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Old 19th Jan 2018, 8:50 pm   #61
carnivalpete
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Default Re: The Best GDO Ever Made?

Many thanks everyone, especially G4EBT for all those aerial ideas and the MFJ link.

Just to clarify my previous note, I am only interested in amateur reception on the ham bands and mostly prefer to find ssb dx on 20m/7Meg. Been doing this since the late 1950's. I would prefer to buy a good GDO but would consider a build project.

I have several switched aerials, giving reception from various compass points, all run through an ATU, which makes a very noticeable difference to reception when the knobs are twiddled (technical expression).

My main issue with the Maplin GDO is the lack of any obvious peaking which makes it very difficult to actually find the sweetest spot. It most certainly does not indicate sufficient difference in results when attempting to adjust aerial dimensions.

As noted elsewhere, there are a number of other GDO clones that appear to be very similar in build, and probably performance, to the Maplin product, so I am hoping that someone here has found something else that is far more sensitive and effective. Even at a much higher cost.

I appreciate that I am largely addressing Rx/Tx experts here, but I enjoy messing with aerials and feel sure there must be an Rx fan around that has this problem cracked.
Kind regards, pete
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Old 19th Jan 2018, 9:36 pm   #62
David G4EBT
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Default Re: The Best GDO Ever Made?

Quote:
Originally Posted by G8HQP Dave View Post
The Ambit kit was from G3WPO. I have one somewhere. Not used it for a while.

I do remember that when I first built it it didn't work too well. Turned out that Ambit had changed the part used for an RF choke. They sent me a replacement choke and that worked much better.
The MK1 wasn't brilliant and I don't think the MKII was much better. The reasons why can be found at the link below, to N4XY's site. He has a thing about GDOs - he has 62 different models from 47 different manufacturers - not counting 7 homebrew units:

http://www.qsl.net/n4xy/gdos.html

The title of the thread being 'The Best GDO Ever Made?' - as with anything from a watch to a car - is highly subjective and depends on the criteria used to define 'best'. A GDO had to provide an unambiguous dip at the frequency to which it is tuned, clearly differentiated from 'false dips' which can also arise. It has to readily oscillate across the full range of frequencies, and given the era in which they were in vogue, the dial markings had to be clear, and the calibration accurate. Nowadays, the latter aspect has less relevance - few would rely on the dial markings of GDOs or signal generators without checking on a frequency counter.

I guess most would add to the list of desirable features the mechanical construction and aesthetics.

For my money, the only one comes up to scratch in all respects is the Eddystone 'EDOmeter' but then it's not really a fair comparison and it certainly falls outside the price point for amateurs so it fails on that attribute. Most GDOs were aimed at the amateur market and hence, were budget priced. (I wonder how many were ever used in anger - not many would be my guess, though they also functioned as an absorption wavemeter). The Edometer was more than just a GDO and was aimed at professionals. The slide-rule dial blows the small circular dials of amateur market GDOs clean out of the water.

An interesting write-up about one here:

http://www.eddystoneusergroup.org.uk...%20Article.pdf

Valve GDOs confer no advantages that I can see over solid state ones, though the Acorn valve was specially designed for VHF so gives a good account of itself in a GDO. That said, the inconvenience of battery requirements for valved GDOs rather rules them out, other than as a nice-to-have vintage 'collectable'.

Back in the days when GDOs abounded, the TECH TE15 and the Trio were popular, and there was a 'generic' one which popped up with various brand names including the Maplin brand. I had a TE15 and Trio - the Trio was better made, but they both worked well enough. They went when I built the PW one which I still have. As with signal generators, they spend more time on the shelf gathering dust than being used.
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Old 21st Jan 2018, 3:11 pm   #63
John M0GLN
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Default Re: The Best GDO Ever Made?

I wonder what causes the performance difference between the models, I have a Maplin YN48C, I don't use it much and I won't dispute the majority view that it is not the best, but I've been comparing the schematics of it with the one for the Eddystone ED902 and there's not much difference in the component count, both have a split stator capacitor, set of coils, 2 transistors, one or two diodes and around about a dozen capacitors and resistors each, it's not like comparing an Eddystone 830 with a Bush DAC90A, I expect the Eddystone split stator capacitor will be silver plated with porcelain end plates and the case and scale to be of higher quality but won't the rest be standard compnents? I've never seen a ED902 or inside the Maplin one, but looking at the the schematics it doesn't look like an impossible task to upgrade the Maplin with a new circuit board and components to copy the Eddystone or will the split stator capacitor and coils let it down?

John
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Old 21st Jan 2018, 3:36 pm   #64
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Default Re: The Best GDO Ever Made?

The tuning capacitor doesn't look anything special to me.

Lawrence.
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Old 21st Jan 2018, 5:55 pm   #65
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Default Re: The Best GDO Ever Made?

As per my post#58, I suspect that the design and construction of the coils is a major factor. Related to that would be detailed design (or lack of it) for the coil plug in connectors.

