Tektronix TDS 540 CRT screen failing when heated up - video inside

[twobeseen]

Hi there,

I got an old TDS 540 oscilloscope and since yesterday it has a new bug: After heating up for about an hour the screen starts to distort and after few seconds it collapses. The the screen comes up anew just to repeat the same over and over. After a cooling period the oscilloscope works fine for another hour.

See here my video:
https://youtu.be/Cxavdhe6vHs

What could be wrong? I searched the internet but nobody describes this crt error.

Is it the crt driver board or more likely the crt itself e.g. the high-voltage flyback transformer? Could it be a leaked cap? Anybody out there who is an experienced television repairman? Of course I know it can be dangerous to play with the high voltage...

In the end I am considering to replace the crt with an LCD replacement but this will be a lot of work if even possible (I found some information about that).....

[OldSchoolTechCorner]

Had fix one that had one where a capacitor leak near the flyback causing a carbon track that cause that same issue. I more suspect leaking caps since common issue with this model and 744a. Also if input current to increase to flyback power supply will to go to protect mode after running for a while. Wonder what the input current of primary side is and temperature of transformer? Could be failing flyback. 

[vk6zgo]

Hi there,

I got an old TDS 540 oscilloscope and since yesterday it has a new bug: After heating up for about an hour the screen starts to distort and after few seconds it collapses. The the screen comes up anew just to repeat the same over and over. After a cooling period the oscilloscope works fine for another hour.

See here my video:
https://youtu.be/Cxavdhe6vHs

What could be wrong? I searched the internet but nobody describes this crt error.

Is it the crt driver board or more likely the crt itself e.g. the high-voltage flyback transformer? Could it be a leaked cap? Anybody out there who is an experienced television repairman? Of course I know it can be dangerous to play with the high voltage...

In the end I am considering to replace the crt with an LCD replacement but this will be a lot of work if even possible (I found some information about that).....

It won't be the CRT--that is the glass thing!!----only computer folks call the whole thing a CRT!

The white highlights still appear when it does its "party trick".

If it is a raster display,the problem may be in the "boost HT" circuit----an overwind on the Line
Output transformer is rectified with a fast recovery diode & filtered to give around 160v.
This supplies volts to both the horizontal & vertical circuits,as well as the amplifier feeding the CRT cathode.
If something is failing there,you may well get the problem seen.

Open the thing up while it is still cold,& direct a source of heat at various points around the circuitry associated with the CRT.
Alternatively,buy some "canned cold" (freezer spray,& when the fault has already happened,selectively cool around the same area.