I mentioned in another thread concerning CAD that I'm not proficient in any CAD packages but I furtle about with MS 'Paint' and Photoshop 'Elements 7' to draw circuit layouts, PCB artwork and component overlays for homebrew projects. I've always been interested in homebrew test gear and have made lots of little projects, most recently the RB wobbulator, a zenner diode tester, and the 'Yorkie' ESR meter - a much modified version of the design by Lawrence Glaister, VE7IT, a Canadian Radio amateur.
My latest project is a 'Coil Coverage Test Unit' based an a 'Suggested Circuit' in July 1969 Radio Constructor Magazine. The wonderful long-running 'Suggested Circuits' series in the magazine was authored by G.A. French. The articles gave the circuit and how it operates, leaving the constructor to build, test and develop the circuit and layout to suit their own requirements.
The basic concept of this circuit is to ascertain the frequency coverage of an RF coil when in a tuned circuit with a tuning capacitor of a given spread of capacitance. The unit has a tuning capacitor built into it with a calibrated dial, which can be set to either end of the capacitance range of the tuning cap to be used in circuit.
In use, a home-wound (or unknown coil) is connected to the unit and a signal injected into the unit from a signal generator. The tuning cap is set to each end of the spread of the cap with which it is intended the coil be used, and the sig genny is swept across the band until there is a dip on the meter in the unit, in the same fashion as with a GDO. The tuned frequency can then be read off the sig genny dial.
As designed, the unit can also be used to ascertain the capacitance of a cap of unknown value, but time has moved on and digital capacitance meters are nowadays so cheap (see other posts on the forum about an excellent kit for under £15.00 which 'Big Al and I have built), that it's pointless to include that facility in this coil coverage test unit, so I've omitted that.
Other amendments I've made are to include an output to a frequency counter which will give a more accurate indication of the actual frequency than the dial of a sig genny. (Again, back in 1969 counters were beyond the pocket of amateur constructors, but are now cheap and commonplace). In the original design there are two resistors which needed to be selected on test - one to set the current drawn by the unit to 4mA, the other to set the meter to FSD. I've substituted those for pre-set pots so the pots can be adjusted as required, then and left in circuit. I've also added an LED and series limiting resistor to give an indication that the unit is switched on.
It's still under development and is just something I think will be nice to have, and it remains to be seen if it will work as desired! It's much easier to test a home-wound coil (indeed any coil) out of circuit than to build it into say a receiver, only to find that it needs turns to be added or removed to get the desired coverage.
It's simple enough to calculate the inductance that a coil needs to have by using any one of the many online resonant circuit calculators. We can then wind a coil and check on an inductance meter if it is of the correct inductance, but really, only when it's in circuit can we find out of it performs as it should.
I've always enjoyed building simple receivers and what sparked my interest in making this coil test unit was when I built the 1959 BBC 'Focus' Gilbert Davis transistor radio design, which originally used the Denco DRR2 coil. I wound my own coil and had to keep putting in into the radio and taking it out to get it right.
I've attached pics show my modest efforts in using 'Paint' and Photoshop 'Elements 7' to draw a circuit layout, PCB layout and component overlay. I've added the circuit from the original article, and I've also added a pic of my version of the BBC Focus radio I built, for which again, I designed a PCB. It's just an unselective amplified crystal set and the original article was very poor too. What a disappointment that design must have been to any youngsters who built it at considerable cost, when for less money, they could have built a one valve TRF which would have been far superior.
The 'Suggested Circuit' of the coil test unit is here:
http://golbornevintageradio.org/forum/sh...p?tid=3254
Hope that's of interest.
My latest project is a 'Coil Coverage Test Unit' based an a 'Suggested Circuit' in July 1969 Radio Constructor Magazine. The wonderful long-running 'Suggested Circuits' series in the magazine was authored by G.A. French. The articles gave the circuit and how it operates, leaving the constructor to build, test and develop the circuit and layout to suit their own requirements.
The basic concept of this circuit is to ascertain the frequency coverage of an RF coil when in a tuned circuit with a tuning capacitor of a given spread of capacitance. The unit has a tuning capacitor built into it with a calibrated dial, which can be set to either end of the capacitance range of the tuning cap to be used in circuit.
In use, a home-wound (or unknown coil) is connected to the unit and a signal injected into the unit from a signal generator. The tuning cap is set to each end of the spread of the cap with which it is intended the coil be used, and the sig genny is swept across the band until there is a dip on the meter in the unit, in the same fashion as with a GDO. The tuned frequency can then be read off the sig genny dial.
As designed, the unit can also be used to ascertain the capacitance of a cap of unknown value, but time has moved on and digital capacitance meters are nowadays so cheap (see other posts on the forum about an excellent kit for under £15.00 which 'Big Al and I have built), that it's pointless to include that facility in this coil coverage test unit, so I've omitted that.
Other amendments I've made are to include an output to a frequency counter which will give a more accurate indication of the actual frequency than the dial of a sig genny. (Again, back in 1969 counters were beyond the pocket of amateur constructors, but are now cheap and commonplace). In the original design there are two resistors which needed to be selected on test - one to set the current drawn by the unit to 4mA, the other to set the meter to FSD. I've substituted those for pre-set pots so the pots can be adjusted as required, then and left in circuit. I've also added an LED and series limiting resistor to give an indication that the unit is switched on.
It's still under development and is just something I think will be nice to have, and it remains to be seen if it will work as desired! It's much easier to test a home-wound coil (indeed any coil) out of circuit than to build it into say a receiver, only to find that it needs turns to be added or removed to get the desired coverage.
It's simple enough to calculate the inductance that a coil needs to have by using any one of the many online resonant circuit calculators. We can then wind a coil and check on an inductance meter if it is of the correct inductance, but really, only when it's in circuit can we find out of it performs as it should.
I've always enjoyed building simple receivers and what sparked my interest in making this coil test unit was when I built the 1959 BBC 'Focus' Gilbert Davis transistor radio design, which originally used the Denco DRR2 coil. I wound my own coil and had to keep putting in into the radio and taking it out to get it right.
I've attached pics show my modest efforts in using 'Paint' and Photoshop 'Elements 7' to draw a circuit layout, PCB layout and component overlay. I've added the circuit from the original article, and I've also added a pic of my version of the BBC Focus radio I built, for which again, I designed a PCB. It's just an unselective amplified crystal set and the original article was very poor too. What a disappointment that design must have been to any youngsters who built it at considerable cost, when for less money, they could have built a one valve TRF which would have been far superior.
The 'Suggested Circuit' of the coil test unit is here:
http://golbornevintageradio.org/forum/sh...p?tid=3254
Hope that's of interest.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