Given that the Model 59 was in service with firstly the US Army, and then the US Air Force, over a period spanning three decades, therein lies the secret of true GDO greatness....Grasshopper .
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Old 21st Jan 2018, 6:58 pm   #66
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Quote:
I wonder what causes the performance difference between the models, I have a Maplin YN48C,
Most of the experience I have with a dip meter is with the Maplin YN48C and despite the fact that I always found it a bit disappointing to use, it always got the job one. I certainly wouldn't swap it for any other dip meter unless I got to try the other dip meter first.

In other words, "be careful what you wish for..."

Having looked at some of the online reviews of the mk1 and mk2 Ambit/G3WPO dip meter I'm kind of relieved that I didn't buy one of these. The YN48C is at its worst on the top frequency range (100-250MHz) as it has lots of subtle false dips to deal with. But on all the other ranges it is OK with no false dips as long as the coil is plugged in snugly to its socket.

Finding a reliable and sharp dip with a test resonator can sometimes require several approaches/orientations but it does seem to get there in the end. To sum it up I'd say it is usually adequate for the job but it's an unrewarding experience because it often requires a fair bit of patience/perseverance to find the dip.

I've never tried using it with an antenna because I can't see the point unless it was for a classic demo of (VHF?) antenna resonance to a student.
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Old 21st Jan 2018, 7:20 pm   #67
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Default Re: The Best GDO Ever Made?

I think there's probably more variability in the manner of use than between the make or model of GDOs when it comes down to how well they perform.

The PW FET GDO I built back in October 1985, which I mentioned in post #31, worked fine straight off, and the plug-in coils, (using 3-pin DIN plugs), were easy to wind. It oscillates readily across the full range from 1.8MHz to 150 MHz and has a sharp dip. Whatever make/models of GDOs people might be interested in, the follow-up article in December 1985 'Using the PW GDO' which explains a range of uses and how to couple the GDO is relevant to any make or model, not just the PW one:

http://www.americanradiohistory.com/...PW-1985-12.pdf

I can't comment on whether the Eddystone one performs any better than the others as I've never owned or used one, but what does set it apart from the rest is that the robust build quality, (for which all Eddystone equipment is renowned), and the slide rule scale show that it was designed and built up to a standard - not down to a price. That puts it in a different category from GDOs aimed at the amateur market as 'shack accessories'. But of course, in a 'beauty parade' of GDOs, the Eddystone will have cost ten times more that the rest, so it's really no contest. For amateur use, that sort of expense just isn't warranted, especially given the small amount of use to which they're put, quite apart from the fact that the price would have put the Eddystone beyond the pocket of the average impecunious amateur in any event.
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Old 21st Jan 2018, 10:25 pm   #68
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Quote:
I think there's probably more variability in the manner of use than between the make or model of GDOs when it comes down to how well they perform.
Agreed. In my case I mainly used it to check the inductance of coils wound on powdered iron toroids. These were typically T37-x or T50-x but I also used T25-x or T30-x a few times. These toroids would typically be use in RF filters in the HF band. I did also use it to measure regular solenoid inductors but not as often.

It generally isn't easy to get a dip with a toroid unless some form of coupling loop is used. However, I found that the coupling loop would shift the resonance slightly so I didn't like this method. Plus it was fiddly to set up the coupling loop.

One neat aspect of the YN48C is that the plug in coils are quite skinny at 1cm diameter so it's possible to fit the loop made by the legs of the test cap and toroid directly over the end of the plug in coil. It can then be slid along the coil as a dip sensitivity control. The image below shows the most sensitive (but widest dip) location and by sliding the toroid towards the meter the dip gets sharper and shallower. This works quite well assuming that maybe 10nH or so is allowed for the extra leg length.

Back in the 1980s I used to use my Trio TS430S HF SSB/CW receiver to then tune through the frequency indicated on the dip meter and this gave me an accurate measurement of frequency on the TS430S display. Today I would use a spectrum analyser or maybe rig up something with a pickup coil to a wideband amplifier and a frequency counter.

To do the same trick with the EDO meter would require very long leg lengths because it looks like the plug in coils are quite wide.

How do other people measure toroids with a dip meter? Is there a better way? I don't like the coupling loop method because it does seem to shift the resonance slightly.
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Old 22nd Jan 2018, 1:45 am   #69
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Default Re: The Best GDO Ever Made?

What we need is a standard test(s) to determine a GDO's order of merit.
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Old 23rd Jan 2018, 7:58 am   #70
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Default Re: The Best GDO Ever Made?

Thanks for your replies to my post #63, I'll leave well alone and not try to convert the Maplin to an Eddystone.

John
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Old 23rd Jan 2018, 12:14 pm   #71
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One further question to ask, what would be the best way to measure the frequency that the GDO is working at? The scale on my Maplin YN48C won't be very accurate but I do have a frequency counter, so where should I sample the signal from, going anywhere near the coil under test with a pick coil must effect the dipping frequency, would I be better taking a signal from the oscillator Q1 assuming I could pick a point which didn't stop it working? The counter is a Philips PM 6661 which has a sensitivity of 20mVrms.
Any suggestions please, they will be welcome.

