Tektronix TDS540 A20 - CRT Driver Issue

[Marco_Mar]

Hi ALL.
Hope someone can help me.
I'm trying to fix a Tek TDS540.
Upon powering on, the unit looks like it is struggling to reach a stable condition because it starts over and over again. I can hear and see the fan jerking.
I was pondering whether the PS was faulty but after pulling it out and loading whit external dummy loads, it seems all right.
Drilling down the issue, I made the scope start after disconnecting the ribbon cable that feeds A20 -  CRT Driver. Of course, I can see nothing on the screen but the sequence of front panel LED lighting and clicking from the attenuator seems sensible. Furthermore, the front panel is responsive.
So my hypothesis is that A20 is drawing too much current forcing the PS to shut down. I can't understand what makes it restart in an endless cycle.

I retrieved the schematics from the net and performed basic checkouts: caps, BJT and diodes look healthy.

 Is it advisable to feed A20 from a 24 V external source to see what's going on?
CRT anode high voltage makes me feel nervous.
Any hints to troubleshoot A20?

Many thanks  for your help

[shakalnokturn]

The flyback transformers can fail with this kind of symptom.
A lot of SMPS's will go into "hiccup" mode when overloaded.
It's a good idea to power the A20 with a current limited power supply, at least you'll get a bit of a clue on how much current it wants to draw.
Keep in mind that if you give it enough current you may give it an excuse to release some magic smoke.
I've repaired one in the past by adapting the flyback transformer from a small monochrome monitor.
I have a TDS540 and 620 around with working display units, the scope acquisition / CPU boards were planning to get repaired some time but I'm ageing and it isn't happening, if you want one for parts let me know, I'll want to cover my initial costs though.

[TerrySt]

You are probably aware that you can plug a vga monitor into the back of the scope.  Or, you can even remove the crt and replace it with a vga lcd panel.  The is a detailed discussion in the test equipment form from a few months back that talks about an Fpga based driver for an lcd panel. 

Terry

Edit:  Sorry, I didn't realize the 540 does not have the vga output on the back.  I don't know if it uses the same ramdac chip to drive the crt that the 524 and 744 uses that makes it possible to drive a vga lcd panel. 

[TerrySt]

First verify the sync and video signals are present from the cpu board.  My 524 lost one of the sync signals and it caused the power supply to cycle as you describe. 
It is possible to carefully remove the crt and crt driver assembly and connect them outside the scope.  That makes it easier to troubleshoot, but proceed carefully. 

Terry

[vaualbus]

You can't on TDS540 model.
The VGA connector is present since revision A.
I recall checking the differences between A and non CPU board regarding the display section, on A revision there are indeed two ramdacs ICs, one used for the internal monitor and one used for the external VGA connector.

[TerrySt]

If the RAMDAC is the BT471 (or ADV471), then you should be able to use the FPGA to drive a VGA LCD panel.

Terry

[TerrySt]

Here is the link that shows the way I connected my CRT and driver externally so that I could troubleshoot it.

https://www.eevblog.com/forum/repair/tds524a-oscope-hvps-what-is-this-part-(e25)/

Terry

[Marco_Mar]

Thank you all for your kind assistance and contributions.

I made up my mind to dismantle the CRT.
On the scope chassis, I spotted some crusty white grime exactly where the rubber anode gasket connects to CRT.
This is the smoking gun (literally): the anode did arc towards the chassis, right?

Next steps
1. power on the A20 CRT driver with an external and limited 24 V PS. Do you still agree?
2. figuring out how to output the video signal, according to your hints (to be studied yet)

Bye

[TerrySt]

Ouch. Definitely some arcing going on there.