TDS794D display/VGA output problem

[d0ng3o]

Hi everyone!

I recently got my hands on a TDS794D oscilloscope with a CRT display problem (the VGA output on the back worked at the time). Since I was trying to save some money, I decided not to buy and install the LCD conversion kit, just to refurbish the CRT driver board. While I had to get to the guts of the scope anyway, I decided to swap out the DALLAS SRAM and TIMEKEEPER chips as well. I went for these parts: https://www.ebay.com/itm/302913384290.

As I worked my way through the scope, I got to the CRT driver board and noticed no significant failures other than that the CRT anode wire had arced to the enclosure and that the high voltage fuse (2.5A 250V, on the CRT driver board) was blown. No leaky caps, no burned/blown components. I then continued by isolating the anode wire with RTV silicone and added some electrical tape to the enclosue in order to further provide isolation. After replacing the fuse, I assembled the scope and gave it a go - that's where the problems began.

At first it seemed to boot up normally but then the scope went into some kind of a rapid boot loop, powering up for a second and then shutting down again. I decided to disconnect the CRT driver board from the main board and then the scope would seemingly boot up normally. But now the VGA output on the back does not work as well. I checked all the voltages detailed in the service manual (https://w140.com/tekwiki/images/3/31/071-0627-02.pdf), they were nominal.

Then tried the troubleshooting process described in the manual, but the only thing I got on the on-board led display was a flashing code of ".b", on which I did not find any information.

So, have I fried the some components on the main board? Is there any hope to fixing it still? Has anyone else encountered anything similar? What should I do next?

Thanks for your time!

[Tantratron]

Hello, I've got similar issues while repairing a TDS794D and TDS540C in the past.

First the anode arcing failure seems unfortunately more frequent issue, for sure it is explained by this very good YouTuber in this video wether for color CRT or mono CRT.



Chances are high your CRT driver is dead which then will trip on and off the main PSU wether your CPU board is OK or not. Disconnecting the J5 connector which derives 24Vdc power bus through the logic board will prove then full boot (no power reset) to work, this happened to me with the TDS540C.

So either you repair the CRT electronic board or go with a LCD kit.

Now there is another topic regarding the CPU (logic) board which you have replaced the NVRAM's to solve another well know issue (battery life then brick of the logic board). Maybe you did not re-program correctly the DS1486 which could then prevent full boot, you need to provide more information.

Anyway there are two topics: CRT failure and logic board boot failure.

Albert