The Best GDO Ever Made?
 

Back in the 1940's, the Boonton Company produced a GDO which some believe to be the best GDO ever produced commercially. They called it the "Megacycle Meter". As originally supplied, the oscillator was a 955 Acorn valve.

As my Christmas present to you all, I now show you this, which I have lovingly converted to a "British schematic". It does not get much simpler does it?

It originally covered 2-400MHz in 7 ranges. Physically, it consisted of two units (i) the psu and modulator and (ii) the hand-held oscillator. At some time, the US Army decided to adopt the instrument and it was then supplied with two oscillator units; the original and then an additional UHF unit which went to 940MHz, this unit using an oscillator with a 6AF4 valve.

Has anybody in the UK ever owned or even seen one of these?

As soon as I have finished my current Tunnel Diode dip meter project, I must get on to this. If the 955 was substituted with a 956, the filament would run on 1.5V, and the HT produced on an inverter, so it would be portable.

Happy Christmas

B

Re: The Best GDO Ever Made?
 

I came across the circuit in an old American magazine. I had a 955 triode and a base for it, so I tried to build one. I never got the cathode choke right and it wouldn't work over the full frequency range, but where it worked it was good.

I later built one around a 6C4 design in shortwave mag and I got that going well.

Valve dippers certainly were less prima-donnaish than transistor ones, but doing a tunnel diode one is brave!

David

Re: The Best GDO Ever Made?
 

David, can you explain why the cathode choke should be critical? I did notice that the parts list in the Army document was detailed except for that one component. Seems like there is a devil in that detail; why should the 6C4 do better; maybe someone could simulate it? In the original, the tuning cap, valve base and coil socket were manufactured as a single assembly.

Re the tunnel diode, what I'm looking at there is trying to copy the old Heathkit design. The diode that circuit used had what appears to be a customer-specific designation on it. The odd ones I have are from a circuit board I picked up in the early 70's, have been collecting dust since then, so may as well try them out. Of course, I heard that the term "Griefkit" caught on in the US :-X .

B

Re: The Best GDO Ever Made?
 

Hi B,

I have one of these GDOs and I must admit that I bought it as much for its looks as for a piece of test gear. It has a full complement of coils and a manual and had been used in the UK by a previous owner as it is wired for 230V AC. I also have a digital version of the manual, in DJVU format.

If you'd like some photos, or other info let me know by P/M.

Regards

Roger

Re: The Best GDO Ever Made?
 

The boonton GDO covers a good wide frequency range, and the cathode choke has to have enough inductance to operate at the low frequency end, and yet not show any resonances right up to the top frequency of the highest band plug-in coil.

At the time I built it I was rather young and hadn't yet learned of these complications. I think I was using a '10mH' sectional choke out of an old radio.

The Shortwave mag design was a completely different circuit with a tapping on the plug-in coil for the valve's cathode making it a Hartley oscillator. I don't think the valve type played any particular part in making it a more reliable oscillator as constructed by me at the time. The lower strays of the 955 would have helped at the top end, but that's all I'd expect.

Decades later I built an FET based GDO, with varactor diodes built into each tuning coil so I could choose diodes to suit each frequency range. This allowed a set of 'bandspread' coils for the amateur bands. A 10 turn turns counting diial like the delay trigger knob on a scope was the tuning scale, with a BNC output for a frequency counter.

When I want a grid-dipper nowadays, I have a few loop probes I made for use with either the network analyser, or my spectrum analyser (which has a built-in tracking generator) The dips are less sharp with this scheme than with a good valved GDO, but they are good enough and can show any stray resonances. Using an old Anzac hybrid coupler (1-2000MHz specs!) I've routinely used this setup around 1GHz for the day-job.

Back in the 60's, tunnel diodes were really futuristic. We knew for certain that our wristwatch-fitted satellite phones would use them! We'd also all be flying around in jet-cars. The future aint what it used to be :-)

David

Re: The Best GDO Ever Made?
 

Oh, for a classy GDO, there's always the Eddystone 'Edometer'.

David

Re: The Best GDO Ever Made?
 

That's a rather nice looking bit of test-gear you've acquired! I hope its usability is as good as its appearance.

[My favourite 'GDO' - I forget its manufacturer or model-number but it was late-1940s from the US and was made for the Military: it wasn't really a "GDO" in the conventional sense since it separated the oscillation and detection functions. 9001 oscillator, 9001 cathode-follower driver feeding the probe-loop another 9001 as buffer followed by a UHF crystal-diode [1N23-style ceramic-encapsulated beastie] as the detector, 12AX7 double-triode as meter-driver].

Re: The Best GDO Ever Made?
 

Curious about the use of that resistor in the cathode choke, I spent a few minutes on Google and found one lengthy 1961 paper about Colpitts oscillators as autodyne receivers for use in nmr equipment https://books.google.co.uk/books?id=...llator&f=false This refers to the addition of a 5 or 10k resistor alongside the choke to improve ‘sensitivity’. Unfortunately, neither the preceding or following pages are included in the Google preview, so some context is lacking. However, it definitely looks like Boonton’s decision to include that resistor has rationale.

