Magic eye tuning indicator - permanently 'full on!' -generic cct? - UK Vintage Radio Repair and Restoration Discussion Forum

Al (astral highway) · 24th Jul 2016, 4:36 pm

Hi folks,

As stated up there in the thread title...I thought others might have the same experience and so useful to gain some insight into how these work, generically. The set is the same old Hacker Serenade that I'm working on but I imagine this is a fairly standard arrangement.

The magic eye tuning indicator is at peak brightness permanently. I imagine there's a leaky cap associated with the device...

If anyone can post a simple generic/ conceptual outline that I'd be very grateful. Thank you!

G8HQP Dave · 24th Jul 2016, 4:46 pm

Try the datasheet for the magic eye, or a similar one.

Al (astral highway) · 24th Jul 2016, 5:31 pm

Sure Dave but I don't and won't have access to the tuner cct and there are no distinguishing marks visible on the magic eye, hence the query...

ms660 · 24th Jul 2016, 5:39 pm

EM84 according to radiomuseum.

Lawrence.

Al (astral highway) · 24th Jul 2016, 5:42 pm

Thanks, Lawrence!

paulsherwin · 24th Jul 2016, 5:49 pm

These are just voltage indicators. If it is permanently fully closed then there is a fixed voltage on the grid. Disconnecting whatever is connected to the grid pin should cause it to fully open. You can apply some volts to the grid with something like a PP3 and watch it close.

You just need to work out how the incorrect voltage is getting onto the grid.

ms660 · 24th Jul 2016, 5:59 pm

Typical circuit shown in this thread:

http://vintage-radio.net/forum/showthread.php?t=97283

Lawrence.

David G4EBT · 24th Jul 2016, 6:00 pm

They all work on the same basis, but have different patterns.

If the display light up, it means the phosphor is still good, but in some eyes - notably EM34s, it rarely is good, and an NOS one costs £75 plus, though whether any sell at that price is another matter.

The pattern of the eye will only change if the voltage on the grid - which is connected to the AGC line of a radio - goes negative. If the set is working correctly, the stronger the received signal, the more negative will be the AGV voltage and the display will respond accordingly. Some eyes need only a small variation in negative voltage to respond - others need several volts. (eg: DM70).

In a set in which the pattern doesn't vary with a strong station tuned in, check the negative voltage on the grid with the set both tuned into that station, then off tune. If there's little variation in AGC voltage, there will be little variation in the eye pattern. Lots of sets that seem to perform well audibly but in which the magic eye seems to be sluggish, are often sets that - though they may have an internal aerial of sorts - benefit from an external one if they have an external aerial socket, even if only a few feet of wire.

If you want to check whether an eye is working or not, just connect say an AA or PP3 battery with the negative to the grid of the eye, and the positive to ground. If the display responds, the eye itself is working. If any eye is dim, it can be due to lack of HT if resistors have gone high, but as a rule it's because the phosphor has gone to magic eye heaven.

Some links which you might find of interest:

General magic eye site:

http://magiceyetubes.com/

Two in a stereo amp:

https://www.youtube.com/watch?v=jXsG...M&spfreload=10

6 magic eyes used in an audio spectrum analyser:

https://www.youtube.com/watch?v=lcvB_oMcVIY

Hope that helps a bit..

Al (astral highway) · 24th Jul 2016, 6:28 pm

Thank you, David! Really interesting and full post. I guess in my case it's clear that the phosphor is good. It's full on without an external aerial and has no internal aerial, so I might suspect some of the passive circuitry - although I will do the simple battery test suggested by you and Paul!

Is it essentially a grounded-grid ,luminescent triode, from what I can see? The extra anode is confusing, though!

Nuvistor · 24th Jul 2016, 7:33 pm

Is the radio working? I ask because if the last IF amp is unstable it will produce a large negative voltage and close the magic eye.
If the radio is working correctly then it's just magic eye circuits.
Frank

ms660 · 24th Jul 2016, 8:01 pm

Is it an FM only radio? If so there's probably no AGC line, the feed to the grid of the tuning indicator will probably be taken from the -ve output side of the ratio detector via a resistor network.

Lawrence.

Al (astral highway) · 24th Jul 2016, 8:01 pm

Quote:

Originally Posted by Nuvistor

Is the radio working? I ask because if the last IF amp is unstable it will produce a large negative voltage and close the magic eye.

Hi Frank, thanks, interesting lead, but I fixed up the tuner just the other day. The magic eye had been permanently on even before I'd sorted it, though, and it remains on now.

