Golborne Vintage Radio

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.

Hi David, you have basically built a transistorised Q meter, which is a very useful bit of kit.
There are quite a few out there for sale quite cheaply and they have an internal oscillator.
The Advance type T2 covers 100KHz to 100MHz and is a very easy to use piece of kit.


Ed

Thanks for reading my post Ed, and for your helpful comments.

The T2 looks to be a nice bit of kit!

I've looked at several 'Q' meter designs over the years in various magazines, one of which was by Raymond Haigh who for some years wrote many high quality articles in mags such as Radio Bygones and Everyday Practical Electronics (in which almost everything these days seems to be PIC based and beyond my comprehension), on a range of test gear, receivers, small amplifiers and so on. Not seen anything of him in magazines for some years.

When I get round to building this unit, if it's a success, I'll post an update.

ED,

I've watched a couple of T2s go through eBay. Judging by the price they're very 'Sought after'.
Isn't there a similar Marconi one?

Alan

(28-03-2013, 05:58 PM)AlanBeckett Wrote: [ -> ]ED,

I've watched a couple of T2s go through eBay. Judging by the price they're very 'Sought after'.
Isn't there a similar Marconi one?

Alan

Yes Alan, Marconi made one, a nice unit from all accounts, there was a high output signal source unit that went with it, come up occasionaly at auction.

Lawrence.

Marconi TF 1245 Q Meter:

http://www.vmarsmanuals.co.uk/archive/32...TF1245.pdf

Lawrence.

Hi Gents, there are at least 3 Marconi Q meters, the one I have is pretty flexible and has alternative signal sources (TF1246 for 40KHz to 50MHz and TR1427 for 20MHz to 300MHZ). Unit itself is TF1245A.
Probably more expensive than the Advance T2.
The really expensive one would be the Boonton unit, definitely the R/R of Q meters.

Together with the meters there are also boxes of standard coils for cal checking purposes. These often come up chgeaply and will work on all Q meters so are worth getting if seen.
I have manuals for all of the above if needed.

I find it a very useful bit of kit if designing or repairing coils. It will also allow you to design coupled coils such as IFT's with ease.

Ed

I want one.

Lawrence.

In November 1978, 'Practical Wireless' featured a constructional project called the 'Sarum': a transistorised Q-meter. I have one of these and it is in my ever-expanding 'round-to-it' pile. Reading the accompanying write-up in 'P.W.', it does look useful, but as is always the case, a thorough exam and test should reveal all, good and bad points.

One day, one day . . . . . . then I'll report my findings here.

Al. / March 30th., 2013 //

I too have got that design somewhere Al. Raymond Haigh also designed a similar 'Q' meter, which again I have details of somewhere.

Right back to my schooldays in the mid 1950s I've always been fascinated by simple test gear projects (and sometimes not so simple, such as a 1976 Radcom 200MHz nixie tube counter, and the Robin 1 GHz counter, and PW Purbeck 'scope). I was in the habit of copying articles and adding them to my 'do list' and one such article was for a Mosfet High Impedance Voltmeter in PW (11 MOhm input impedance, compared to AVO 8 at 20k, and Avo 7 at just 1K). It will measure up to 500V DC, and has an AC probe to measure up to 28V P-P at frequencies up to 200MHz, so is ideal for peaking RF circuits. It uses a 100uA meter, and I came across a meter for £2.00 at a rally which has a 5" x 3.5" scale - ideal for the project. I recently turned up the article again and the PCB (7 x 5 cms), and was quite shocked to note that it was in Dec 1985 PW. It seems like only yesterday, and is yet another example of the long gestation period that some of my projects have!