2465 time base knob issue


 

When I pull the SEC/DIV knob to set the B time base I am only able to freely rotate the knob about eight tops before it starts to bind and drag the A time base setting along with it. Once it has bound it will not rotate in either direction without dragging the A time base along and I must either rotate it completely counter clockwise to unbind it, or I must physically hold the A time base skirt motionless while rotating the main knob.

My 2465 had some damage to the SEC/DIV knob that bent the shaft for the CAL/VAR inner knob, and I tried to straighten the shaft, but was unable to do a good job because I can't figure out how to disassemble the entire knob.

I bought a replacement front panel assembly to experiment with, and it turned out (luckily?) to have the same binding issue. I was still not able to figure out how to disassemble the knob. Even after I remove the VAR knob, and loosen the two set screws on the main knob on the main knob, the main knob will not come free. However, when I retightened the set screws on the main knob I noticed that the binding of the A and B time bases had lessened considerably or gone away entirely (it was hard to tell which, and when I loosened and tightened the set screws again the binding returned).

I had a similar issue on a 7B53A horizontal plug-in that I recently fixed up, but in that case there was no difficulty in removing the main knob. With the main knob removed it was easy to straighten both the main (hollow) shaft and the inner (solid) shaft that were both bent from a mishandling. Carefully straightening the hollow shaft fixed the binding problem on the plug-in, but it was also obvious that the hollow shaft was bent because the main knob did not meet the transparent skirt and a perfectly right angle.

The SEC/DIV knob on the 2456 seems to be perfectly perpendicular, even though I know that it was bent, and the inner shaft is still slightly bent, despite my attempt to straighten it. The main SEC/DIV knob shows a hairline crack as a result of the VAR knob being bent over.

This is more of an annoyance than a show stopper. I still have use of the separate time bases, it's just inconvenient if I want more than a 100:1 ratio, but I'd really like to fix this, as the scope is in otherwise excellent condition.

-- Jeff Dutky


 

I think I fixed it, but I'm a bit puzzled by how I fixed it. I tried loosening and retightening the two set screws on the main knob, doing so in an "unbalanced" manner. After a couple of tries the knob no longer binds. I don't really understand what I did, except that the two set screws must control the centering of a hollow shaft inside the knob, and my fiddling got it centered well enough.

-- Jeff Dutky


DaveH52
 

Whenever I have to fiddle with those skirted knobs, I slide a piece of cardboard or two between the skirt and the front panel, to provide uniform spacing, then remove it when I'm done.


 

The skirt on the 2465 isn't like the skirts on earlier instruments: it's sort of separate from the rest of the knob and appears to maintain it's own spacing from the front panel. I've used exactly that technique when working with the VOLTS/DIV and SEC/DIV knobs on 400-series scopes, however, because it's devilishly easy to get them too close to the front panel and rub off the printing on the back side of the skirt.