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David G4EBT · 21st Feb 2015, 6:17 pm

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.

21st Feb 2015, 6:17 pm	#1
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,761	'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. Attached Thumbnails __________________ David. BVWS Member. G-QRP Club member 1339.