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Old 23rd Oct 2014, 3:07 pm   #21
PE9ZZ_JO22KI
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Default Re: EM34 magic eye

Don't forget CRTs have a much larger area and are only written to very briefly (when the electron beam passes a particular spot). CRTs will burn if deflection fails.

A magic eye is full on all the time, dramatically reducing life time. I don't believe ion burn causes the eyes to go dim. Voltages are too low for that I'd say.

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Old 23rd Oct 2014, 3:16 pm   #22
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Default Re: EM34 magic eye

At risk of going off topic, I've never come across an EM84 type with the phosphor on the envelope glass that has faded, just my luck or is there a correlation?
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Old 23rd Oct 2014, 3:34 pm   #23
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Default Re: EM34 magic eye

They do hold up well- but I have seen some very weary and dim EM84s. Perhaps it's a better scheme to have phosphor on glass, rather than on a metal backing. I reckon it's not a bad idea to remove them for day-to-day background use and keep them to low hours for show- bearing in mind price ande availability. Some sets with variable selectivity only applied HT at minimum bandwidth for precise tuning.
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Old 23rd Oct 2014, 3:45 pm   #24
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Default Re: EM34 magic eye

I have got a resistance bridge with a magic eye for balancing.
It has a switch to turn it off while the instrument is powered and waiting for use.
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Old 23rd Oct 2014, 9:05 pm   #25
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Default Re: EM34 magic eye

Quote:
Originally Posted by merlinmaxwell View Post
Is it because most CRTs have an aluminium backing to the screen?
I thought the aluminising was to improve brightness by reflection, that's what it was advertised as.
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Old 23rd Oct 2014, 9:15 pm   #26
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Default Re: EM34 magic eye

I saw aluminising sold as a means of avoiding phosphor burn and allowing more energetic beams. In scope tubes it came along with PDAs.

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Old 24th Oct 2014, 9:22 am   #27
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Default Re: EM34 magic eye

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Originally Posted by dave walsh View Post
During the sixties magic eyes were common in domestic tape recorders but I was never sure if this was to fit in with a more sleek item of equipment for home use, to make it look attractive [as per valve radios] or expedient in some other way.
I think originally, the main advantage of using a magic eye is that it was as you say that it was cheaper than a mechanical meter, but also that being a valve, it did not require any particular drive electronics. A low-impedence meter would have required at least a cathode follower to provide reasonable drive, so if you needed a valve you might as well use one that can do the indication as well.

Plus the public would have been used to the concept of magic eyes from radios. And using an overlap point in the resulting image is actually a rather accurate way of seeing when things go above a certain level.

As a side effect, a magic eye is inherently peak reading, and since distortion happens at the peaks, it makes sense for a domestic recorder to have a peak reading recording level indicator. For professionals, who can better judge the amount of peaks in the programme material, and have a better grip on tape distortion, a VU meter might be better to get maximum usage from the tape.

Tandberg stayed with magic eyes a bit into the transistor technology era for precisely this reason (Tandberg model 12). Once they went to meters, they had appropriate drive circuits so that they were peak reading (most noticeable in 70's machines such as the 3300X or 9100X where PEAK READING is even printed on the meter). This was the late 60's, so by then an additional transistor to drive the meter cost peanuts.

In terms of cost-cutting, it has always struck me that European recorders most often had magic eyes for recording level indicators, whereas American machines at least in the 50's tended to have a couple of neon lamps for this purpose, one flashing at 0dB, and one a bit over. That must have been even cheaper than a magic eye, and I think the US market especially in that era was more price sensitive then in Europe.
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Old 24th Oct 2014, 10:53 am   #28
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Default Re: EM34 magic eye

I've recently been lucky with acquiring an EM1 for my '39 Philips that I've had for years and the eye never worked. I got it for a tenner (plus postage from S America) It is absolutely lovely, so bright and the first time I've ever seen one working.
As it is brighter than required, I'm interested in Restoration73's idea of perhaps half-wave rectified mains fed to the target. What are peoples' thoughts on this? Possibly a switch to cut off target volts while playing the set for long periods?
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Old 24th Oct 2014, 11:03 am   #29
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Default Re: EM34 magic eye

Re: post no 27,

... and remember that 50s and 60s tape oxide formulations were not so forgiving of transient overloads - so a fast responding magic eye made sense...
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Old 31st Jan 2015, 8:15 pm   #30
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Default Re: EM34 magic eye

Quote:
Originally Posted by Tyso_Bl View Post
At risk of going off topic, I've never come across an EM84 type with the phosphor on the envelope glass that has faded, just my luck or is there a correlation?
It's not entirely true that EM84s don't degrade!

I have two EM84s, one from an Elizabethan LZ29, the second from a Robuk RK4 (first version), and both are showing some phosphor degradation - in bright artificial light or daylight you can see a brownish/grey tinge to the illuminated parts of the phosphor when no power is applied!

Surprisingly, the Robuk had the worst one even tho' its target-anode (pin 6) is not operated at H.T. voltage but at about 155-165V with a 100K feed resistor.

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Old 31st Jan 2015, 9:33 pm   #31
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Default Re: EM34 magic eye

Higher HT, non-intuitively is lower wear!
Yes the later blue-green and phosphor on glass to degrade, just same as a new VFD panel in DVD/HiFi/Cooker does (same triode tech but lower voltage, like a DM160). It's just that the Emerald green (EM34 and similar) degrade MUCH faster.

The Russian Octal doesn't have same Eye pattern as an EM34, but otherwise is similar. If the gain is too high then the driving grid can have an attenuator. It needs an adaptor or base rewiring anyway. About 1/10th price or less for NOS.
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Old 1st Feb 2015, 12:21 am   #32
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Default Re: EM34 magic eye

Why is higher HT lower wear, Mike W? I'm intrigued...
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Old 1st Feb 2015, 12:55 pm   #33
Mike. Watterson
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Default Re: EM34 magic eye

The problem is though that the "crt" grid isn't accessible (it's usually tied to cathode), so to reduce the target beam current you need to reduce heater voltage.
On the other magic eye thread I put the link to Radiomusuem.org article.

I'm sceptical myself, but the person making the claim is a "tube guru".

I'll look again for the link
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Old 1st Feb 2015, 5:27 pm   #34
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Default Re: EM34 magic eye

Lighting up dark eyes
http://www.radiomuseum.org/forum/lig...eye_tubes.html

Life span question (phosphor)
http://www.radiomuseum.org/forum/lif...magic_eye.html

How it works
http://www.radiomuseum.org/forum/how...ors_works.html

Operation of very different principle DM70 / DM71
http://www.radiomuseum.org/forum/dm70_operation.html


Other links

http://www.radiomuseum.org/forum/rep..._the_6e5c.html
http://www.radiomuseum.org/forum/rep..._the_6e5c.html

http://www.radiomuseum.org/forum/mag..._patterns.html

Russian 6e1p Angel pattern
http://www.radiomuseum.org/forum/how...4_or_em87.html

Best dedicated site on Magic Eyes (He is also a Radiomuseum.org member)
http://www.magiceyetubes.com/patterns.htm

Comprehensive history of Tuning Indicators
http://www.radiomuseum.org/forum/his...c_eye_led.html
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