Kenwood DM-81 GDO; Anything Special?

[Bazz4CQJ]

A couple of years ago, there was a thread on the forum addressing the question of what was the best GDO ever made https://www.vintage-radio.net/forum/....php?t=142437.

There was quite of lot of support for the Measurements Model 59 (also known as the "Megacycle Meter"). This was manufactured in substantial numbers for the US Army and used a single Acorn valve.

One GDO which didn’t get any mention was Kenwood’s DM-81, a transistorised unit made in the 1970’s. Just a few days ago, I was looking at N4XY’s website, which is devoted to GDO’s, where he makes the comment that he rated the DM-81 as the best battery-powered (i.e. solid state) GDO he’d come across https://www.qsl.net/n4xy/gdos.html

The DM-81 uses no less than 4 transistors, with a bipolar transistor as the oscillator https://pa0fri.home.xs4all.nl/Divers...al%20DM-81.pdf. The circuit is quite interesting, primarily in the meter arrangement.

Has anyone got a DM-81, and if so, is it what you might expect from Kenwood, i.e. rather good?

B

[Superscope]

The DM-81 is beautifully made, like all Kenwood equipment from that era.

I haven't used the DM-81 in anger for many years. Not sure if anybody still uses GDM's
as better equipment is available these days.

The only problem I had with mine was the lack of Coil G (83-250Mhz),
which was hardly Kenwood's fault. I bought mine Second Hand and it had one Coil missing.

I always planned to wind one, but it was one of those Jobs I never got round to.




Ian

[Bazz4CQJ]

Originally Posted by Superscope

The DM-81 is beautifully made, like all Kenwood equipment from that era.
Ian

Beautifully made... and finely developed too I imagine. The oscillator is a 2SC1215 which I think has an ft of ~1000Mhz (original Panasonic devices available online for ~£2.50 ea.). Meter sensing is done via a FET buffer amp (2SK19) connected to the "top" of the oscillator tank circuit via an RF detector circuit (two UHF mixer diodes).

As Kenwood's excellent user-manual gives all the component details, including layout of the two small PCB's, I fear that a 'project' may be coming on .

B

[Radio Wrangler]

It looks awfully familiar. Isn't it a badged version of the generic Japanese GDO that also got branded Altai amongst other names?

Think I've got one kicking around somewhere with its coils and packaging.

David

[Aub]

Yes, I've got a Maplin one that looks very similar.

Aub

[Radio Wrangler]

I've just checked. They are different. The Trio/Kenwood one is built with an extruded aluminium outer sleeve as a case, the Altai is two interlocking folded aluminium U-channels.

Bipolar inside though.

David

[Jon_G4MDC]

I always thought mine was money well spent.

That said I have another GDO with a built in counter which means that any old inductor can be used without the need to calibrate.

The 83-250MHz coil for the DM-81 is a loop on PCB as shown.

[Bazz4CQJ]

My feeling was that the "generic family" of Japanese GDO's were mostly just two transistor units. Typical selling price these days of those Japanese units is £15-20 whereas the DM-81 seems to attract appreciably more money than that (£60-90 ?). Some (all?)of the generics use fairly cheap-looking solid dielectric tuning capacitors - I wonder if the DM-81 uses a decent air-spaced ? The comment on N4XY's museum site is not the only one on the web suggesting that the DM-81 is a cut above the rest.

I guess that if you (or I) were building a copy, you'd add a counter module, bringing it up to date and making life a lot easier.

In the past, I've seen repeated suggestion that one feature commonly found in "good" GDO's is the use of coils of a large (~25mm) diameter but the DM-81 has quite small coils. Did they make a compromise on that just to achieve a very nice design with the coils stored inside.

B

[Jon_G4MDC]

OK Bazz, well after 30 years or so you have persuaded me to open it up!
I have never done so before because I have never needed to.

Here are some views inside. I like that VC. I have done no circuit examination but it looks like the two gangs are worked differential wrt ground.

I expect somewhere on the web there is a schematic but I have never seen one.

Osc board looks like two devices, one a 2SC1215, the other one is unid so far.
Meter board has a 2SA719 with a 2SK19.

Very well made I think. Coils are about 10mm diameter which when trying to couple with other coil formers, on say a multiplier strip, is a big advantage.
You can be pretty sure that you have coupled with the intended victim not with all the neighbours.

[Bazz4CQJ]

Thanks for those picture Jon. I think they fully confirm that even if there are strong external similarities to other makes, the DM-81 is quite different to anything else.

Regarding coil size, it could be the case that they had optimised other aspects of the design, and so they could use smaller coils, which do enable better control of coupling.

I think the answer to my OP is that Kenwood's DM-81 probably is a noteworthy GDO and that N4XY's comment that it's the best of the transistorised units has substance to it .

