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Old 20th Dec 2017, 10:54 pm   #1
Steve Browne
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Default Magic Eye on Precision Aneroid Barometer stopped working

If anyone can help me return this to working order I would be very grateful. The DM70 magic eye is powered by a single 1.5volt D cell. Up until a week ago it was working well then it became unreliable, sometimes failing to light up when the button was pressed to take a reading. In dim light the heater element can be seen glowing when the valve is supposed to operate. I presume the DC/DC converter has broken as no HT is being produced. There are no user serviceable parts as it is encased in a block of resin.
I do not have a suitable circuit and am not electronically savvy enough to come up with one myself. I am able to assemble and solder components but cannot produce printed circuits. The space inside the instrument is limited.

If I can't get the magic eye working I would look for a circuit which changed an LED from red to green when a contact was broken but would much prefer to keep it as original as possible. It is a very accurate instrument but at the moment is completely unusable.
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Old 21st Dec 2017, 5:17 pm   #2
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

It is obviously ex military. I wonder what the H.T. supply was. Difficult part to replicate. Maybe a separate H.T. supply via some PP3's connected via a mini plug socket. Military Forums might be able to help. J.
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Old 21st Dec 2017, 8:44 pm   #3
Dave Moll
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

According to The Valve Museum, the anode voltage for a DM70 is 85V.
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Old 21st Dec 2017, 9:11 pm   #4
David G4EBT
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Quote:
Originally Posted by Heatercathodeshort View Post
It is obviously ex military. I wonder what the H.T. supply was. Difficult part to replicate. Maybe a separate H.T. supply via some PP3's connected via a mini plug socket. Military Forums might be able to help. J.
One application of the DM70 was in a 9V battery operated Nombrex L/C Bridge. I forget which one as I'm way from home right now, but I cleaned up a rather poor circuit for the Bridge, but can't attach it just now. The HT was derived from a transistor oscillator and voltage doubler, using two silicon diodes I'm not sure what the Anode voltage was, but I suspect it was no more than 65V and the display glowed brightly.

The filament voltage is 1.4V and doesn't take kindly to excess voltage. The size of the exclamation mark display varies according to the grid voltage. When used as a tuning indicator in radios, as with other types of magic eye, the grid is fed from the AGC line. The more negative the AGC voltage, the smaller the display.

Unlike many magic eyes (EM34 etc), the DM70 display is long-lived and they seem not to go dim over time.

I think a good place to start would be to see what the minimum HT voltage needs to be to get a decent display. I wonder how the barometer causes the voltage on the grid to vary to change the display?

Difficult to replicate the circuit with it all encapsulated in resin. At least the DM70 seems to be intact, given that the filament can be seen glowing. HT on the anode will give a full display, negative volts on the grid will shorten the mark till it all but disappears.

A year or so ago I restored a Unitra Figaro radio which uses a DM70. The set has an internal ferrite rod antenna, and on stations at good signal strength the display didn't vary much. Only when an external aerial was fitted did the AGC voltage go sufficiently negative with a station on tune, for the display to vary markedly.

Good luck in your endeavours Steve.
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Old 21st Dec 2017, 9:18 pm   #5
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Idea...
Fit a few 12V car key fob batteries in series to get the HT, no need to switch as the DM70 won't take current until the filament glows.
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Old 21st Dec 2017, 9:49 pm   #6
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Quote:
Originally Posted by David G4EBT View Post
Quote:
Originally Posted by Heatercathodeshort View Post
It is obviously ex military. I wonder what the H.T. supply was. Difficult part to replicate. Maybe a separate H.T. supply via some PP3's connected via a mini plug socket. Military Forums might be able to help. J.
I wonder how the barometer causes the voltage on the grid to vary to change the display?

Good luck in your endeavours Steve.
From memory (I had one of these barometers) the DM70 is used to monitor a set of switch contacts only and it illuminates either as a dot or an exclamation mark depending whether the switch is open or closed.

One of the switch contacts is attached directly to the aneroid capsule assembly and the other contact is on the end of micrometer arrangement. To take a reading the micrometer screw is manually moved to the precise contact point. The micrometer screw rotation is linked by gearing to a 3 digit 'odometer' that shows the pressure in millibars.

Because the aneroid capsule does not have to drive a needle or do any real work the measuring accuracy is good, mine would easily detect 3 feet change of height.

Ian
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Old 21st Dec 2017, 10:11 pm   #7
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

That's a precision barometer, I think the user turns a vernier knob to balance the atmospheric force on the aneroid with a spring or else weights on a variable arm. The magic eye is a balance indicator air pressure is read off the vernier (cyclometer) scale.

Probably from a military airfield for measuring "QFE" the baro pressure to set your altimeter to so that it will read zero feet at the altitude of the runway.

David
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Old 21st Dec 2017, 10:20 pm   #8
David G4EBT
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Interesting Ian.

