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Tek 455, bad crt ?


lameturtlez@...
 

Hello,

Today I turned on my lovely 455 just to find a pretty fat and kinda fuzzy trace on screen. Does not look good and well defined as it used. I try to raise the intensity and refocus only to find that while raising the intensity the screen dims then brightenss then flickers, when the pot reaches full intensity the brightness is fully up with a trace that fills almost a whole division .
I've given a lot of care to this scope, all the elctrolytics are replaced with quality ones, everything thoroughly cleaned and adjusted.
All the voltage rails are measuring spot on, nothing shorted.
Is this a bad crt ?
I know the problem is pretty hard to describe so I took a video of it, if anybody thinks it's useful and if we are allowed to post links in this group I will upload it somewhere.

Kind Regards
LT


lameturtlez@...
 

Here is a video showing the problem.

https://youtu.be/0vVEwHMaEGs

I left the scope on for an hour or two and the problem seems to be getting worse, now some portion on the left of the trace is barely illuminated.
Crt is really expensive and I want to make sure what it is before ordering one but I'm really not familiar with crt failure to recognize.


Dave Voorhis
 

On 25 Oct 2020, at 19:28, lameturtlez@... wrote:

Here is a video showing the problem.

https://youtu.be/0vVEwHMaEGs

I left the scope on for an hour or two and the problem seems to be getting worse, now some portion on the left of the trace is barely illuminated.
Crt is really expensive and I want to make sure what it is before ordering one but I'm really not familiar with crt failure to recognize.
That doesn’t look like the double-peaking of a worn CRT, and it sounds like the problem came on suddenly rather than the typical slow degradation of a dying CRT. Unless the unit was dropped — which could damage the CRT — I’d suspect failure elsewhere first.

It actually looks rather like adjusting the brightness on my 475, which has a faulty brightness pot. It's stiff to turn and brightness varies in fits and starts rather than smoothly.


Tom Lee
 

I agree with Dave Voorhis. Few crt failure modes are sudden, and those that are generally have quite obvious effects (either they go dark, or go to full brightness). Deflection is unaffected in those modes.

Your crt is fine. Your problem lies elsewhere.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/25/2020 12:28, lameturtlez@... wrote:
Here is a video showing the problem.

https://youtu.be/0vVEwHMaEGs

I left the scope on for an hour or two and the problem seems to be getting worse, now some portion on the left of the trace is barely illuminated.
Crt is really expensive and I want to make sure what it is before ordering one but I'm really not familiar with crt failure to recognize.




Mlynch001
 

I would second what Tom is saying. This is not typical of a “bad” CRT. I would look closely at U550, which is the HV multiplier, DC restorer and other circuitry, all built into one module. This is a known failure component. Good Luck!

--
Michael Lynch
Dardanelle, AR