Prolonging the life of a Magic Eye.

[G6Tanuki]

Backstory: I've dug out of my collection a Pye PCR that's at some time been retrofitted with a magic-eye tuning indicator (an ex-military VI103 to be precise). This will become my "kitchen radio" for the next few months, largely to be used on shortwave.

This 'eye' is still rather heathy and green - I want to preserve this so was wondering about switching it off when not actually tuning the radio.

The obvious thing being to remove the HT from the target so it's no longer being bombarded - I'm thinking about fitting a microswitch to the "tuning lock" lever so when the tuning's locked the eye is deactivated.

Which got me thinking - it'd be a SPCO microswitch - should I wire it to earth the target (or connect it to the cathode, which sits a few volts above earth because of its bias resistor) when the eye is off?

Or leave the target floating?

I was always taught that bad things can happen if you leave any valve electrodes floating.

[n_r_muir]

Hi,

how about switching the heater off?

[Andrew2]

Doesn't running the heater with no HT applied risk poisoning the cathode?

[kalee20]

I have a tape recorder (EM34 Magic eye), on playback the HT is removed but heater left powered.

The recording head is quite worn - it's had a lot of use! - but the eye is still bright. And I know it has never been replaced, as my father bought it new.

Hopefully this answers your question!

[Refugee]

I have got an AC resistance bridge that has an EM34 and it has a switch to extinguish it.
I aught to be looking for a cap decade box to make it more useful to me.
It leaves the heater on. This it a later model addition as all the circuits on the web do not show the switch.

[Boater Sam]

As you only need the eye whilst tuning I have thought about putting in a proximity detector near the tuning knob to only enable the full HT when a hand is near the control. Leaving a few volts of HT on should avoid cathode poisoning.
Replacing EM80 and EM81 with a EM87 is also a possibility, they last much better, never seen a dead one.
The EM84SS is an interesting alternative, I have all the bits, must get on and do it one day.

[TonyDuell]

In the Leak Trough Line tuners (at least the Trough Line 3), the magic eye (EM84) is turned on when you switch the AFC off. The idea is that you turn off AFC to tune it, the eye comes on and when you have tuned it, you turn on AFC. The manual suggests this is to reduce burning of the magic eye phosphor.

From memory, the switch simply removes the HT+ from the magic eye target. But I am sure the circuit is on-line somewhere if somebody wants to check.

[Sideband]

I wouldn't have thought that removing HT from the target would do any harm. The triode part would still be operating. It's the target that degrades with use. I have several very dim EM34's that test good for actual gain but just don't display very well.

[kalee20]

True, although the triode and the eye-part often draw current from different parts of the cathode. And in any case, the aforementioned tape recorder switches HT off the whole caboodle, and it doesn't seem to have poisoned the cathode - rather, the thing still lights up very well!

[MrElectronicman]

Originally Posted by TonyDuell

In the Leak Trough Line tuners (at least the Trough Line 3), the magic eye (EM84) is turned on when you switch the AFC off. The idea is that you turn off AFC to tune it, the eye comes on and when you have tuned it, you turn on AFC. The manual suggests this is to reduce burning of the magic eye phosphor.

From memory, the switch simply removes the HT+ from the magic eye target. But I am sure the circuit is on-line somewhere if somebody wants to check.

I have a Leak Trough Line Tuner, and the EM84 in it is just as bright as another EM84 that I have which has only seen very limited use.

Yes the AFC switch removes the HT from the EM84 when turned on.

[G6Tanuki]

Originally Posted by Andrew2

Doesn't running the heater with no HT applied risk poisoning the cathode?

From whjat I've been able to determine, "magic-Eye Death" is all about loss-of-fluorescing of the target not cathode-emission.

[G6Tanuki]

Originally Posted by n_r_muir

how about switching the heater off?

While sensible in theory, it would be a spectacular pain to have to switch-to-tune and then wait 10 seconds or so before the tuning-eye was ready. TBH I'd be worried about the stress repeatedly power-cycling the heater would cause!

[G6Tanuki]

Originally Posted by Boater Sam

As you only need the eye whilst tuning I have thought about putting in a proximity detector near the tuning knob to only enable the full HT when a hand is near the control. Leaving a few volts of HT on should avoid cathode poisoning.
Replacing EM80 and EM81 with a EM87 is also a possibility, they last much better, never seen a dead one.
The EM84SS is an interesting alternative, I have all the bits, must get on and do it one day.

The PCR already has something better than a 'proximity-detector' in the form of the aforementioned tune/lock mechanism. You flick it to tune, tune the station, then when it's tuned you flick to 'lock' and then the tuning-capacitor's mechanically clamped.

A thin bit of springy metal can easily be fitted to convey the tune/lock-mechanism status, and so shut-down the Eye when it's not needed.