Tektronix 475 crt arcing - Page 1
Forum section: electronics / repair

author: Warsp
author_rank: Contributor
timestamp: September 25, 2023, 03:10:30 am
content: Hi all.I have been having a problem with the CRT on my tektronix 475 arcing to its shielding. Previously the anode wire was arcing, but after fixing it, it started to arc somewhere else. Currently it sounds like a taser whenever it turns on and is usually accompanied by the screen flashing and sometimes a faint circular pattern on the screen. I think it might be arcing from the body of the CRT, but I don't think that could happen since the body of the CRT is grounded. Could I just fill the shielding with spray foam or cover the inside of the shielding with insulating paint to permanently fix the arcing?https://youtu.be/N3wBouVvvYE

author: Paceguy
author_rank: Regular Contributor
timestamp: September 25, 2023, 10:27:17 am
content: Have you tried corona high voltage dope ? I used that stuff back in the 70's-80's on arcing problems on TV flybacks and crt anodes. A couple of coatings usually solved the problem. GC Electroncs and MG Chemicals still make that stuff and it's available online thru multiple sources.To pinpoint where it's arcing, it's best to do it in a dark room, and usually you can see the arc. By looking at your photo, I would suspect that white area where the anode wire connects to the crt and the red anode wire itself. A hairline crack in insulation somewhere is what I would suspect.

author: Warsp
author_rank: Contributor
timestamp: October 16, 2023, 07:41:31 am
content: Here is a pretty late update. I have pretty much covered what I believe to be almost every surface of the crt anode and all of the surrounding areas with silicone tape and corona dope, and the problem has only gotten worse. Somehow, I think it was arcing less when the entire anode wire of the crt was covered in carbon dust than it is now with my "repair". Is there anything else that could possibly fix this or is it time to cut my losses and buy a new crt or better, another parts machine. It will really be a shame though if I have to throw out the crt since it is in such good shape besides the arcing.

author: Gyro
author_rank: Super Contributor
timestamp: October 16, 2023, 05:59:27 pm
content: It sound as if the wire insulation may be getting a little bit 'porous' and applying a resistive layer to the outside is helping to spread the charge and dissipate the point source corona. short of taking the end connector off the wire and putting the whole thing in PTFE sleeving, it sounds like a good pragmatic solution that should prevent the insulation from degrading further. It's certainly not worth condemning an otherwise perfectly good CRT.

author: Warsp
author_rank: Contributor
timestamp: October 16, 2023, 07:23:01 pm
content: That is probably what is happening. It also appears that not only is the anode wire arcing, the entire bare ceramic part near the screen is also arcing. I am going to start stripping the corona dope off the wire and go from there.

author: jonpaul
author_rank: Super Contributor
timestamp: October 16, 2023, 09:23:22 pm
content: carbon tracking. One arc caused by dust, dirt, age of insulation breeds more as the insulation carbonizes, forms a track of carbon and a path for future faults.Clean CRT/DAG/anode conn/lead with 99% ISO alcInspect with mag and light for carbonreplace leads and connectors as neededJ

author: Warsp
author_rank: Contributor
timestamp: October 21, 2023, 08:24:12 am
content: Here is what will probably be the final update. I have tried cleaning the area around the anode with IPA, acetone, and even abrasives but despite all odds, the ceramic part of the crt is still arcing to the chassis. I have also found that not only is the anode wire arcing, the boot on the crt also has some hv leakage. At this point, I have concluded that the crt is beyond repair. If the crt is somehow repairable, it is probably not worth the many hours I would have to spend, and the materials I would have to buy to repair it. Even if I can patch all of the "holes" in the anode, it is more than likely that the anode would just find another way to start arcing again. I am still open to any suggestions on ways to fix this, but I am also going to start looking for replacements.

author: jonpaul
author_rank: Super Contributor
timestamp: October 21, 2023, 04:11:15 pm
content: try stronger solvents and ultrasonic cleaner bath if available.Jon

author: Gyro
author_rank: Super Contributor
timestamp: October 21, 2023, 04:52:35 pm
content: Quote from: jonpaul on October 21, 2023, 04:11:15 pmProbably a really bad idea, the EHT lead is captive to the CRT!

author: Andy Watson
author_rank: Super Contributor
timestamp: October 21, 2023, 05:49:56 pm
content: Do you have the means to test the EHT voltage? IIRC there is a test point on the negative cathode supply at -2450V. The EHT oscillator is stablized via feedback from the cathode voltage via a thick-film resistor - which has been known to fail. If the resistor is open, or any of the feedback loop is compromised, there could be excessive voltage present.

author: Warsp
author_rank: Contributor
timestamp: October 21, 2023, 06:16:17 pm
content: I tested the hv and the correct voltage was present. The tube used to only arcs when the anode was completely discharged but now it ticks when it is on as well. As for solvents, paint thinner was the strongest thing I had and it barely did anything. The only thing that sort of had an effect was sanding the dirty area but the dark stains on the ceramic are really tough to get out. The main issue with using any solvents on the tube is that the DAG will dissolve and have a tendency to smear into the clean area, making it very difficult to clean near the DAG.

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2023, 06:29:03 am
content: I would stop fixing and find a newer scope the 40..60 yrs old 475 all eventually fail on the HV transformer and HV multiplier as well.Even with a new CRT it will fail in a few years max. 465B, 475B are later models but 12465B is the best and last TEK CRT scopejJon