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Author Topic: Please Help - Noob Troubleshooting Arcing  (Read 196 times)

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ArcadeWilly

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Please Help - Noob Troubleshooting Arcing
« on: February 17, 2020, 07:24:53 am »
Hi guys, long time lurker but first time poster.

I've gotten into the arcade hobby fairly recently and built my first MAME machine. Of course I opted for a CRT, and I read up about multisync monitors. They seemed like the best fit since I wanted to play arcade games from various years as well as a few different consoles. I picked up a Wells Gardner 27D9204, I'm aware they don't have the best rep but too late now. Unfortunately it's had some problems, I've read up a lot but still find myself a bit lost/uncertain.

I don't have any practical experience working with CRT's. I've repaired/swapped components on consumer electronics before (including LCD displays) and I'm reasonably good at building vape coils (mostly focuses on Ohm's Law) but would definitely call myself a beginner to electronics.

The symptoms have been a cracking sound and picture briefly shrinking before returning to normal. I would think it's arcing somewhere, so I've read it could be dust, frayed insulation on flyback/HV wire or somewhere under the anode cap.

The other night I started with a dust clean using canned air. A few spots on the tube have stubborn dust so I thought I might have to discharge and wipe it off with a rag. I've never discharged before and I'm a bit terrified but saw a really good video linked here which made me feel a lot more confident. I picked up what I need to build a discharge tool and looking forward to diving in! I was still curious about a few things though;

- I have heard tubes can build charge. Do I need to establish an ongoing ground while working? I have seen some people warning of this clip the anode to the frame which didn't really make any sense to me.
- I assume I only need to make contact with one prong under the anode cap to discharge, not both?
- Do I need to check for any connection between chassis/frame, or is it simply enough that the frame is bolted to the tube ears?
- Is there any issue with magnetic tips? Can I simply sand them back if they're a problem? (I read to sand back where solder will be applied)

While I was air cleaning I actually noticed a little uninsulated wire blowing around which was touching a neck bolt and one of the pins above the yoke. There are some other wires wrapped on the plastic behind the yoke but they're a distinct copper or sage colour, not silver. I don't yet know if it's attached to anything since I wanted to discharge before going near it but I assume that could be part of the issue. Will update once I've gotten in there. Seems like the monitor was tubeswapped with a Thomson TV.

The flyback seems to have dust caked on, will try wiping it off but if that's not enough is there a recommended cleaner? Is the flyback any danger once the tube is discharged, or any necessary caps to discharge?

Aside from being paranoid I'd like to discharge to check if there has been any arcing underneath the anode cap. I've been told to apply corona dope if that's the case but can't find much information. I was wondering how much to actually apply, and if it needs to be applied to the cup or tube if that's the issue?

I haven't reseated or removed the neckboard and wasn't able to get much dust out. It's doable but need to remove part of the frame! Is there any serious risk of necking the tube or just a case of being careful/patient?

I haven't been able to do the dark room check since it was reasonably intermittent (aside from the last time I powered it on - it was constant but haven't powered on since then) and I'm in extremely limited space, so wouldn't even be able to get a foot away from the monitor which makes me a bit nervous.

I've attached some pictures of the yoke wire but can take more if needed (most were too big to attach but can compress + attach in the morning). Sorry for the somewhat long post and possible stupid questions, but hoping I can learn quick and start helping others soon!

buttersoft

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Re: Please Help - Noob Troubleshooting Arcing
« Reply #1 on: February 18, 2020, 05:23:20 pm »
Old, well-used tubes can bleed charge for days. Less each time, but i'd be discharging before handling if five minutes have passed, until you know how this particular tube performs. Remember that you may never get any arc snap or sound if the tube has a bleeder cap on it.

One prong is fine, they're both connected.

A tube is a big capacitor. Discharging is the act of bringing the aquadag outside, or black graphite coating around the back of the tube, in contact with the aquadag inside, via the anode button i.e. connecting the two plates of the capacitor. The outside coating is connected to a grounding strap, usually, and from there will be connected to the neckboard via ground wire, and this may be connected to the frame too, if the setup has one. This may or may not be connected to mains earth, depending on the setup, and in theory you don't need to unplug from the wall if the power is off, but you'd be stupid not to. You clip onto the grounding strap with one end of your lead, and connect the other end to the anode button or the wire prongs of the shatter hook on the end of the flyback lead that connects to the anode button, as you noted above.

Hmmm, magnetic tips. Magnets and CRT's are an interesting mix. Never leave a permanent magnet anywhere near your CRT unless you mean to because you're trying to manipulate the colours. Temporarily is generally fine, if you're not talking about some enormous commercial subwoofer that's going to distort the shadowmask. I'd err on the side of caution though, and use a nonmagnetic tip if you can.

The loose wire round the yoke isn't that unusual. If it's a tinned copper loop, make sure not to break it. Otherwise just tuck it out of the way so it won't contact anything.

I wouldn't clean the flyback with anything stronger than a rag with a bit of isopropyl, or just compressed air and a toothbrush. Don't bang it around either. The potting gets microscopic cracks in it with time, and you don't want to hurry the process along. You certainly don't want to risk dissolving anything, or even starting to. If the flyback is arcing it's done, and you need a new one.

There can be many causes of arcing, but cleaning is a good first step. Often, if dust around or under the anode cap is a problem the monitor will sound like frying bacon. And if it's less severe you often get a corona you can only see in the dark. Not sure what's happening with yours. Clean the dust off the tube, clean the inside of the anode cap with isopropyl on a cloth, rub a little spark-plug grease or other high-dielectric grease around the edge of the cap to help seal it, and clean around and inside the anode button. If there's any rust you want to remove it, which might mean shoving a wad of steel wool into the anode button and working it around. TBH even if there's no rust giving the shatter hook a rub with some fine sandpaper will help it make contact.

If the cleaning doesn't fix it, you really want to watch what's going on in the dark and try to pinpoint the arc.

« Last Edit: February 18, 2020, 05:30:52 pm by buttersoft »

ArcadeWilly

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Re: Please Help - Noob Troubleshooting Arcing
« Reply #2 on: February 22, 2020, 10:48:30 pm »
Thanks for your extensive reply!

The screwdriver only seemed to have a light coating which sanded mostly off by hand (light shadow left behind) but will try to grab something without it, or worst case see how it cleans up with an electric sander.

I had a tech out a few months back because I wasn't comfortable adjusting the raster rotation and it had made a LOT of crackling the last time I'd run it which scared the life out of me. He said it was likely from dust but only really wiped it down. He did say the flyback looked to be in really good nick but to watch out since they're getting hard to come by, so I'm praying it just needs a good clean.

Been a bit flat out but just waiting on the corona dope to be delivered which should be here in the next two days! I actually had a lot of trouble trying to find any videos on reapplying, they were all in other languages or just videos of people arcing anode to aquadag so might try to put something together.

Also a really huge thanks for the post on CRT_Emudriver configuration! Read through it a few times and some of your other troubleshooting posts which helped a lot.