John
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Old 23rd Jan 2018, 3:33 pm   #72
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Quote:
Originally Posted by John M0GLN View Post
One further question to ask, what would be the best way to measure the frequency that the GDO is working at? The scale on my Maplin YN48C won't be very accurate but I do have a frequency counter...
John
When you get a dip on the GDO, you just leave it set as it is, then bring the coil close to an 'RF sniffer' attached to the counter input, till you get a reading on the counter. If you look at post #31, in the third pic you'll see an RF 'sniffer' - just a couple of turns of insulated were connected to the input to my frequency counter as in the pic below. In the instance of the pic at post #31, it was sniffing the VHF coil showing that the GDO was tuned to 150HHz. That's how I calibrated the GDO dial in the first instance, but whether using a GDO or signal generator, I never rely on the dial markings.

Hope that helps John.
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Old 23rd Jan 2018, 4:02 pm   #73
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Default Re: The Best GDO Ever Made?

It does help, thank you for that David, I have a coil very similar to the one you have but I wasn't quite sure of the best way to use it, but now I shall do it as you suggest.

John
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Old 23rd Jan 2018, 8:44 pm   #74
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Picking up on a couple of queries which arose back at the start of this thread;
  1. the RFC/resistor in the cathode; this is shown in the attached picture - it looks like maybe two layers of thin cotton-covered wire on top of a half watt resistor? My best guess would be a fraction of a mH (200-300uH?). Picture acknowledged to http://blog.aa6e.net/2009/10/my-new-...el-59-gdo.html
  2. The question of why two resistors (6k8) in series on both the anode and the grid? As Laurence suggested, maybe capacitance issues, but it turns out that there were two schematics in circulation and one of them just shows a single 7k6 resistor in each position. There are pictures on the net showing oscillator heads with 7k6 resistors, but I've not yet found one with two 6k8's.

Other resistors in the HT line come in to play in obtaining an anode voltage of 105V.

Measurements Corp made good equipment... but their configuration control was probably typical of 1947... "could do better"?

B
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Old 25th Jan 2018, 8:54 pm   #75
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It was during the late 1960s when I extended my SWL activities to VHF - 2m. Amateur Radio, to be precise. Prior to that, I had formed a close friendship with the local TV / radio repair man (Tony) who ran his own business and I learnt a lot from him on radio theory and in practical matters. He was also an SWL who had experimented with VHF kit himself, but that was before the AmRad G8 licences became available, so he found 2m. to be simply dead. He gave me a VHF GDO that he had built - the cct. of it is below, as are a couple of photos. It features an Acorn 955 triode.

The coil has lost its original natural bright copper colour after all these years and to be quite honest, I haven't yet tested it to see if it still works. But that's not really the point - it did once - and performed very well, too. It was also useful as a signal source. The knob and tuning dial are not original.

I haven't used this item in decades, but it has become a treasured possession, simply because my friend effectively introduced me into the World of Radio & Electronics, which led to a lifetime's employment and a lasting hobby. He eventually got his 'ticket' - G4IOT. Sadly, Tony passed away during 2005. I will never forget his unbridled kindness to me. He was married but childless: perhaps that was the reason. Occasionally, when I'm working at my bench, I can hear him telling about a better way of doing whatever it is I'm then struggling with. Some memories never fade.

Al.
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Old 25th Jan 2018, 9:00 pm   #76
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And next, an addition to the above post: a home-built absorption wavemeter. My attempt (then) to meet one of the AmRad licence conditions. (It did look a lot smarter once, unlike now! ) Its back panel has been removed for the purposes of the photo. The case is Perspex. The diode is a 1N914.

Al
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Old 25th Jan 2018, 10:03 pm   #77
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Definitely looks like your man Tony was a fully paid-up subscriber to the "Big Coil" school of GDO design . All it needs is a carbon mike in the cathode and you could probably work most of Somerset, Al!

B
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Old 25th Jan 2018, 11:10 pm   #78
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Ah yes: the coil. All a matter of Q - getting enough of it!

First thing I must do now is see if it works - and fix it if it doesn't. It could then be an instructive exercise to remove that coil and measure its Q. (I have an Advance Q-meter); polish off the corrosion - and measure the Q again. Finally, if the GDO still works , check its calibration and adjust as necessary.

Al.
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Old 25th Jan 2018, 11:13 pm   #79
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Could that coil be made of tubing? Silver plating might be nice?
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Old 26th Jan 2018, 1:05 am   #80
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Might well be. From what I recall of Tony's workshop and tools, plus his approach to things practical and his access to all sorts of bits 'n' bobs, etc., I reckon it is tubular. But we needn't guess: to measure its electrical properties, it will have to be de-soldered from the variable capacitor - then I should get the answer.

As for silver plating, I don't have the necessary kit for that.

Al.
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