It's not a 'must have' but obviously it would be great to restore the set back to as authentic working condition as possible, especially as this seems probably technically trivial.

Al (astral highway) · 24th Jul 2016, 8:52 pm

Quote:

Originally Posted by pwdrive

Is it an FM only radio?

Hi Lawrence, yes, FM stereo tuner.

I haven't got the cct diagram but I've sussed out where the AGC business is located and I'll take a look. Possibly one or two caps are candidates for leakiness and acting somewhat like resistors instead.

Nuvistor · 24th Jul 2016, 10:02 pm

The circuit I have for the RG16 Serenade does not have AGC, the magic eye is controlled as in Lawrence's post, from the ratio detector.
There is a 2.2M resistor connected to chassis from the grid of the EM84, that going very high,O/C could cause the fault.

I cannot find the circuit on this forum but I think I got mine from the Hacker Yahoo groups site.

Frank

Al (astral highway) · 24th Jul 2016, 10:06 pm

Quote:

Originally Posted by Nuvistor

T, ...the magic eye is controlled as in Lawrence's post, from the ratio detector.
There is a 2.2M resistor connected to chassis from the grid of the EM84, that going very high,O/C could cause the fault.

Thanks, Frank, I'll try to join the Yahoo group. My RG16 does have some sort of AGC as there's actually a dedicated turn-control that is externally adjustable.

However, thanks for spotting that it isn't relevant to the Magic Eye - I'll a look
at the ratio detector circuit once I've got the cct at hand.

Nuvistor · 24th Jul 2016, 10:16 pm

The control is a sensitivity control for the first IF amp, it adjusts the cathode voltage on the valve, looks like a potential divider network from HT to chassis.
Definitely no AGC on my circuit just the sensitivity control.

Frank

Sideband · 24th Jul 2016, 11:14 pm

I've been working on a Barker 88 as and when time allows. This set is the opposite of yours where the eye hardly moves when tuned in to a station. It's a new eye as the original was dead. I rigged up a test circuit using a PP3 battery and a pot so I could vary the grid voltage. I can quite easily open and close the eye by varying the pot so I need to look for an AGC fault or maybe the set has low sensitivity. The AGC voltage is hardly varying when tuned in to a station.

G8HQP Dave · 25th Jul 2016, 1:30 pm

Quote:

Originally Posted by astral highway

Is it essentially a grounded-grid ,luminescent triode, from what I can see? The extra anode is confusing, though!

No, more like two triodes. The first triode is a voltage amplifier. Its anode is directly connected to the deflection electrode of the display 'triode'.

Al (astral highway) · 26th Jul 2016, 8:34 pm

Ah, now that I see how these work, it's actually in the minimally illuminated position (1/4 illuminated bilaterally) and doesn't respond to variations in signal. I replaced the associated caps upstream with new ones, but no difference. New ones are quite expensive

Nuvistor · 26th Jul 2016, 8:39 pm

Does the voltage vary across the AM suppression capacitor when tuning a station, if the voltage is small it may just require a decent aerial.
With a dipole in the attic I can get 25 volts across the capacitor on the national stations. On the aerial wire supplied with the set it can be as low 4 or 5 volts. This is a different radio than yours but similar design idea .
Frank

24th Jul 2016, 4:36 pm	#1
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Magic eye tuning indicator - permanently 'full on!' -generic cct? Hi folks, As stated up there in the thread title...I thought others might have the same experience and so useful to gain some insight into how these work, generically. The set is the same old Hacker Serenade that I'm working on but I imagine this is a fairly standard arrangement. The magic eye tuning indicator is at peak brightness permanently. I imagine there's a leaky cap associated with the device... If anyone can post a simple generic/ conceptual outline that I'd be very grateful. Thank you! __________________ Al

24th Jul 2016, 4:46 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Try the datasheet for the magic eye, or a similar one.

24th Jul 2016, 5:31 pm	#3
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Sure Dave but I don't and won't have access to the tuner cct and there are no distinguishing marks visible on the magic eye, hence the query... __________________ Al

24th Jul 2016, 5:39 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? EM84 according to radiomuseum. Lawrence.

24th Jul 2016, 5:42 pm	#5
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Thanks, Lawrence! __________________ Al

24th Jul 2016, 5:49 pm	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,944	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? These are just voltage indicators. If it is permanently fully closed then there is a fixed voltage on the grid. Disconnecting whatever is connected to the grid pin should cause it to fully open. You can apply some volts to the grid with something like a PP3 and watch it close. You just need to work out how the incorrect voltage is getting onto the grid.

