Wobbulator and DFM

[Gerry O’Hara]

I recently constructed a combined wobbulator and digital frequency meter (DFM) unit to assist in the alignment of sets (a wobbulator is a swept-frequency oscillator, the centre-frequency of which is set at the centre of the receiver IF passband and sweeps either side of this frequency to allow accurate alignment of a sets IF. It is also handy for aligning FM discriminators). I have been using a Wavetek Model 164 sweep generator for this purpose – a very nice and versatile instrument – but found it not to be very stable on the higher IF frequencies (eg. 10.7MHz).

I found a simple wobbulator circuit in the April/May issue of ‘Radio Bygones’ magazine (https://www.radiobygones.com/issues.html) and decided to construct it, together with adding a frequency counter into the same box for convenience. The standard wobbulator circuit ranges cover from below 400kHz to 14Mhz, but could be modified for lower or higher frequencies (eg. I needed a range around 37MHz for a project I am working on). I had previously made-up a DFM from a kit (an FCC-1 from Norcal, https://www.norcalqrp.org/fcc1.htm) that works up to around 50MHz or so, and this seemed ideal for the purpose. The two circuit boards were constructed and built into a small cast aluminum case I had picked up for free at a recent CVRS meeting. I made the switching such that the DFM can be used independantly of the wobbulator. I’ve found that the wobbulator oscillator has adequate stability for the type of service I have in mind and the DFM works a treat. Incidentally, the FCC-1 unit has facility for programming-in different frequency offsets (plus many other features) and so can be used as a digital frequency display on virtually any receiver by connecting the DFM input to the sets local oscilator (not bad for $40).

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[cvrs]

at our past radio club meeting Elmer gave us an informative tech session , during his session he suggested that people not try to attempt to adjust the IF’s as they are not often the cause of trouble, and that many radios were better off if the radio is left with it’s IF cans not tweaked because it is possible to allign then in such a way that the radio is able to recieve stations, but at the same time, to introduce a condition where the recievable bandwidth is made so narrow that the sound quality is affected.
I have not alligned a great number of radios but in most allignment instructions, that I have gone through ( in cases where there were good instructions) it seems to call for the IF’s to be alligned first , then the bandspread and tracking adjustments follow.

Now I am trying to wrap my head around how this works. The way I understand it the IF transformers are designed to oscillate at a predetermined IF ( usually 455) Other things are incorporated into the design of the coil such as the number of turns, the spacing of the coils, the capacitance and the type of wire which may afeect the "Q" or efficiency of the coils.

ok I can mess up the reception by making it so the transformers are misalligned in such a way that they are not resonating at 455 and thus not letting signals through, but in order to change the way the coil reacts as far as the width of the range of frequencies which are passed through would I not need to also alter some other factor such as coil spacing or capacitance? I have heard that it is best to use a meter ( not volume) to peak the IF’s for maximum output , but with the volume control turned low as to not involve the AGC circut.

He may have meant to not change the capacitors in the IF cans as they could likely be (more stable) mica capacitors which are reliable and not usually replaced, as doing so could introduce problems by un necessarily replacing a good cap with one of perhaps slightly different capacitance,thus causing the above effect.

I have seen some radios that have paper caps used in the cans , and have usually replaced them as a matter of course in recapping.

I have heard that there are some capacitors which are less likely to cause issues with leakage , and I wonder if these fall into that area. caps like used in the power supply for example have lots of potential across the plates, but caps in an IF can aren’t really in a circut where much current is flowing, thus not much voltage drop across the coils in the IF cans, so perhaps this is a factor in the capacitor’s likleyness to be a failure point.

I guess a little knowlege brings more questions and , when I saw the post about the wobbulator I had to question wheather this is an instrument that one could use to aid in alligning, to avoid clipping audible frequencies?

Should paper caps in IF transformers from early thirties sets be changed ritually for leakage? or are they better left alone? Any comments/ opinions appreciated.

my delemma is that when restoring a set , I my main objective is usually to improve the performance as much as I can without attempting to redesign the circut. If I am to resist the urge to tweak the IF adjustments, how would I know if this may have made a drastic improvement in volume , reception , tone quality etc.

I am not satisfied with leaving the screws alone, this reminds me of my dad telling me not to screw with the air fuel mixture on the old outboard. I remember him so often saying "that carb has never been adjusted, don’t turn the @$## screws".. 🙂

Phil

[Gerry O’Hara]

Hi Phil,

Yes, the wobbulator is an IF alignment tool and provides a way to visualize the actual passband of the receiver you are working on. This the ‘too-sharp peak’/’two-humped curve’ extremes that Elmer warned against should not occur. Ed and I intend to bring it along to the next meeting and give a demo.

[Ed Stone]

Just a note to remind folks that Gerry plans to give a presentation and demonstration at the CVRS meeting in Burnaby on January 18 of how to align a radio using the wobbulator described in this post. I will bring along the Eddystone EC10 described on one of my other posts for the demonstration.

Ed

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[Gerry O’Hara]

A copy of Gerry’s presentation can be downloaded in pdf format from the ‘Reference’ section to be found under the ‘Electronic Restoration’ section of this Forum.

[Bill W-S]

Hi Gerry,
I have wanted one of them for some 45 years! How do I get the plans to build it? I can’t get any information from the website you listed. Bill PS I tried once to build one using an electric motor turning a tuning capacitor connected to a Hartly oscillator, it didn’t work. Bill

[Gerry O’Hara]

Hi Bill,

The wobbulator article is in Radio Bygones Issue #82 (April/May, 2003) – back-copies of the Radio Bygones magazine, pubished in the UK, can be purchased from the website I provided the link to (for $5 each I think).

Radio Bygones can also provide the printed circuit board for abot $10, though not a kit of parts – most are standard items that can be purchased this side of the pond, though I did buy the TOKO coils and the varicap from one of the suggested suppliers (Jab Electronic Components in the UK, https://www.jabdog.com/).

I have made a couple of wobbulators prior to this one and have found this is much better than my previous ones. I also find it better for ‘wobbulating’ than a commercial (Wavetek Model 164) sweep generator I have. Please note that you will need a scope with a ramp output, though if your scope does not have this facilty it should be straightforward to tap into the timebase circuit (as I did in one of mine).

Gerry

[Gerry O’Hara]

Update – I tried adding the 40MHz range using a small tapped coil mounted directly on the range switch, but found that the maximum frequency obtainable was some 25MHz due to self-inductance of the leads from the circuit board to the range switch. In order to reach higher frequencies I think you would have to replace one of the coils on the circuit board and use very short leads (or hard-wire the board for one range), or alternatively, build a single-range unit. While I had the box open I did undertake a few modifications to increase the span of each range slightly and to improve tuning precision, stability and repeatability:

– replaced R4 (22k) tuning pot with 20k high-quaility 10-turn pot
– replaced R3 (1k) fine-tuning pot with a high-quality 220ohm pot
– replaced R2(4.7k) with a 3.3k resistor
– replaced R5 (33k) with 15k resistor
– rewired the on-off switch such that both the wobbulator and the DFM switch on together (I found I often left one of the units on and ran the batteries down! – also, it was a nuisance having the wobbulator on-off switch on the tuning pot as the frequency had to be re-adjusted every time I used the unit.

Gerry

[Gerry O’Hara]

If you are interested in using a wobbulator for IF and FM discriminator alignment – see my new article on the Eddystone User Group website at https://eddystoneusergroup.org.uk/Restoration%20projects/Restoration%20page.htm (look for ‘Technical Short #23’).

Gerry

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