Tektronix TDS540 Screen get twisted and garbled after 30minutes of use

[lugaw]

The screen seems to be getting bigger and bigger after 30minutes of use and after that it get twisted.  After cooling if off it goes back to normal.  It doesn't seem to affect the other function of the scope.

I haven't open it up yet to test voltage. What could be the issue? Is it a simple power supply or the flyback transformer is dying?

image_tek2.jpg is the normal screen on startup and image_tek1.jpg is when it has expanded a bit after 20mins.

[CaptDon]

Um, it would have been nice if what you were trying to display was the same in both pictures. Without the on-screen menu and so forth in the second picture it is a bit hard to compare apples to oranges???  Try it again, leave all the settings alone and show us before and after.

[factory]

The text for "Tek running" in the top left has partially disappeared off the screen, looks as if the whole imagine has increased in size.

David

[TheBay]

It does seem to be a EHT issue, it would be useful if you could measure it at the PDA.
Then it could either be a LOPT fault or the H drive circuitry.
Does the LOPT feel hot after it has been on for a while?

[shakalnokturn]

The H drive and LOPT are in the same circuit on the TDS540.
Anyway +1 for the LOPT being defective, I had the same scope with exact same symptoms.
You may also find that the current draw rises and regulated 21.5V (? Off the top.of my head) droops a little as it warms. You'll notice the regulator's heatsink is very hot.
ISTR posting on groups.io about mine.

[lugaw]

Hi all, thanks for the reply what is LOPT, EHT and PDA? Sorry new at crt circuits.

Yes the TEK disappears after 30mins and then after that at random minutes it will scramble and I don't have a picture of it since I don't use it anymore till that happens.  I haven't opened it yet since it is so large it consumes my whole table.  Can I purchase those things you mentioned if they are broken?  I really want to keep the scope because of the bandwidth my other scope is just a 100mhz Rigol

[shakalnokturn]

LOPT: Line OutPut Transformer, also  commonly referred to as flyback, HV transformer...

EHT: Extra high tension, the negative voltage that runs to the separate connection (anode cap.) on the side of the CRT. This is the voltage that gets the electrons to hit the screen. Always discharge this to ground before touching the connection, the CRT can store a small charge but at several kV.

PDA: Post deflection acceleration, I wouldn't say the acronym is appropriate here, it usually refers to older design "cathode ray oscilloscopes" (CRO) where the CRT uses electrostatic deflection rather than electromagnetic deflection (outer coils as in the TDS540 or televisions).
In CRO's with PDA the electrons were accelerated by a couple of kV's to pass the deflection plates then a higher voltage (10-20kV) would help them strike the screen.
On the TDS540 there is only one HV used for acceleration.

The LOPT can be found, though not easily, usually you'll be looking for a spare on evil Bay or full parts scope.
I adapted a B/W monitor's LOPT to fit into mine, didn't want to risk 50€ + shipping + customs from US for a part that may have the same problem as original.

[lugaw]

Thanks shakalnokturn for the clarifications!

Do you have model number for the substitute transformer you used?

Has anyone tried replacing the CRT with an LCD?

[shakalnokturn]

I went through my archives and threw a zip together.

The waveforms from the TDS were taken with the original defective FBT (FlyBack Transformer) but while still cold and behaving, I needed these as reference to look for a decent substitute from a different working monochrome system.

1st FBT was the one my PCB design was built around, it ran a few hours and quite hot. Powered down one evening, powered up the next day it was dead. No opens, no shorts, just dead... couldn't figure that one out, in the end I mounted it back in the original monitor to confirm it was dead there too.

2nd FBT was from similar equipment, they're likely both long obsolete anyway, I just use what I have at hand to get the job done... Note the jumper wire on one of the PCB's to accommodate for pinout difference. This one ran cool, there was some minor geometry variation from cold to warm, with a little tweaking I was able to get everything on screen as intended.

[bdunham7]

Do you have model number for the substitute transformer you used?

When you open yours up, can you post a clear picture of the transformer and the part number or markings that appear on it?

[radioguy123]

This does sound like the flyback is going bad. Isn't this a common problem with the mono displays ?

[andy2000]

I'd check for leaking caps on the display board, and in the power supply.  The leakage might not be visible unless you remove the caps to inspect underneath. 

Unless there's an unusual defect with this model, I wouldn't suspect the LOPT/flyback transformer was the problem.  They don't usually fail like that.  Typically they either short, or start arcing.

[james_s]

EHT: Extra high tension, the negative voltage that runs to the separate connection (anode cap.) on the side of the CRT. This is the voltage that gets the electrons to hit the screen. Always discharge this to ground before touching the connection, the CRT can store a small charge but at several kV.

It's the positive voltage that goes to the anode connection. You may be thinking of an analog scope with electrostatic deflection, those typically have the cathode at a negative potential.

[shakalnokturn]

My bad, positive it is of course.