If you enjoy building and / or repairing and restoring old radios, one of the most useful pieces of equipment you can have is a wobbulator. They are fairly simple in concept; an oscillator is swept from one frequency to another repeatedly, synchronised with the timebase of an oscilloscope…or better still, driven by the ‘X’ output from the oscilloscope. It’s important to note that many cheaper oscilloscopes don’t have an ‘X’ output, so we need a sawtooth generator.
The oscillator is also passed through the circuit under test, normally a bandpass filter or IF strip, and the level of the signal at the output of the circuit under test, is fed to the ‘Y’ amplifier input on the oscilloscope; the level of the signal is derived using a detector of some form, often just a diode and capacitor. Most wobbulator designs were either based on valves with motor driven capacitors, or were limited in their functionality, only covering a small range of frequencies.
Back in April/May 2003, Radio Bygones published a design for a very versatile wobbulator in their issue number 82. The design was by Raymond Haigh…you can find the article here. This design is compact, solid state and portable.
I won’t repeat the article text on this page, feel free to download and use the article to build a wobbulator. Note that the article is Copyright Raymond Haigh 2003.
One big problem is that there are four Toko coils used in the design that are now obsolete; the coils are mentioned in the article as being available from Jabdog…this is no longer true. Tony Nailer of Spectrum Communications, has kindly provided a set of equivalent coils which he sells on eBay. Rather than give you the item number, as it may change, here’s a link to Tony’s eBay shop, spectrumcomms. Or just search on eBay for ‘raymond haigh wobbulator’ and his coils come up. Tony has lots of other modules and bits and pieces…take a look at his other items whilst you’re there.
The original TOKO parts were:
RWO6A7752EK (green core)
154FN8A6438EK (violet/deep red core)
154FN8A6439EK (yellow core)
KXNK3767EK (pink core)
The varicap diode used to tune the oscillator in the wobbulator, is specified as a KV1236, which is actually a snap apart pack of two. The KV1235 is the same but in a pack of three. Both are available at the time of writing on eBay and either ‘pack’ has what you need.
A final word of warning, apparently on the PCB overlay in the article, the range 3 & 4 coils need to be swapped over. I haven’t built my wobbulator just yet, hence ‘apparently’. If you accidentally build it with them in the wrong place, I’d advise simply swapping the wires around on the range switch, rather than risk damaging the coils by physically swapping them.
For those of you who have an oscilloscope without an ‘X’ or ‘Horizontal Output’, Colin Armstrong designed a suitable circuit using a 555 timer and a few other bits; note that the 555 used was a CMOS version and other versions may not work. The circuit diagram for that is here.
There is quite a bit of discussion regarding this design on the internet, especially on the Vintage Radio Forum. I’d suggest using your favourite search engine to look for ‘Raymond Haigh Wobbulator’ and you should get a lot of hits.
At one time you could get PCBs for this project…they are no longer available so don’t ask me or others who have been involved in the project. You could make your own PCB or use Veroboard or something similar.