CRT Repair - Any analog wizards here? EV910B Runaway HV

Communities DIY Corner
1

tl;dr Where should I start looking to diagnose a runaway HV (45kV+)? Can I power up a CRT mainboard completely disconnected from the tube without damaging anything?

Been working on this CRT monitor (EV910B) for a week and I’ve landed at a very exciting problem.

Cleaning the board, adjusting B+ and HV got the picture looking quite good, but redraw lines would appear/disappear sometimes after measuring the HV at the anode cap. Then the screen would look different in the next morning and need further adjustment. Sometimes there wouldn’t be a picture until restarting a few times. Sometimes it would need a good whack to bring the picture back to full screen, but I alleviated that problem by reflowing the connections at the flyback and the neckboard connector.

Over the weekend the tube started arcing HARD from the anode cap to the frame / conductive coating on the tube. The highest HV I measured was 45kv. Screen is blank.

X-ray protection is connected in the diagram, but my model number (EV910B) is an older revision than the EV910C that this manual is for. As far as I can tell, the x-ray pin on both main ICs isn’t connected to anything.

I’ve checked every electrolytic capacitor on the board and they all measure good. I’m currently working my way through testing every component, starting at the flyback, because I don’t REALLY know what else to do.

B+ measures at ~185, but I’ve been killing power before the arcing starts so that may well be changing.

To help drive my deeper understanding of how the circuit works, what section of the board should I be investigating first?

I would really prefer not to smoke the tube if I haven’t already, is there any special risk to damaging the board if I power it on without the tube installed?

Service Manual: LG - EV910 - Monitor -- Opweb, the free download archive if you don’t trust me or lg–ca-84–service–ID7190.zip (6.4 MB) if you do

Here are some pictures I took during disassembly / after cleaning. Might not be much use for troubleshooting but they help me get cables back in the right place.

20241216_144100
20241216_1441001920×3950 604 KB
20241216_152744
20241216_1527441920×3950 881 KB

20241218_044237
20241218_0442371920×3950 598 KB
20241216_143046
20241216_1430461920×3950 367 KB

20241216_143057
20241216_1430571920×3950 422 KB

3 Likes
2

The only thing I could suggest is to have a look at retro tech on YouTube to see if he has ever covered anything like this and maybe reach out to Steve and see if he is willing to help?
Sorry for the not too helpful answer, but I hope you manage to get it sorted.

1 Like
3

The only sources for CRT repair content on yt that I know is: @adriansdigitalbasement.com on Bluesky

He really explains how stuff works, perhaps you find some hints in one of his videos

1 Like
4

Another youtube resource would be joe’s pinball repair!

1 Like
5

I’ve got a set of 100x probes for my scope in the mail, once that gets here I can measure the output of the regulator that drives the flyback. As far as I’ve been able to logic out:

  • If it’s output voltage is high, then it’s busted
  • If it’s output frequency is too high, something driving IT is busted.
  • If it’s neither, then I have no idea

This has been a fantastic excuse to buy some HV gear.

2 Likes
6

New probes did their job, the output of the regulator driving the flyback was about twice as high as it was supposed to be. Didn’t find any other glaring faults.

By some miracle I was able to source a replacement regulator, we’ll see what happens next week when it arrives.

2 Likes
7

Removed the old part and compared it with the new one, found what seemed like a blown diode within the part. Installed the new one and no change :man_facepalming:

Haven’t had any luck sourcing a datasheet for it, that would be a huge help.

1 Like