B

[Jon_G4MDC]

Here is the rest of the story.

It is clear to see now how they are working the VC and it solves the type number of the transistor which had the markings sideways on.

Thanks for making me curious enough to find out about it.

[David G4EBT]

During my active years in amateur radio, (broadly, from 1975 - 1995) when I was making and tuning my own traps for trapped diopole antennas, trapped vertical antennas and a G4MH style 2 element 3 band mini beam, a GDO was invaluable. Initially I had a 'Tradiper TE15' which was very commonplace. Later, I bought an as new Trio DM-81, primarily because it was cheap. Then in 1985, I built the October 1985 Practical Wireless Gate Dip Oscillator, which I still have. I disposed of the TE15 and DM-81 long ago.

'Anything special?' is a subjective term.

In a 'beauty parade' of GDOs, the DM-81 would probably win, based on its attractive design, build quality and the convenient method of storing the coils, reducing the likelihood of any being lost, though they do get lost and that obviously affects the value significantly because as with missing radio back panels, knobs, or cracked dials, unless you are very lucky you won't find replacements.

In terms of functionality (convenience of use and effectiveness), the GDO needs to cover the desired frequencies, and provide an unambiguous dip at the resonant frequency of the circuit under test. They all do that, however simple the design because when at the resonant frequency, the oscillator stops working, hence the dip on the meter. That said, the effectiveness is more about the skill of the operator in knowing how to loosely couple the GDO so that unless at the resonant frequency, neither the circuit under test or the GDO don't adversely affect each other. Sometimes the GDO coil needs to be parallel to the coil under test, other times, at right angles. It takes a bit of practice to get the hang of that.

Other aspects of the design are does it have a large clear easy to read dial? Many, such as the Ambit G3WPO GDO homebrew (or kit) - very popular in its day, has a very small dial with tiny numbering. Also, the accuracy of calibration is clearly important, but less so if a frequency counter is to hand. Indeed, on a homebrew design, a counter is the most convenient way to calibrate the dial.

The Tech TE15 scores slightly over the DM-81 for tuning in that whereas like most GDOs, the DM-81 dial is directly rotated (it has a serrated edge), the TE15 has a slow motion drive. They were both accurate enough, but neither of them performed any better than my PW homebrew one. GDOs aren't like say hi-fi amplifiers, where there can be clear measurable differences supported by oscilloscopes and from my perspective, performance wise, there's nothing that puts the DM-81 above any other GDO. GDOs are 'one trick ponies' - they just have to dip, nothing else, and they do, but yes, in terms of appearance, build quality, and the convenience of coil storage, the DM-81 scores over others.

I admit to being a tad biased - I was a bit of a fan of Kenwood equipment.

For more than 20 years I had a Trio TS830 HF transceiver with matching ATU, filtered speaker, CW filter, headphones and low pass filter. A joy to use, but as I haven't been on air since 2003, it all had to go as it was gathering dust and has gone to a good home where it's used daily. All in the past - if we're not careful, the older we get the more we look in the rear view mirror and if we do that too much, we run out of road!

GDOs are a bit like signal generators - many of us think we ought to have one, but few ever use them. Most of us have a liking for test gear, and don't stop at one multi-meter, signal generator or scope. As my daughter in law would say - 'you can't have too many pairs of shoes or handbags'.

GDOs aren't really in vogue but often turn up at radio rallies due to SK sales, given the age profile of those that have owned them. Often like new in their original box, having been used little, if at all.

Some links might be of interest:

https://www.americanradiohistory.com...WM-1957-11.pdf

http://www.vk6fh.com/vk6fh/fetdipmeter.htm

PW Gate Dip Oscillator:

https://www.americanradiohistory.com...PW-1985-10.pdf

How to use a GDO:

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

http://www.agder.net/la8ak/m11.htm

https://docplayer.net/20761383-A-mod...scillator.html

Tech TE15 info:

https://www.google.com/search?q=tech...TopbfyifHRtgM:

A few pics:

Pic1 My PW GDO
Pic 2 The coil set I made for the PW GDO, with a 7 MHz calibrated test coil in front.
Pic 3: A homebrew 7 MHz antenna trap using double sided PCB as a capacitor with brazing rod for terminations.
Pic 4: The trap sealed up.
Pic 5 - nothing to do with GDOs, but a homebrew balun for the dipole.

Hope that's of interest.

[Terry_VK5TM]

Back in the time of the DM-81, it was unique to Kenwood (Trio, which is/was the same company, labelled it the DM-801), it wasn't a clone of anything else although there are many similar circuits.

