Magic eye EM84 tube calibration ?

[Lef de art]

Hello forum.
One question for anybody how may knows.
After i have changed all caps and 2 resistors close to the EM84 tube in a tube5 reel to reel "Geloso G258 " the indications on the tube is all the time in the half. Before it was in the start position. Now it stays in the middle with no reason.
Any ideas ?
The sensitivity is ok.

I have uploaded 2 pictures with the before and the after situation.

The capacitors and the resistors are changed with correct in value components.

What im missing?

Thanks in advance guys

[snowman_al]

You might get more response if you post this in the 'Vintage Tape' section.

Not sure I understand the question fully. Does the 'eye' close at all?
Start by checking the voltage at pin 1 of the EM84.
It should change depending on the signal supplied.

[Lef de art]

Sorry for my bad English

Yes the eye close when i increase the volume.
The problem is that after resistors/capacitors change when the eye it is open dose remain in the middle.
In the picture you can see the start point of the eye before and the start point now.
Now it starts from the middle. Dose not totally open when the sound is at zero.

[Sideband]

Originally Posted by Lef de art

Dose not totally open when the sound is at zero.

Maybe that is normal. I have never seen any EM84 'totally open' with no signal (zero sound). All the ones I have seen have around 3-5mm of green florescence showing at each end.

[Lef de art]

3-5 mm is ok i suppose. But i have about 1cm from each side .

[Ed_Dinning]

Hi Folks, try looking at the electrodes with a scope. Hum can cause this effect

Ed

[Andrew B]

http://www.r-type.org/exhib/aaa0079.htm

Hope4 this helps, the PDF data sheet will help you understand how it works.

[snowman_al]

Originally Posted by Lef de art

... Yes the eye close when i increase the volume.
The problem is that after resistors/capacitors change when the eye it is open dose remain in the middle.
In the picture you can see the start point of the eye before and the start point now.
Now it starts from the middle. Dose not totally open when the sound is at zero.

I understand.
As sideband says there is always 4-5mm of the green phosphor glowing (at each end) under no input conditions. That is with pin 1 of the EM84 at 0 volts.
Any small negative voltage on pin 1 will start to close the 'eye'. (Like hum as Ed says.) So check the voltage on pin 1.

The other thing that may influence the fully open position might be low HT volts. (Could be that a higher voltage will deflect the beam wider, so make the ends smaller?)
I see on the diagram there is a 0.1M ohm pot in the HT line to the EM84. Circled in red in the picture. Does that have any effect?

The last thing is does this coincide with the reduced mains voltage you spoke about previously?

[Lef de art]

I will make some more tests and i will inform you guys.
Im was wondering....there is a dim close to the Em84.
You suggest to adjust it or i will make the situation more complicate?

[snowman_al]

Originally Posted by Lef de art

I will make some more tests and i will inform you guys.
Im was wondering....there is a dim close to the Em84.
You suggest to adjust it or i will make the situation more complicate?

You should check pin 1 first...

Then please measure the HT voltages on pins 7/9 and pin 6 before you change anything. I assume 'dim' = trim pot? Be careful there should be ~ 230 volts round there.

You count clockwise from the component side for the valve pin numbers. (In case you did not already know.)

[Lef de art]

Ok i will do it today, at the end of the day.

And now a silly question

For measuring the voltages in pins, i must use as ground the chassis or other pin ?

Example...for measuring the pin 1 i must use as ground the chassis and the pin one?
The same for pin six? Ground on chassis and pin 6?

[Station X]

With few exceptions voltages in valve equipment are measured with respect to (wrt) ground or chassis.

[Chris55000]

Hi!

The 100k (0.1M) in series with the target anode lead to pins 6 and 9 of the EM84 is to adjust the eye so it closes for maximum undistorted recording level, and is normally adjusted by feeding in a specified amount of audio to the input of the recording amplifier, the amount of audio is normally given in the maker's service sheet!

If a sheet isn't available, as was often the case with old Continental machines, the simplest way I would suggest is to record some "talk the hind legs off a donkey" Radio 4 or Radio 5 on tape for about 20 seconds with a five/ten second gap between each, noting the position of the volume control at each test, increasing the recording level with each segment until the replay starts to sound badly distorted, then once you've found the recorded part where there's bad distortion in the replayed segment, turn the record level back slowly one mark on the panel, record/replay again, and as soon as you have a clear recording, leave the record level control as the highest undistorted level you can get, then feed in the programme signal you used for this test, adjusting the "magic eye's" target anode preset so the lit segments are just touching on the peaks of the speech.

This trial and error process takes a bit of time but can usually be done without too much bother in most cases!

Regarding the OP's fault, hum in the amplifier will certainly cause a spurious deflection on the eye as Ed suggests, another cause is high–value resistors in the grid–circuit of the EM84 itself!