Presumably then, when the switch is open, there's no voltage on the grid so the exclamation mark is full, then when the switch is closed, a negative voltage was applied to the grid to reduce the display to a dot? (A tall order to design the circuitry to raise 1.5V to the HT required to illuminate the display to the required level. Oscillator + tripler or whatever?).
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Old 21st Dec 2017, 10:38 pm   #9
M0FYA Andy
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

I've taken the liberty of posting your picture on the Aviation Forum, asking if anybody has any information on the unit.
Andy
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Old 21st Dec 2017, 10:59 pm   #10
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Instructions here:

https://www.analogweather.com/upload...uctions001.pdf

Helmut Singer is still in business so far as I know.

Lawrence.
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Old 21st Dec 2017, 11:23 pm   #11
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

I have used these back in the day when i was in the merchant navy. They were supplied by the Met office as I remember it and issued to those vessels that were "Ocean weather ships" where we reported the met observations back to Bracknell every 6 hours (GMT). From these observations the familiar weather map would be compiled.
As noted by RW and others they were quite simple devices and relied on an accurate set of contacts that were adjusted to open/close by means of the micrometer knob on the side and you read the BP "on the break" i.e. as the exclamation mark turned into a dot or vice versa. This is shown on the photograph where there is a label to that effect.
I'm not sure if I kept my study notes for that instrument as we certainly did cover it in the 2nd mates/ONC nautical science syllabus but I shall dig about in the old suitcase in the loft. I guess it was as simple as the switch applying a sufficient voltage to the grid to make the display show on the phosphor coating.
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Old 21st Dec 2017, 11:36 pm   #12
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

http://www.sage.unsw.edu.au/currents.../html/s68.html

A quick google brought this up, not the same model as the type used by the Met office/Admiralty etc but the operating principle holds. that one's been converted to give an LED display rather than use the DM84.

Andy
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Old 21st Dec 2017, 11:42 pm   #13
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

And here we see one used on the RRS Charles Darwin by the 2nd mate.

A.
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Old 21st Dec 2017, 11:48 pm   #14
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

I guess the contacts are very small, high-precision things, maybe crossed taut wires, which can't cope with much current (maybe a milliamps at most), otherwise why not just have a battery and torch bulb?

That being so, there is probably a negative supply also generated, so that the control electrode is biased either to -8V or so, or to zero - giving a positive indication with a few microamps drawn through the sensitive contacts.

A simple transistor circuit would allow a LED to be switched while subjecting the contacts to a few microamps and a few hundred millivolts, though this would be without the fun of the DM70.
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Old 22nd Dec 2017, 12:00 am   #15
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

I would modify the oscillator out of an old tape or video recorder.
They produce quite a bit of voltage off load. Use a UF4007 and get the negative voltage from the cathode being connected to the positive terminal of the battery so that it acts like a tap. I bet that is how the original did it.
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Old 23rd Dec 2017, 10:19 am   #16
M0FYA Andy
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Nobody has come up with a circuit on the Aviation Forum, but this suggestion has been made -
"It's piece of Met Office invented kit. Every Met Office had at least one. Email the Met Office instrument section. They will have the cct diagram - or know who does. You won't get much help from Negretti & Zambra."

Andy
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Old 23rd Dec 2017, 5:28 pm   #17
Steve Browne
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

Thanks for all the helpful comments. I think I will try Merlinmaxwell's key fob battery idea first if there is enough space for them inside. It would be a pity to take the LED route (says a man who has just spent two hours soldering in replacement filament bulbs to an ancient Christmas light set)!
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Old 23rd Dec 2017, 6:58 pm   #18
bikerhifinut
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

That would seem the best way forward.
I noticed that one of the other manufacturers variants used a pair of hearing aid batteries to get about 70V HT, so merlins idea of a few of those 12V alarm fob jobs could be the answer. It depends on how important it is for the unit to look authentic inside the casing (ie removing the encapsulated circuit) or whether you just want to keep the DM70 magic eye display.
I am sure one of the forum members here could come up with a suitable and simple circuit to replace the existing one as it seems to be based on a simple "microswitch" principle i guess it would be about as simple as a resistor to adjust for the correct amount of grid volts to trigger the display?
I wonder who serviced these items for the Met office?

A.
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Old 23rd Dec 2017, 7:46 pm   #19
M0FYA Andy
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

If the encapsulated circuit is to be removed, it would be worth dismembering it to try to work out the circuit. Is there anything which will soften/dissolve the resin without dissolving the components?
One for the chemists on here perhaps.
Andy
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Old 23rd Dec 2017, 9:00 pm   #20
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Default Re: Magic Eye on Precision Aneroid Barometer stopped working

The old methylene chloride paint strippers did a good, if slow job on many epoxy encapsulants. It did remove paint, markings and some plastic from components too so frequently you had a real mess to decipher, if that was the intent. Dont know about the current safer formuations.

Ken
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