CRT tube making arcs on glass diode - doesn't power up - Page 1
Forum section: electronics / repair

author: highcap
author_rank: Contributor
timestamp: August 09, 2025, 08:40:05 pm
content: Hi friends, so I have this old 17" CRT, powered it again after some 18 months and now it does continuous relay clicks and power light blinks yellow, however at times is trying but can not fully power up the tube.Upon inspection, with the case open (when the monitor goes from clicking to trying to power up the tube) there's arc snaps going right within, or on the leg of a glass diode near the HV transformer.Removed the diode and it checks open both sides.Could this be the only cause of the arc, and what type of diode is it for the replacement?Thank you for any info.

author: fzabkar
author_rank: Super Contributor
timestamp: August 09, 2025, 11:50:14 pm
content: I believe that's a spark gap.Disable the line stage and check the B+ voltage. It'll probably be of the order of 100V - 125V. It's best to load this supply with a dummy load, eg a 100W 240V incandescent lamp.Check the tuning capacitor and damper diode. These will be connected to the collector of the line output transistor (also called horizontal output). If the capacitor goes open, the flyback pulse will be excessively high.

author: inse
author_rank: Super Contributor
timestamp: August 10, 2025, 11:14:05 am
content: What does the silkscreen on the PCB read, Dxxx or SGxxx or other?I second fzabkars suspicion and propose to check all the high voltage/pulse rated capacitors for cracks.

author: highcap
author_rank: Contributor
timestamp: August 11, 2025, 11:37:44 am
content: Thank you both for the advice.I have found the HOT and could trace the damper diode which looks like a transistor but has 2 legs, on the radiator next to the HOT, and it measures fine, though I could not figure out where the tuning capacitor is. I have checked the high voltage caps (100-200v) and they checked fine, I mean no short, as I don't have an ESR meter.I have added more detailed pics, maybe I can get more help...I'm yet to do the suggested check of disabling the line stage, cause I'm still too newbie and I'm afraid not to mess up things by not following advice correctly, but I will progress over time hopefully.The markings on the spark tube is SG702. SG probably from spark gap.Thank you all.

author: inse
author_rank: Super Contributor
timestamp: August 11, 2025, 06:30:38 pm
content: Ok, this is obviously a PC multisync monitor…Did you check those blue ceramic disc capacitors C266 and those close to J519 for cracks?BTW, I would also clean the PCB from that grime - dust that has become moist may also lead to arcing and leaking.Has it been stored in a basement, probably?

author: highcap
author_rank: Contributor
timestamp: August 14, 2025, 05:42:55 pm
content: It's an Eizo.I have checked the blue disc caps and they're fine.Yes the monitor wasn't properly stored... it was even more dirty when I got it and I have cleaned the boards with isopropyl alcohol as it was the only thing at hand.Since the last cleaning and test a while back, the HOT radiator developed some kind of salty leak that touched with the board, and after seeing the monitor enters standby safety mode, I have opened the case and cleaned that plus re-cleaned the boards. Obviously not a great job, will need to get some special solutions.

author: shakalnokturn
author_rank: Super Contributor
timestamp: August 17, 2025, 12:51:54 am
content: It would help to know what output or CRT pin that spark gap ties to.In my experience the resistors in the voltage divider in the HV transformer's regulation often fail leading to secondaries going too high.

author: xenoxaos
author_rank: Contributor
timestamp: August 17, 2025, 01:39:20 am
content: I'd definitely start with a thorough cleaning. Making sure everything is discharged...hot soap and water and a paint brush to scrub it all. Rinse thoroughly and then rinse again. If you have really bad water get a gallon of distilled/deionized water from the grocery store (usually around the baby food/formula area) and rinse it again with that. Then give a quick squirt of IPA and let it run off the board. This should remove any remaining gunk and help the water dry faster. Set in front of a fan and forget about it for a few days and then turn it a different angle. And then forget it again.Water and electricity aren't friends... but that doesn't mean you can't clean and scrub dirty boards as long as there's no charge anywhere and it's completely dry before trying again.If I have a board that I had to do a lot of work on and theres a lot of flux and crud remaining... I'll throw it in an ultrasonic cleaner to make it nice and clean.