[Radio Wrangler]

Kenwood is a brand name operated by the Trio corporation of Japan. They prefer to sell gear under the Kenwood but in the UK they were worried about the Kenwood name already being registered for electrical equipment (food mixers) so they branded the same equipment 'Trio'. They seem less worried nowadays so put Kenwood on stuff here as well.

From the viewpoint of nice clear dips, I've always found the valve designs better than semiconductor ones.

The raised value of the Trio/Kenwood GDOs seems to come from people wanting one just because the rest of their amateur radio gear being Trio/Kenwood. I've used them but not thought them special.

Probably the biggest price hike for collectable GDOs has to be the Eddystone Edometer. It looks nice, but plenty others equal its useability and function.

David

[Bazz4CQJ]

Originally Posted by Radio Wrangler

From the viewpoint of nice clear dips, I've always found the valve designs better than semiconductor ones.

The raised value of the Trio/Kenwood GDOs seems to come from people wanting one just because the rest of their amateur radio gear being Trio/Kenwood. I've used them but not thought them special.

David

The claim that valve GDO's are in someway superior to transistors certainly does have a following, as per the thread we ran two years ago. I'm certainly slightly suspicious about GDO's using FET oscillators, though this is no more than opinion based on one or two quirky events. When I first saw that the DM81 used a FET, I jumped to the conclusion that it was the oscillator; wrong!!!

Re the question of the higher prices being paid for the DM-81, I think thatthe design, calling up 4 transistors and 3 diodes, aspires to do well, having the meter respond to the RF voltage of the tank circuit rather than a DC property of the oscillator, the construction with the RF board nicely sandwiched between the tuning cap and the coil socket (no long leads) are all pointers to some trying to produce something that would be better than the run-of-the-mill competitors. However, the question could reasonably be asked about did this pudding eat any better, and only the owners can comment on that.

B

[Radio Wrangler]

The amplitude level of an oscillator is interesting.

Tons has been written on oscillator frequency, but very little on oscillator amplitude. The only material I can bring to mind is in 'Communications Circuits and Analysis' by Clarke and Hess.

For an oscillator to run at a controlled level (IE not dying out and not hitting the rails) there has to be a mechanism of diminishing gain with increasing amplitude. If I load an oscillator such as by putting the coil near a resonator on/close to the same frequency, a little power will be taken and the oscillator will both lose amplitude and increase its gain in an attempt to restore the amplitude. For an oscillator with good level control, the level will be a poor indicator of 'grid dipping' because it tries to stabilise itself. It's counter-intuitive, but looking at the DC conditions to see how hard the oscillator is trying becomes the better indicator.

Valves are less abrupt in their non-linearity than transistors, and oscillators tend to have softer level control. So their level becomes the better indicator of coupled resonator dipping. The Grid DC is simply a self-rectified amplitude indication.

David

[Bazz4CQJ]

Originally Posted by Radio Wrangler

It's counter-intuitive, but looking at the DC conditions to see how hard the oscillator is trying becomes the better indicator.

David

Ooh, I see your point; although looking at the RF level may work, looking at the DC conditions may well be the more sensitive approach.

Two years ago, I started building a copy of the Acorn valve "Megacycle Meter". I got side-tracked into playing with solid-state vibrators as a means of powering it from a 12V battery, and it fell in to the deep pile of unfinished projects. Perhaps I should get back to it?

B

[Bazz4CQJ]

The DM81 is not the only GDO to measure tank circuit voltage; the Millen 90652 transistor GDO does that too.

That might have appealed to manufacturers, as the GDO will then easily switch to acting as a sensitive absorption wavemeter / field strength meter, making the model look like a good buy.

B

[avocollector]

Intriguingly GDO's on the local internet auction site in NZ often fetch relatively high prices - I've missed out several times by thinking that $50 max bid (about 25 UK pounds) would be more than enough to secure one. Anything up to $100!! at times.

[Bazz4CQJ]

Originally Posted by avocollector

Intriguingly GDO's on the local internet auction site in NZ often fetch relatively high prices .

Yes, I believe the same thing is true in the UK. Some say that there are now other instruments which supersede the GDO, but for hobbyists, and particulary in the context of vintage radio, a GDO can serve not only as a GDO, but also as a signal generator, absorption wavemeter, and at a stretch, an L/C meter.

Regarding the discussion started in earlier posts about the meter circuitry for a solid-state GDO, I've now looked at a fair number of such circuits and it does look very much as though it is quite usual for such GDO's to meter the RF voltage rather than a DC property of the transistor. If anyone knows of anything different, please let me know. The exception seems to be the FET-based GDO which I built in the early 1970's which has a transistor sensing the oscillator drain current and that transistor drives the meter and I designed that circuit . I'm not sure if a copy of that circuit can still be found.

B