Evidence of open deflection plate connection


John Atwood
 

What is described here was found while fixing an HP 180A scope mainframe, but is applicable to Tek scopes as well.

I wanted to test an old HP 180A mainframe (rackmount type). I didn't have a working vertical + sweep plugin, but I had a mostly-working 8558B spectrum analyzer plugin. When powered-up, I got the familiar white noise trace at the normal horizontal width, but the height of the noise seemed low and the vertical position control had little effect. The trace was a little blurry, as well. In an effort to clarify the trace, I started manipulating the focus and astigmatism controls and found something really unusual: adjustment of the astigmatism control (but not the focus control) moved the whole trace significantly up and down - almost the full height of the screen - something which should not happen.

Pondering this, I realized that this is evidence of an open vertical deflection plate connection. The purpose of the astigmatism control is to equalize the electrostatic fields between the horizontal and vertical deflection plates. If they are all normally connected, the fields are balanced and changing the astigmatism voltage doesn't deflect the trace. However, if one plate is unconnected and floating, the other plate has nothing to work against, so the deflection is essentially between the connected plate and the astigmatism electrode, a very unbalanced situation.

I checked the connection to the vertical deflection plates at the side of the CRT, and they seemed good. The problem was found in the sliding edge connector that connects the plugin to the vertical plates. One of the spring contacts was slack. After bending up this with needle-nose pliers, everything worked as normal. This sliding connector is unique to the 180 series of HP scopes, but there are opportunities for open deflection plate connections in Tek scopes, especially at the wimpy connectors to the side of the CRT.

So, if you see the astigmatism control noticeably move the trace, check the deflection plate connections!

- John Atwood