Re: tek 465 display problem

On a second look at photo #1, you might have magnetic field interference, though, personally I doubt it, the screening in one of those is just too good. Make sure there are no possible sources of this (other instruments, electric fan, etc) near.

I doubt very much it's the CRT. That sort doesn't have distributed plates where you could get part of one plate moved with respect to the others. Also the "re-entrant" horizontal scan points to timebase interference.

Fred Abse wrote in news: snipped-for-privacy@cerebrumconfus.it:

The distortion is in very specific areas of the display and seems to be mostly in the upper left quadrant. I thought there was a shadow mask similar to a TV CRT on those CRT's....then again maybe not. In any case, running the trace off the top or bottom and cranking up the intensity to where the whole face glows dimly reveals a area which is brighter than normal that more or less corresponds to the distorted area.

The tube must have been dropped really hard to knock the mask loose if it exists.

Rich Andrews wrote in news:Xns93D382A7AF80Dmc2500183316chgoill@216.168.3.44:

To check CRT geometry,put in some sinewave signal of 1 cycle per division,amplitude enough to reach from top to bottom of graticule.Move position controls and watch for distortion of sine wave. The usual manner is to use time marks from a calibration generator,which most people do not have.It's possible for hard shocks to distort the mesh lens at the end of the electron gun structure,or to fracture the glass EG rods,and have glass chips make 'hot' or dark spots on the phosphor.

No, there isn't. There is a PDA mesh a bit back from the faceplate but that probably won't be ferromagnetic.

That probably doesn't mean much, since we already know there's a velocity irrregularity at the LHS. BTW, I wouldn't recommend doing that too often :-(

What you need to do is switch off the timebase (set to X-Y) and with a (not too bright) single spot, move the spot over the screen area with the position controls, one at a time to see if you can trace out straight lines. If you still get funnies, try and see whether it appears to be interaction between axes, since the X and Y amps are still in circuit. Then it's time to get another 'scope and see what is happening at the plates.

It would be very unlikely for the CRT shield to have become magnetized, since it's made of mumetal, which is as near to non-retentive as you can get in a high permeability alloy. Expensive, too, it's mostly nickel.

Have you checked the power supplies for voltage and ripple within tolerance? It could be tired decoupling capacitors.

Does the instrument look to have been dropped?

Fred Abse wrote in news: snipped-for-privacy@cerebrumconfus.it:

Agreed, but I felt it was a quick and dirty test to rule out the vertical and horiz amps. What if I reversed the deflection plate leads. If there is a problem with one amp, then would not the problem will move to the other side of the display?

If I took a dc battery and a pot, would that not totally rule out the electronics?

Did that and using the hor. pos. pot to trace out a line yields a a very bumpy line at the areas where distortion occurs.

It has been a while since I worked on a scope, the last one being a 555. Now there is a formidable monster! Decoupling caps were an issue then and it seems they still can be. To be honest, no. I need to get a manual.

I had thought that if PS ripple were an issue, any abberation would occur at the same vertical axis. A trace at the bottom 1/4 of the screen is relatively pure.

Actually, no. It looks straight and dent free. Now if someone had changed the tube out and resurrected a dropped 475, I couldn't say.

Aren't unusual problems kinda fun? It certainly makes one drop back to basics.

posted

"bg" wrote in news:bh71e9$o55$ snipped-for-privacy@terabinaries.xmission.com:

The bottom signal is clear and straight. When you move the trace up on the screen to the top 1/3, it is distorted.

"bg" wrote in news:bh94cu$kjh$ snipped-for-privacy@terabinaries.xmission.com:

I am going to try that. Until I get a second scope, if I reverse the vetical leads on the CRT and the problem stays at the top of the crt, then vert. amp issues would be ruled out then, yes?

I wonder if I still have that handheld 1" tape demagnetizer here somewhere....

Rich Andrews wrote in news:Xns93E88502CF9D3mc2500183316chgoill@216.168.3.44:

The mesh lens is a dome-shaped,very thin,fine screen,and if the scope is shocked in certain axis,the lens could distort.Perhaps by setting the scope down hard on it's end,in transporting it,but I am not certain of that.

Jim Yanik wrote in news:Xns93E8BE0112CC6jyanikkuanet@

scope

I am surprised how little there is to hold the crt in place. Changing the CRT is about the easiest job in servicing that series.

Jim Yanik wrote in news:Xns93EA800031DBFjyanikkuanet@

from

I put them back in and didn't lose any either. (:>)