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Old 21st Feb 2015, 6:17 pm   #1
David G4EBT
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Default 'Coil Coverage Test Unit'

Back in 2013 when winding a replica Repanco DRR2 coil, I thought it might be handy for future use to build an updated version of a 'Coil Coverage Test Unit' which was 'Suggested Circuit No 224' by G.A. French in July 1969 Radio Constructor magazine. I've at last got round to finalising that project. The basic idea is that if you have a calibrated variable capacitor and put a coil in parallel with it, then feed it with a signal from a signal generator, sweeping the generator across the dial, and when the circuit reaches resonance, there will be a dip on the meter of the test unit, at which point, the frequency can be noted from the signal generator.

Thus for example, if the tuning capacitor in the equipment for which the coil is being designed covers say 50pF to 500pF, the tuning cap of the Coil Test Unit can be set to those two extremes, and the frequencies at which a dip on the Test Unit meter is observed at those extremes will indicate the frequency coverage of the coil. If the frequency range is too low, turns can be removed from the coil to bring it to the desired frequency. If the frequency is too high, turns can be added. Obviously it's much easier to do this with the coil out of circuit than when it's built into the equipment.

'Suggested Circuits' were just that - they were not finished projects so there were no layouts or constructional details. Hence, I designed and made a PCB to my own requirements, and developed the circuit to suit my needs. The original design had the facility to check the value of unknown capacitors, but capacitance meters nowadays are so cheap that there seemed no point in incorporating that requirement, so I dispensed with the switch and sockets shown on the original circuit.

Back in the late 1960s, frequency counters were beyond the reach of most amateurs, but that isn't so nowadays. I therefore added a frequency counter socket to give a more accurate readout than relying on the signal generator dial. The original design used a long since obsolete silicon diode - a Lucas D000. After seeking advice from others, I used a 1N4002 as being close enough to the Lucas spec. On the original circuit, both R1 and R4 were 'select on test' so I decided it would be easier to incorporate two pre-set pots for adjustment purposes - R4 being to set the meter to FSD, R1 to set the collector current to 4 or 5mA. I fitted a link so that the collector current could be monitored with a mA meter in circuit while R1 was adjusted, then when set, I fitted a link in place.

The air-spaced tuning capacitor ranges from 50pF to 550pF. (Few tuning caps which tune to 500pF or more, will tune much below 50pF at the lower end). I designed a front panel and calibrated the scale of the tuning cap in situ. (The scale isn't linear). It doesn't matter whether the meter goes to full scale, so long as it goes across far enough to enable a dip to be observed at resonance. I had what I thought was a 100uA edge-wise meter, but in fact it's 200uA, so doesn't go to full scale. Its merit is that it's a small edge-wise meter (approx. 35mm x 18mm) so is ideal for the job. (I've searched for a similar 100uA or 50uA meter but the only one I've come across is from Anders - not too expensive at 6.75, but the killer punch is the post - 12.00, so for now, this one will have to do). The null is even more dramatic to observe on the 'scope. I made an acrylic cursor for the knob, and screwed it to the rear of the knob with c/s 6BA screws.

As has become my habit for housing home-brew test gear projects, I made a comb-jointed box from some scrap mahogany. (18cms wide x 14cms high x 10cms deep).

I've tested the unit on several homebrew coils and it seems to be working nicely.

It's not the sort of thing that many will want to build, for the limited use to which it will be put, but it's no chore to me as I enjoy building simple test equipment, adequate for my needs.

I've attached the modified circuit, the PCB layout I designed, the component overlay and a couple of pics of the finished project. (The PCB is 12cms long x 7 cms wide).

Parts List:

R1: 2M7 PRE-SET POT
R2: 470R
R3: 270R
R4: 10K PRE-SET POT
R5: 470R- 1K (TO SUIT LED)
C1: 0.01 uF
C2: 500 pF VARIABLE CAPACITOR
C3: 0.01 uF
C4: 0.01 uF
C5: 0.1 uF
D1: 0A79 GERMANIUM DIODE
D2: 1N4002 SILICON DIODE
TR1: BC168C or BC184L
METER: 100 uA
LED.