24th Jul 2016, 5:59 pm	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Typical circuit shown in this thread: http://vintage-radio.net/forum/showthread.php?t=97283 Lawrence.

24th Jul 2016, 6:00 pm	#8
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,761	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? They all work on the same basis, but have different patterns. If the display light up, it means the phosphor is still good, but in some eyes - notably EM34s, it rarely is good, and an NOS one costs £75 plus, though whether any sell at that price is another matter. The pattern of the eye will only change if the voltage on the grid - which is connected to the AGC line of a radio - goes negative. If the set is working correctly, the stronger the received signal, the more negative will be the AGV voltage and the display will respond accordingly. Some eyes need only a small variation in negative voltage to respond - others need several volts. (eg: DM70). In a set in which the pattern doesn't vary with a strong station tuned in, check the negative voltage on the grid with the set both tuned into that station, then off tune. If there's little variation in AGC voltage, there will be little variation in the eye pattern. Lots of sets that seem to perform well audibly but in which the magic eye seems to be sluggish, are often sets that - though they may have an internal aerial of sorts - benefit from an external one if they have an external aerial socket, even if only a few feet of wire. If you want to check whether an eye is working or not, just connect say an AA or PP3 battery with the negative to the grid of the eye, and the positive to ground. If the display responds, the eye itself is working. If any eye is dim, it can be due to lack of HT if resistors have gone high, but as a rule it's because the phosphor has gone to magic eye heaven. Some links which you might find of interest: General magic eye site: http://magiceyetubes.com/ Two in a stereo amp: https://www.youtube.com/watch?v=jXsG...M&spfreload=10 6 magic eyes used in an audio spectrum analyser: https://www.youtube.com/watch?v=lcvB_oMcVIY Hope that helps a bit.. __________________ David. BVWS Member. G-QRP Club member 1339.

24th Jul 2016, 6:28 pm	#9
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Thank you, David! Really interesting and full post. I guess in my case it's clear that the phosphor is good. It's full on without an external aerial and has no internal aerial, so I might suspect some of the passive circuitry - although I will do the simple battery test suggested by you and Paul! Is it essentially a grounded-grid ,luminescent triode, from what I can see? The extra anode is confusing, though! __________________ Al Last edited by Al (astral highway); 24th Jul 2016 at 6:41 pm.

24th Jul 2016, 7:33 pm	#10
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Is the radio working? I ask because if the last IF amp is unstable it will produce a large negative voltage and close the magic eye. If the radio is working correctly then it's just magic eye circuits. Frank

24th Jul 2016, 8:01 pm	#11
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Is it an FM only radio? If so there's probably no AGC line, the feed to the grid of the tuning indicator will probably be taken from the -ve output side of the ratio detector via a resistor network. Lawrence.

24th Jul 2016, 10:02 pm	#14
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? The circuit I have for the RG16 Serenade does not have AGC, the magic eye is controlled as in Lawrence's post, from the ratio detector. There is a 2.2M resistor connected to chassis from the grid of the EM84, that going very high,O/C could cause the fault. I cannot find the circuit on this forum but I think I got mine from the Hacker Yahoo groups site. Frank

24th Jul 2016, 10:16 pm	#16
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? The control is a sensitivity control for the first IF amp, it adjusts the cathode voltage on the valve, looks like a potential divider network from HT to chassis. Definitely no AGC on my circuit just the sensitivity control. Frank Attached Thumbnails

24th Jul 2016, 11:14 pm	#17
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? I've been working on a Barker 88 as and when time allows. This set is the opposite of yours where the eye hardly moves when tuned in to a station. It's a new eye as the original was dead. I rigged up a test circuit using a PP3 battery and a pot so I could vary the grid voltage. I can quite easily open and close the eye by varying the pot so I need to look for an AGC fault or maybe the set has low sensitivity. The AGC voltage is hardly varying when tuned in to a station. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

26th Jul 2016, 8:34 pm	#19
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Ah, now that I see how these work, it's actually in the minimally illuminated position (1/4 illuminated bilaterally) and doesn't respond to variations in signal. I replaced the associated caps upstream with new ones, but no difference. New ones are quite expensive __________________ Al

26th Jul 2016, 8:39 pm	#20
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Magic eye tuning indicator - permanently 'full on!' -generic cct? Does the voltage vary across the AM suppression capacitor when tuning a station, if the voltage is small it may just require a decent aerial. With a dipole in the attic I can get 25 volts across the capacitor on the national stations. On the aerial wire supplied with the set it can be as low 4 or 5 volts. This is a different radio than yours but similar design idea . Frank