TDS542A Flickering Screen Repair

[silviustro]

Hello everyone


A few months ago i bought a Tektronix TDS542A. I managed to get it relatively cheap (excepting import fees of course) because the seller advertised it as having an "Acquisition FAIL" message in the self test section.

To my surprise, once I turned on the scope, I found it worked perfectly, no faults at all. I still took it apart and gave the entire thing a good clean, it was full of dust after all. I took a good look around the main boards and saw no signs of corrosion or leaked caps, so i postponed the recapping operation because i wouldn't be using it too often anyway.

I left it sitting for a couple months and decided to turn it on again to get a measurement, to my surprise, it actually returned an "Acquisition FAIL" message, and after 2 minutes, something I haven't seen before in one of these scopes.

The screen started to flicker rapidly. No visible artifacts, just a rapid flicker accompanied by a vertically scanning bar and color shifting.

Video of the screen flicker in action: https://youtu.be/smf7parbLRg

I took a video of the screen and then shut it down. Time to replace all the electronlytics I guess. It took me almost two days but i replaced every single capacitor in both the power supply board and the display board.

I reassembled the scope and turned it back on, i thought the problem had been resolved, but after another 2 minutes, the flicker started again.

How should I proceed? I still haven't replaced the SMD capacitors on the processing and acquisition boards, as i thought the problem was on the display board itself.

Schematics and manuals: https://1drv.ms/u/s!AnLlNyWb7uqghEhDepjOKs6jya15?e=wsOpiG


Thanks in advance.

[Ground_Loop]

I had the same thing happen to my 784.  Assumed the color shutter crapped out and replaced the whole thing with a NewScope LCD. Now works perfectly.

[silviustro]

NewScope LCD's look great. But they're quite expensive at over $300. I would prefer a way of repairing the shutter if that's the problem.

I don't think the shutter itself is defective. Maybe the circuitry that drives the shutter?

[madao]

only Replacing of capactitor doesn't help.

You must washing   Shutter eletronic board on display  (a pcb with EPROM)
leaking elektrolytics is a problem and pcb must be clean!

Greetings
matt

[shakalnokturn]

It looks as if the shutter isn't syncing correctly with Vsync, you should be able to check that easily.

Of course as said above it's essential there is no electrolyte damage on the PCB to start with.

[silviustro]

The screen doesn't flicker when connected to a VGA monitor, which is a good sign because it means it's isolated to the display portion of the scope and not the processing board.

After a few minutes I noticed the LCD shutter on the CRT stabilizes and stops flickering. Which components can exhibit this behavior? older diodes?

[Miti]

After a few minutes I noticed the LCD shutter on the CRT stabilizes and stops flickering. Which components can exhibit this behavior? older diodes?

Dry capacitors.

[silviustro]

I already replaced all the electrolytic capacitors on that board, I haven't replaced the film or ceramics yet though.

[james_s]

You should NEVER power up a device that has leaking capacitors, the electrolyte will very rapidly corrode traces and vias when power is applied. Even just sitting there unpowered it is corrosive and needs to be dealt with quickly but powering it up greatly accelerates the process. It gets underneath parts and does a great deal of damage. There is no reason to replace ceramic and film caps but the SMT electrlytics all need to be changed, the boards thoroughly washed and carefully inspected. You have to remove every trace of electrolyte and repair all of the damage, it can be a big job if it was neglected.

[silviustro]

Yes, I do realize my mistake. I am in the process of replacing every single capacitor, but I haven't found a single one that has leaked yet. Even the small SMD caps which are famous for leaking and corroding the traces, none of them seem to have leaked. There's no discoloration around the traces, and no visible damage around the area, continuity seems fine as well.

I still have the Acquisition board to go, but on first inspection the caps don't look leaky either.

[silviustro]

I actually had the idea of doing the VGA test before continuing to replace more caps. The test was successful, but it brought with it some shenanigans. I plugged in the VGA cable and turned on the partially assembled scope, all worked fine with the exception of the Acquisition FAIL message when I first booted up and the CRT shutter flickering like before.

Great! I thought, now I know it's the shutter; "I shall look for my phone and take some photos to document this, where did i put it?" after a 5 minute search I found it and returned to find a rather pleasant but confusing surprise. The shutter was no longer flickering.

I guess a specific component on the small LCD shutter board is failing intermittently before heating up enough? a diode maybe? I will attempt to power the CRT display board outside the scope and perhaps probe the diodes, or perhaps just unsolder them and test them that way.

[drforbin]

Ever find out what was wrong with display?