The original Radio Constructor 'Suggested Circuit No 224 in July 1969 Radio Constructor can be found at this link:

https://9f589047042d12b866cecc16cd9c...f.html#jul1969

Hope it's of interest to someone on here.
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Old 21st Feb 2015, 6:23 pm   #2
David G4EBT
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Default Re: 'Coil Coverage Test Unit'

For the curious, here's an inside view of the unit. (The tuning cap is from my spares box - nominally 2 x 250pF in parallel).

The copper sheet on which it is mounted is a scrap piece of 22mm copper pipe, annealed, slit along its length and flattened out.

'Waste not - want not' is the motto here at 'Bleak House'!
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Old 21st Feb 2015, 7:43 pm   #3
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Default Re: 'Coil Coverage Test Unit'

Hi David, What a lovely piece of equipment you've constructed! It's like a useful work of art.
David
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Old 21st Feb 2015, 7:50 pm   #4
Mike Phelan
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Default Re: 'Coil Coverage Test Unit'

Excellent, David

As to how you find time ...
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Old 21st Feb 2015, 9:06 pm   #5
David G4EBT
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Default Re: 'Coil Coverage Test Unit'

Not sure that I do - it has taken two years for this little project to come up to the top of the pile Mike!

Thanks too, to David - 'Backtoreality' for your kind comments.
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Old 21st Feb 2015, 9:37 pm   #6
HamishBoxer
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Default Re: 'Coil Coverage Test Unit'

Very nice Well Done without a doubt.
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Old 21st Feb 2015, 9:51 pm   #7
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Default Re: 'Coil Coverage Test Unit'

What an excellent little project, superbly realised.

Have you ever considered submitting a project like this to one of the vintage magazines? I notice you are a member of the BVWS, and a 'Radio Bygones' subscriber, I am sure your projects would be welcome with open arms!

As a recent 'article author' myself, there is a real pleasure in seeing your own work & pictures on a magazine page.


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Old 22nd Feb 2015, 12:31 am   #8
David G4EBT
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Default Re: 'Coil Coverage Test Unit'

A good point to raise Simon.

I have in fact has several articles published in the BVWS Magazine - most recently in Autumn 2014, in which I had two articles - a simple coil winder powered by a sewing machine motor for such tasks as re-winding field coils, the second article on making valve base adaptors to extend the range of valves that can be tested on a Taylor 45D Valve tester.

Summer 2013 carried two articles - one on making a 'universal router jig' for making slotted replica radio backs, and a second article on converting side-contact EBL1 valves to the rarer octal based EBL31 as used in the Ecko A22 rado. (I had an earlier article published which also concerned EBL31 valves).

I must say that Carl Glover, who edits the magazine, is very supportive and encouraging and works wonders in setting out the articles to best advantage. As to the current thread - the Coil Coverage Test Unit, I'm not sure what the copyright situation is with regard to the original article in Radio Constructor, or whether given that it was a 'Suggested Circuit' which I've developed further into a completed project, (which after all was the original concept of 'Suggested Circuits') would mean that copyright wouldn't be an issue. It's something I could raise with Carl if he feels that the project would merit space in the Bulletin.

It could of course be built on stripboard or tagboard or whatever and built into a project box. I know that not everyone is geared up for making PCBs or comb-jointed wooden cabinets or would want to - it's just my take on things. Incidentlly, I drew the front panel using MS Paint and Photoshop Elements 7, which is about the limt of my rudimentary CAD skills.

Ideally, I'd like to track down a small 100 uA or 50 uA meter similar to the 200 uA one in the first post above.

Lots of square ones available cheaply, but so far, not smaller edgewise ones.
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Old 22nd Feb 2015, 12:57 am   #9
Ti Pwun
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Thumbs up Re: 'Coil Coverage Test Unit'

Very useful piece of kit. And timely, too, given that I'm in the process of finishing my coil winder and I'll need to wind coils for the frequency generator I'm building. Definitely worth building one of these units.

