I's pretty obvious in your drawing where the various lines connect, but it might confuse the less experienced who might want to have a go.
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Old 3rd Nov 2020, 8:16 am   #10
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Default Re: Haigh Wobbulator with Timebase

Hi Terry,
thanks for the suggestion.
I have always followed the convention of staggering the connections. If a line comes to another line (at right angles) and stops, its a connection. If a line comes to another line at right angles and continues it does not connect but crosses.
If two lines approach a line from opposite directions and need to connect to the line they must be staggered so it is clear that they join, not cross.

However dots may be clearer and I will take up your suggestion in future when posting schematics I generate.

Peter
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Old 3rd Nov 2020, 8:52 am   #11
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Default Re: Haigh Wobbulator with Timebase

Quote:
Originally Posted by Bazz4CQJ View Post
You're doing a good job Peter - maybe you should offer a kit of parts, but I guess the market may be limited .

B
On second thoughts, I have some spare parts, I will put together 4 kits of coils and semiconductors and offer them on the appropriate section of the forum.

Peter
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