Looking at pictures of this GDO, one striking feature is the size of the coils, I’d guess that they are wound on 1 inch polystyrene tube, and the simplicity and wide separation of the plug-in pins is noteable.

To Roger, yes I’ll PM you. I wonder how well your unit had survived before it arrived with you?

B

Re: The Best GDO Ever Made?
 

I really like this one.
Who are Delica? I have never heard of them.

The main benefit is any old coil will do, including the random loop of wire pictured. It is self calibrating.

Re: The Best GDO Ever Made?
 

Mmm, I tend to keep notes of any that I encounter, but that is new to me. There are a few clones around, but that has a distinctive appearance. Do you have any info on it or just the device itself? After-thought, is that something like one of the MFJ analysers??

B

Re: The Best GDO Ever Made?
 

I too have a Boonton 59 Megacycle meter along with standard and UHF heads. so far I have not used it in earnest can't report on how good it is.

Perhaps not the 'best' GDO in the world but one that looked interesting used an EM87 as both oscillator and indicator. The attached PDF is a short article on it's operation.

Al

Re: The Best GDO Ever Made?
 

There seems to be a view that the units which use FET's can do "odd things", that the valve ones are more dependable and that the Boonton is especially good. My own FET-based unit is something I built a very long time ago and has generally been fine. I've found it doing strange things when using it to trim aerials to length. It dips at frequencies which don't make sense and the dips are different (strangely narrow) from its normal behaviour.

The EM87 has come up in previous threads; very interesting but it somehow fails to trigger my need to make one :shrug:.

You say you haven't used the Boonton much; is that a recent acquisition?

B

Re: The Best GDO Ever Made?
 

Here in the US, the military AN/PRM-10 is highly regarded. However I don't think that it is the one referred to in post #8.

N4XY has quite a collection. See www.n4xy.com

73 John

Re: The Best GDO Ever Made?
 

Hi John, that AN/PRN10'schematic looks really close to the old Boonton design - 955 Acorn at the core. Overall design is similar; separate PSU/modulator and RF head. I wonder if Boonton maybe sold the rights and a new, slightly modernised version was developed? There are some differences which are worth a closer look in terms of constructing a clone.

N4XY certainly sets a standard for GDO collecting.

B

Re: The Best GDO Ever Made?
 

For something as wideband as a GDO, that's absolutely impossible.

A more likely explanation is that it provides a resistance in the cathode circuit
to reduce gain at low frequencies and provides a good bypass at higher frequencies above its parallel resonance where it goes capacitive.

Of course, at DC its whatever it is.

Re: The Best GDO Ever Made?
 

Absolutely impossible it may be, but there are kHz to GHz chokes in the bias feeds of many network analysers, and they definitely have to be free of resonances in the range. Not easy to design.

David

Re: The Best GDO Ever Made?
 

Hi Baz, in reply to your earlier question I initially bought the GDO for my test equipment collection. I have always been interested in setting up an old AM amateur radio station for the VMARS net etc. If and when I get round to it perhaps the GDO will get some proper use then.

Re AN/PRN10, Boonton released a later version of the 59, the 159. I cannot find anything on the net at the moment but they do turn up from time to time on US Ebay. From what I remember the head looked the same as the 59 but the base unit was re-styled with rocker switches instead of rotaries. Perhaps PRN10 refers to the 159.

There is a US army version of the 59 manual that can be found here.

http://www.hparchive.com/boonton-nj.htm

Al

Re: The Best GDO Ever Made?
 

Yes, GriefKit was used here. It came to common usage when Heath supplied the kits with diodes with the cathode band on the ANODE! Also when the resistors had the wrong color coding on them.
I built many of them, and the worst was an audio generator & a dot bar generator. Even after they sent me replacement parts, they still didn't ever work quite right.
FrankB

Re: The Best GDO Ever Made?
 

I may have spoken too soon; that AN-PMR10 is a bit different. The tuning capacitors are linked via gear wheels to a pot which changes the valve bias according to frequency. The bias circuit also incorporates different resistors inside the plug in-coil assemblies.

Cloning that gear wheel arrangement could be tricky. I traded in my Mecanno set some time ago. Alternatively, losing the tuning capacitor and replacing it with a dual gang pot, with one section driving two varicap diodes for the RF tuning and the other section adjusting the bias could offer 'hours of fun'?

B

Re: The Best GDO Ever Made?
 

One major correction - The Measurements Model 59 Megacycle Meter has no association whatever with the Boonton Radio Corporation, which was (and is) an entirely separate company. Measurements was located in Boonton, New Jersey, which is shown on the label, but that firm had no connection with Boonton Radio Corporation. Measurements was started by a group of engineers who jumped ship from Ferris Instruments when Malcolm Ferris died in the late 1940's and his widow hired an accountant to run the company, who was reputedly clueless and ran the company into the ground.

The Megacycle Meter is properly referred to as the Measurements Model 59.