I love the way you go a step beyond functional and make nice enclosures for them with great looking front panels - I totally agree that they would be excellent projects for submission to the various mags.

Good stuff. Thanks for posting this.
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Old 22nd Feb 2015, 3:10 am   #10
Colin Ames
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Default Re: 'Coil Coverage Test Unit'

Very nice David. Your projects always turn out so well.

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Old 22nd Feb 2015, 8:47 pm   #11
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Default Re: 'Coil Coverage Test Unit'

Quote:
Originally Posted by David G4EBT View Post
I have in fact has several articles published in the BVWS Magazine - most recently in Autumn 2014, in which I had two articles - a simple coil winder powered by a sewing machine motor for such tasks as re-winding field coils, the second article on making valve base adaptors to extend the range of valves that can be tested on a Taylor 45D Valve tester.
Well, I have read your excellent articles and enjoyed them. I'm sorry David I didn't realise that these were yours, please accept my apologies.

I concur that Carl works wonders with the text & photos, it really does turn out to be a very professional presentation.


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Old 30th Mar 2015, 7:23 pm   #12
David G4EBT
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Default Re: 'Coil Coverage Test Unit'

I mentioned in post #8 that I'd try to track down a small 100uA or 50uA edgewise meter to replace the 200uA meter that I used, but that quest came to nothing.

Not only that, neither could I find any small 'SEW' type meters (about 40mm square) that used to abound. The smallest panel meters now seem to be 62mm wide x 55mm tall. Furthermore, the tuning cap I'd used shorted out its vanes when fully closed and I couldn't discover where the short was, so I needed to fit a replacement tuning cap, which meant a new front panel calibrated to suit that cap was called for. I found a suitable tuning cap in my spares box, which ranged from 35 - 550 pF in my spare box.

As a wider and taller front panel was needed to accommodate the larger meter, I scrapped the original test unit and built another one (and yet another comb-jointed box!). I sourced a 50uA from e-bay (4.23 'buy it now' post free) - and while I was at it, I thought it would be useful to add a 'scope socket on the front panel so that the null can be observed on the scope as well as the meter. Not in any way necessary, but for the sake of a slight alteration to the PCB layout and an extra socket on the front panel, it was no great effort.

The more sensitive meter means that the unit will work well on a signal generator with low output, and with the meter at full scale deflection the null is very pronounced and unambiguous.

I appreciate that it's not the sort of instrument than many home-brewers would have at the top of their list of projects to build, so I realise that it will be of limited interest, but nevertheless, I've attached the updated circuit, PCB artwork and component overlay, and some pics of the 'MK2' version.

A point worth mentioning should anyone did want to built such a unit, is that the tuning cap must be 'floating'. (IE, it must not be grounded). Initially, I'd put it on a metal bracket attached to the front panel, which is made from scrap double sided PCB, then when it wouldn't work, the penny dropped that I'd ground the tuning cap. I therefore fitted it on insulated stand off pillars and all was well.

The front panel was printed on coloured card, encapsulated, then a clear protective panel of 2mm acrylic was cut, drilled and fitted in front of the printed panel.

A lot of fun (for me, anyway) for a negligible outlay. All my projects are strictly 'MFJ' - 'made from junk'!

Hope that's of interest.
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Old 23rd Apr 2015, 9:14 pm   #13
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Default Re: 'Coil Coverage Test Unit'

A very interesting article.
You have done a great job of building it David.

I have been winding coils for my SW receiver, and this could come in useful.
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Old 24th Apr 2015, 11:21 am   #14
Mike Phelan
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Default Re: 'Coil Coverage Test Unit'

Up to your usual standards, David.
Documentation, pictures and a non-homemade appearance!
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Old 24th Apr 2015, 5:15 pm   #15
David G4EBT
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Default Re: 'Coil Coverage Test Unit'

Thanks for you kind comments and encourgement guys - much appreciated!
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