strange effect Tektronix 2245A

[Smatek]

The oscilloscope operates normal and it is almost perfect.
Unfortunately it makes the following effect:
If I turn off the oscilloscope, the fan slowes down and the scrren goes off. But after four
seconds follows a very hushed  "psst" sound and the screen is showing a small sharp drawn square. The square seems to have the same proportion like screen. It is very bright. The left and the right line of the square are much thicker.

What is broken? Where do I start the search? Maybe it is a known error on CRTs.

I have all the documentation of the Tektronix 2245A inclusive of service
Manuals. This effect was not described in there.

Thanks for any hint.

[Hydrawerk]

Make a video, please.

[vlf3]

The fault is probably in the blanking stage, where the CRT control Cathode pin is not being turned off... if no other problem exists however; although it also sounds like the EHT to final anode CRT is not discharging, regarding your PSST or even Hiss sound. 

First of all, with no power supply connected; take a test lead and crock-clip one end to the chassis, the other end connect a 1K resistor... then probe the CRT final anode for 5 seconds, and all the other CRT base pins, for any residual high voltage; then do your inspection and metering for continuity.

Do a simple look around the CRT earthing, it's outer screen first, if there is one... so it has a good earth connection to the chassis; then check the CRT base connections too, that they are in good electrical condition, by pulling off the base connector, and see if their is no oxidation or signs of green/copper oxide around the pins, even check for good solder connections at the CRT tag pins too.

It's also worth checking any deflection PCB's if they have plug-in edge connections, pull-out and clean !

Other than that, it's down to electronic fault tracing, good luck.

[c4757p]

Interesting - I want to hear what the cause is if you figure it out. I had a CRT computer monitor (one of the ancient monochrome ones) a long time ago that developed the same problem. Ended up just discarding it - this was around 1998 and it was attached to a computer from 1982 - but I'd love to know what it was doing.

[houdini]

so you cant just unplug or put it in a power bar with a switch why?

[Smatek]

Thanks for the quick reply. I suspect corrosion. Attached you will find the circuit diagram. May somebody has a glue.

https://www.sugarsync.com/pf/D1713961_028_23651039

Thanks for the good and obviously proven instructions. On weekend I will give a try.

[vlf3]

Have your circuit download, and see their is no CRT screen to chassis so, unless you find a connection or oxidation problem, it's most likely to have an electronic blanking or brightness fault ? might need more circuit stages back from the deflection stage.