"Breakthrough" from the h.f. bias oscillator picked up on the grid wiring to the EM84 or the section of the ECC83 used as a rectifier diode for the indicator will as well, but this fault wouldn't show on playback normally!

Spurious oscillation or instability, in the earlier a.f. stages would also be picked up by the eye as well – shorting the grid (pin 1) to cathode (pin 3) would prove this – this should cause the indicator to open fully.

I might add that the eye will tend to close if the 0.1M target–anode preset itself goes too high in value, but a straightforward meter test will pick that up, as will low h.t. voltage caused by a high–resistance metal bridge h.t. rectifier – do a quick check with a meter to make sure the h.t. is up to standard!

If you suspect breakthrough from the bias oscillator, check the wiring to the indicator's base hasn't been disturbed too much and ended up too close to the oscillator components, whilst hum or instability in the amplifier warrants a check of earth connections, the contacts of the record/playback changeover switch, and the various electrolytic capacitors decoupling the h.t. line – most valve tape–recorder amplifiers use about three, one for the first valve, another for the second valve, and the third one is normally the smoothing electrolytic for the output valve screen grid supply.

Very occasionally an indicator valve can itself go faulty to cause a spurious deflection but I've only seen it in very old end–viewed eyes, never the EM84/EM87 type!

Chris Williams

[Lef de art]

These are the measurements.

Pin1= - 0.6V
Pin 6= 235V
Pin 7= 56V
Pin 9= 56V

Chris I have two more spare EM84 and the eye looks the same with all of them, so for sure is not a valve problem.
The problem is that the eye dose not totally open.
Ramains in the middle. I have two trim components in the chassis. I believe the one close to EM84 is for controlling the eye.
I haven't yet tried to adjust it.

[joebog1]

I have never seen an EM84 with a totally open eye!! They usually have 7 or 8 mm "edges", or sides. The actual moving eye is only about 15mm in the centre of the phosphor.

Joe

[Lef de art]

Hello Joe. Indeed the eye can't be totally open.
But...

In my EM84 i have a total length of 3,7cm .
1,6 for each part.
The busy part in a mute situation before was 3mm for each edge.
Now is 1cm from each part.
Totally 2cm . It is to much.
More than half is closed in a silent mode.

[snowman_al]

Originally Posted by Lef de art

This are the measurements.

Pin1= - 0.6V
Pin 6= 235V
Pin 7= 56V
Pin 9= 56V

... I have two trim components in the shassi. I believe the one close to EM84 is for controlling the eye.
I haven't yet tried to adjust it.

The voltages on Pin 6, 7 and 9 are fine. No need to alter the trim pot.

The idle (no signal) voltage on pin 1 should be 0 volts. Any negative voltage will start to close the eye.
Try shorting pin 1 to ground next. Does that change the open position?
Then switch off and remove the mains lead. Wait 10 minutes then measure the resistance from pin 1 to ground. You should get ~ 3.3Meg ohms?

PS. We should ask which specific components you replaced and why. And that you have carefully checked they are correct and soldered properly...

[Lef de art]

When i ground the pin1 the edges get smaller. Approximately half size. The correct size.
I have try to adjust the trim close to EM84 but just lower the phosphor light. It was in the maximum position so i left it there again.
I have removed all the new capacitors and replaced them with the old one but still no effect in the length of the edges. So i have replaced them again with the new one. I have made the same wirh the resistors but again nothing...

I really can't understand how to remove this -0.6V from pin1. I don't know from where it comes!
I have even removed all the tubes and still the -0.6 is there !!!

Showman_al i didn't understand what you want to do with the ~3.3Meg

[snowman_al]

Originally Posted by Lef de art

... I really can't understand how to remove this -0.6V from pin1. I don't know from where it comes!
I have even removed all the tubes and still the -0.6 is there !!!

Snowman_al i didn't understand what you want to do with the ~3.3Meg

If you make a resistance measurement from pin 1 to ground / chassis, what do you get? (Switch off and pull out the mains plug before doing this.) It should measure about 3.3M ohms.

The 3.3M resistor is there to 'hold' pin 1 to ground. Without it the grid of the EM84 is allowed to 'float' and will build up a negative voltage. Maybe the -0.6 volts you have?

[Chris55000]

Hi!

The triode section of a "magic eye" is a straight or sometimes variable–mu high impedance triode operated at low current, and like a.f. amplifier triodes, will develop it's own negative grid voltage with zero signal due to "contact–potential" self–biassing effects!

What you could try is to connect the ends of a 10k potentiometer across the cathode–bias resistor of the EL90/6AQ5 output valve, then disconnect the lower end of the "magic eye's" 3M3 resistor from chassis and reconnect it to the slider of this added pot., which can then be adjusted to tap off a small positive bias to offset the –0.6/–0.7V standing "contact potential" bias on the EM84, allowing the bars to open fully!

Try this tip out – it might solve your problems!

Chris Williams