Tektronix 556 dual beam Oscilloscope, only upper beam works

[naja452]

Thank you in advance to anyone that has insight into fixing this problem.

I work with computers and electronics a lot and have, at work come across a very old oscilloscope. I don't really have any use for it but am having a lot of fun figuring out how it works and the applications it can be used for. I have in using it, discovered that it is a dual-beam (not the same as dual trace I am told) and only the upped beam works.

If I set the controls for the upper beam to show the signal from either the left or right module (dual trace A1A plugin) it works just fine. Only the lower beam will not display anything. It did work off and on for the first hour but has since gone to only a green glow that covers the entire display.

I will attach some pictures so you can see what I mean. Be patient with me if you can I am a new school electronics geek and have only a little knowledge of all the old analog stuff as I tear this apart. Thanks!

below is a picture of the upper beam showing the traces from module A



Below is a picture of the upper bean showing the traces from module b



below are pictures of the lower beam both on lowest and highest intensity. you can see that the highest is just a green blur across the screen.

 

[Mattylad]

That's a lovely looking scope, it would look excellent on any workbench of mine (If I had one) - much better than the small plastic cased ones you get nowadays. (Although it may not work as well lol).

I love big old test gear - makes you look like a mad scientist lol

Can you swap the modules around to test if it is the module or the connections from the module to the scope?

Have you found the service manual yet? (See here

 

[naja452]

Thanks for the reply, and yes I have found the manual. I have been poring over the schematics trying to find something wrong with it but can't. I have tried switching the modules around but no luck. As I have seen it appears that the signals from either module can be routed to either beam (upper or lower).

And the beam appears to have the problem and not the modules themselves. I know this because I have fed a sine wave into all inputs and displayed the out put on the upper beam. No luck with the lower beam though.

 

[BradtheRad]

It did work off and on for the first hour but has since gone to only a green glow that covers the entire display.

Did you at any time hear snapping sounds (arcing)? Did the beam look 'jittery'?

Did the beam disappear suddenly, or did it fade gradually?

Did it wander up or down and disappear offscreen, or did it remain at the same vertical level as it stopped appearing?

Does the glow change its location or brightness? Can you tell if the beam might be hitting the side of the tube?

Any observations you make will help to track down the problem.

The first things to suspect are the high voltage generator, or the vertical or horizontal deflection circuits.

Can you measure volt levels on the test points of the working beam, and compare them to the non-working circuitry?

 

[naja452]

here's a quick description of a few of the issues that I could see right off the bat.

 

[BradtheRad]

Your horizontal sweep appears to be working somewhat. However it lingers at the left too long as it begins a sweep.

Do you see an out-of-focus waveform? I can't be sure if it's an effect of the camera. It is nearly vertical, as though it is at maximum amplification. Could it be 60-cycle hum?

From your description the beam has become so out of focus that it only shows up as a glow on the screen.

To reach a diagnosis you must gather more observations. Test any and every adjustment. Each thing you try might allow you to track down the problem.

Can you focus the beam? Move it vertically up or down? What happens when you change vertical gain?

Can you turn off the horizontal sweep, so you have a stationary beam? Can you adjust the horizontal position to bring the beam onscreen?

Can you get inside the scope? Tap the tube(s) in different spots. Fiddle with a few wires.

When you have more than one issue, a bad ground comes to mind. You have the added glitch of the horizontal sweep waveform not being quite right.

 

[pplus]

I am a new school electronics geek and have only a little knowledge of all the old analog stuff as I tear this apart. Thanks!

If you were more experienced and had a high-voltage voltmeter, then I would recommend the procedure to perform "4 Adjust High Voltage - Lower Beam" n 6.12 manual (thanks Mattylad) and compare the voltage at designated points with those in the Upper Beam (red dots) . When off the oscilloscope off, check the integrity of the filament (green dots).