400 tuning eye issue

[tonyg88]

You can still get eyetubes that are new stock (made in Yugoslavia I believe). I think they are EM-87's.

Thanks I had no idea! good to know!

 

[dcgillespie]

Kapp -- check that you can see the heater glowing in the eye tube. It should have a normal yellow/orange glow like any other tube does, and off about the same intensity of other tubes its size.

If the heater is glowing, do you have a DVM to measure the voltages on the pins of the tube? There are really only four possible reasons for an eye tube to be dark:

1. The heater is not lit.
2. There is no B+ voltage supplied to the tube.
3. The eye tube is bad.
4. The tube is functioning normally, but the beams are well contracted due to other circuit issues, so that the tube appears to be dead.

Let us know!

Dave

 

[kappclark]

FWIW - Yesterday I replaced the FM tubes V1 - V6, and the stereo beam is back ! (I had replaced the original EM84A with a EM87 several weeks ago, with no blue glow at all) .. now the 400 is even more fun to play with

 

[dcgillespie]

Congrats on getting it resolved! It sounds like you had two issues going on -- a combination of #3 and #4 in my last post. Replacing the eye tube resolved its dim appearance, but other tube issues were preventing its beams from drawing together, so that even though it was now good, it still appeared to be dead. Replacing them then was the last piece of the puzzle.

Good luck with it!

Dave

 

[eduarsan1]

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

This is a good point to find out a replacement of the EM/84A/87 due to prices and also stock, where why not think about the 6E2 tube that is also a good replacement a little more sensitive but works.

Thoughs???

Luis