Tek TDS754D High voltage arcing/discharging - Page 1
Forum section: electronics / repair

author: Jwalling
author_rank: Unknown
timestamp: May 10, 2017, 02:38:21 pm
content: Hi all,So I've got a Tek TDS754D whose high voltage is actually discharging and arcing (with a loud crack!) from beneath the anode clip to the to the interior or exterior of the CRT tube. Several times the neck of the tube actually lit up with a pretty bright flash. I verified with another working (but dim and unusable) tube that the same thing happens.My thinking is the the high voltage is far too high, and that the only suspect would be the flyback transformer is at fault with shorted windings on the primary. I don't have a high voltage probe anymore, and frankly, while this is happening wouldn't want to be messing around with it anyway!It probably killed the CRT already. Does anyone concur with my diagnosis or could it be something else? Would this likely have killed the CRT neck board as well?Thanks.

author: pigrew
author_rank: Frequent Contributor
timestamp: May 10, 2017, 03:27:07 pm
content: In one TDS784D failure I saw, the flyback transformer exploded and left soot on the chassis. The rest of the CRT board looked visually OK. I only saw it after it failed, and we didn't repair it. I don't know what the root cause was (and I have little experience with CRT repair).

author: Gyro
author_rank: Super Contributor
timestamp: May 10, 2017, 05:02:04 pm
content: It's very unlikely to be a shorted winding in the flyback transformer - that would result in exactly the opposite (lack of flyback!).A couple of possibilities:1. Contamination of the glass / underside of the anode clip with dust etc. It's probably worth cleaning both very thoroughy and applying some anti-tracking grease. I would issue the standard warnings about EHT voltages, discharging the CRT etc. but it sounds as if you're already past that point.2. The other reason for excessive EHT (if it actually is) is the supply voltage to the line output stage being too high. Check the rail voltage against the manual, check for faulty regulator etc.All the other faults I can think of would cause low or missing EHT. Are you actually getting a normal display, apart from the arcing?

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2017, 05:11:59 pm
content: I used to see this a lot with CRT projection TVs. Usually it was dust built up on the tube that then got moist and the HV would flash over, once that happens there's a carbon track and it continues to flash over. Start by removing the anode cap and clean thoroughly, then you might apply a bit of dielectric grease, you can get it from any automotive parts store near the spark plug wires and electrical stuff.It may be worth checking the HT but normally if that's too high the picture will be excessively bright and shrink. Due to regulations, CRT displays are designed so that it's very difficult for the HT to go too high, I don't think I've ever seen it in a "modern" post-1970s CRT display

author: Jwalling
author_rank: Unknown
timestamp: May 11, 2017, 10:15:26 am
content: Quote from: Gyro on May 10, 2017, 05:02:04 pmDidn't want to run it like this and risk further damage.As it turns out, the TI video chip is popped as well. With a known good HV assembly and tube, there is no video. Checked it with a scope on the two pin video output connector, and it's dead. This one is weird, the slightest touch on the plastic package on top edge of the chip on the opposite side of pin 1 affects what's seen on the CRT, but it's still garbage. I could replace the chip, but with the other problems, I decided to drop an LCD in and call it a day.Thanks all.

author: tautech
author_rank: Super Contributor
timestamp: May 11, 2017, 10:22:37 am
content: Seen this before in old dusty and grubby CRO's and as James and Gyro mention it leaves a carbon track maybe both on the CRT and the anode cap. Clean as best you can and add some HV dielectric grease. In my early days with CRO's I once got away with just silicone sealant.

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2017, 03:54:31 pm
content: If you're interested in parting with the CRT you removed, I wouldn't mind having a spare for my scope. I'm a big fan of those NuColor displays, they're very novel.

author: Jwalling
author_rank: Unknown
timestamp: May 12, 2017, 02:30:30 pm
content: Quote from: james_s on May 11, 2017, 03:54:31 pmI'm going to keep it. I work on these all the time and I know I'll use it at some point.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 13, 2017, 10:52:59 am
content: If it has an flyback, the flyback casing sometimes crack, and the coils are open to air, when powered we hear an crippling sound due to an arc/spark making its way thru, sometime i had luck in patching the hole with hv sillicone sealant, but eventually the flyback need to be changedCheck for small, very small cracks on it ??

author: Jwalling
author_rank: Unknown
timestamp: May 13, 2017, 01:11:02 pm
content: Quote from: coromonadalix on May 13, 2017, 10:52:59 amAs I said, the arcing was at the tube, not the flyback.

author: denimdragon
author_rank: Regular Contributor
timestamp: October 04, 2017, 07:40:24 pm
content: Jay, does it sound like this:This is a 754D as well.

author: texaspyro
author_rank: Super Contributor
timestamp: October 04, 2017, 08:05:07 pm
content: Quote from: denimdragon on October 04, 2017, 07:40:24 pmI know that sound... my friend has been eating Taco Bell bean burritos again!

author: denimdragon
author_rank: Regular Contributor
timestamp: October 04, 2017, 08:53:54 pm
content: Quote from: texaspyro on October 04, 2017, 08:05:07 pmThose were my thoughts when I first heard it yesterday. Where abouts in Texas are you?

author: texaspyro
author_rank: Super Contributor
timestamp: October 04, 2017, 10:23:57 pm
content: Dallas

author: Jwalling
author_rank: Unknown
timestamp: October 05, 2017, 09:54:33 am
content: From memory, I believe it did sound like that. It did not kill the tube, BTW, but if left running like that for awhile it might have. I've never seen a neck light up like that.

author: james_s
author_rank: Super Contributor
timestamp: November 15, 2017, 09:09:01 pm
content: The only time I've seen serious arcing in the neck of a CRT was when the tube had been cracked and was up to air.