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400 tuning eye issue

jvstratman

jvstratman

Well-Known Member
One of my 400s tuning eye is completely unresponsive. It appears slightly illuminated but doesn't react at all to changes in signal. In other words, it doesn't open and close. The tube works fine in a different 400. Both 400s have good reception with an antenna but the one in question doesn't pull in a station if I use myself as an antenna by touching one of the antenna strip screws. The 400 that the eye tube works flawlessly in does pull in stations by touching the screw.

Will the eye tube react this way even though the FM sounds good with an antenna attached? Again, the eyetube is completely unresponsive to open/closing but appears slightly illuminated.

Thank you.
 
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Most likely yes. If the tube is not responsive in the FM Mono setting to a reasonable signal presented to the unit, then something is not performing properly in the tuner circuits proper. If it reacts properly in the Mono setting but not in the FM Stereo setting, then something is not performing properly in the MPX section.

There were two different versions of the 400, with the break point at serial number 48000. Which one you have will determine which service manual you need or use as there were significant differences in the FM tuner section between the two versions.

Dave
 
The eyetube is unresponsive in both, mono and stereo. Although it's not opening and closing, it appears to be slightly lit all the length but never varies even slightly when rotating the tuner.

Strange thing is that this 400 recieves a nearly dead quiet signal most of the time. It drifts a little. hmm...

Another thing; I was wiggling the tube socket and noticed that if I pulled the socket halfway off the tube would light strong in the "weak signal" fashion. In other words, strong illumination except 3/4" or so in the middle. That also didn't vary with tuner rotation. I then cleaned the socket with deoxit and cleaned the pins and now can't replicate that phenomenon. Weird!
 
If the eye tube wont respond in mono, then clearly there is a problem in the tuner section proper. Start by cleaning all the tube sockets and testing all the tubes in that section. Tube sockets in the tuner section can become a major source of tuner problems over time, that a simple (but good) cleaning resolves. Besides using a good cleaner, use small pipe cleaning brushes or dental brushes to get down into each receptor pin in each socket, but make sure you don't spread the receptor pin so that it no longer grabs the tube pin as a result.

Dave
 
Will do.

What is the proper way to connect an antenna to the terminal strip? Strip terminals left to right are Norm, gnd, Loc

Single wire:
Dipole:
Cable via a 75 to 300 ohm xfmr:

What does Loc stand for?

Thanks, plugging away...
 
Normal strength connect ot NORM & GND.

If signals are very strong causing distortion, connect to LOC & GND
 
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I'm resurecting this thread because I finally decided to figure out why this tuning eye would light up looking normal and then fading to barely noticable. I set up another 400 next to it and started checking resistance and voltages. There it was, the violet wire from the eye tube socket was hooked up to the terminal next to where it belonged on the chassis. Feels good to have it working now.

On another similar note, I have (2) EM84A tubes that turn on approx 1/2 brightness and work for about 45 seconds and then go completely dim as if the were turned off. Anybody else ever experience this?
 
I am interested in this thread...I have a 400, but find the "STEREO BEAM" hardly readable -- perhaps in a dark room I could see the magic ... I know it works when tunng a station, just wondering if there is a reason it is so dim ... I checked behind, thinking I could clean the tube, but see no way of doing this ... doesn't make the sound any worse !
 
Pop the little spring off the top of the bracket and lift the tube with bracket attached. Maybe your tube is rolled down? When you lift the bracket, a small curved square piece of plastic which the tube rests against could dislodge. It has the narrow slot in it that the tube eye aligns with. Could be a weak eye tube.
 
If the beams are pulling in appropriately as they should when they are supposed to, but simply dim (or very dim), then invariably 99.99% of the time that will be due simply to the fact that the tube is worn out. All of the various indicator tubes employ (among other things) a florescent "target" (similar in function to a plate element) that glows when struck by electrons, allowing you to visually see a changing negative voltage level within a circuit. Therefore, they were often used as recording level indicators and tuning indicators. But the target's florescence coating is by far the least hearty component of these tubes so that as they age, they will start to dim -- finally even dramatically so -- well before the tube itself ever ceases to actually function otherwise (primarily a function of cathode emission). That's why the failure mode of these tubes is always that it works fine, but is just very dim. If that's the case, replace the tube.

I hope this helps!

Dave
 
Yes Dave - it does appear the tube is worn out -- it was very dim from the start. and now nothing -- I found an EM87 on ebay -- (thank you for the detailed article I found on AK about this being a perfect replacement for the EM84A) ... I remember as a kid marveling at the magic eye VU meter which was on my brother's Woolensak real to reel tape deck --
 
I have read that the 6E2 tube will also work. They are quite inexpensive on the web. I recommended this to a fellow with a very dim tube in his 500B and the 6E2 worked fine. The display band is evidently slightly wider than the EM84. Here's an AK link describing:

https://audiokarma.org/forums/index.php?threads/magic-eye-solution-for-500b.761616/

Another option if the EM84/EM87 type tubes become hard to get. I keep a couple on hand.
Dave
6E2 tube.jpg
 
SO - update -- I got EM87 (used) on Ebay -- carefully replaced the tube (wow those clamps are tight ! ) -- still no signal or magic eye --

so assuming the tube was good, is there a simple test ? I was thinking I might have 'budged' some of the wiring when I was re-stringing the tuner last week..-- should have removed the tube prior to restringing, but learning from mistakes makes this whole thing worth it - right ?
 
Have you cleaned the tube sockets (including the eye tube) with Caig Deoxit or equivalent?

When your on FM, does lightly wiggling the RF tubes cause any static? My understanding is these sockets can develop intermittent connections which might cause your issue. Those more experienced, correct me if I'm wrong please.

One of my own PM treatments is to clean all the tube pins with a small enough brass brush as to shine-up the pins without bending them.

I use a small dental pick to re-tension all the sockets pin receivers.

Then I spray the sockets with Deoxit and insert a tube a couple times then use compressed air to dry the socket.
 
I have the deoxit downstairs -- will apply tomorrow ... not sure abt FM tubes, but can wiggle tomorrow as well..get most noise when in Stereo mode ... sometimes sounds like the unit is gargling briefly .. (first thought that came to mind) ... putting in mono fixes issue, and the tube sound is still there ...
 
For the most part yes. Like everything else, if it's abused it can die early. But run in spec, they'll last a good long time. I have eye tubes built in the 40's (6e5's) that had very little time on them and work very well. I've also got a few EM-84, 84A, and 87's that are plain wore out from voltages being off and overdriven. So it's a balancing act. The tube has phosphors that light up when electrically charged much like an older CRT Tube in a TV (mind you this is before LED/CCD/Plasma flat screens. And the phosphors DO wear out. (The PIONEER Fluoroscan meters use a fluorescent type lamp that wears out and dies, most are 1 foot in the grave and the other on a wet banana peel now.)
 
Thanks, Did fisher run these hard? I ask because i always wanted a B but its in the back of my mind that the cool factor of that model is the eyetube which could be getting rare.
 
You can still get eyetubes that are new stock (made in Yugoslavia I believe). I think they are EM-87's.
 
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