CRT's...  (Or How can I make myself fly across the shop?) - Page 1
Forum section: electronics / beginners

author: Skimask
author_rank: Super Contributor
timestamp: January 04, 2013, 11:11:14 pm
content: Picked up a "contract" to do light work repair CRTs on Dart board games, as well as other machines, for a local provider. I say light work meaning fixing the obvious...cold solder joints, obviously blown parts, etc.Regarding CRT safety these days, is this document still relevant?http://www.crtsolutions.com/CRTSafety.pdfI found a handful of other references...PDFs, documents, etc, and they pretty much all cover the same items.Anything major I'm missing? Besides the experience of being thrown across the room

author: vk6zgo
author_rank: Super Contributor
timestamp: January 05, 2013, 03:12:22 am
content: It is fairly relevant,although a bit paranoid!Modern CRTs are extremely unlikely to implode,due to their construction.Very early CRTs were made of much lighter construction,& could implode easily,with associated danger.Over many years,I replaced hundreds of CRTs safely.Those tubes that we didn't get rebuilt were chucked in a big "dumpster".This did not cause them to implode,neither did breaking the tube necks with a long length of steel pipe!The only exceptions were some very old ones from the late 1950s,which did make a bit of a fuss,but even they were not what we had been led to expect.A "zap" from a EHT lead will be quite nasty,but the chances of getting one are fairly remote.Just make sure the monitor is turned off,grab a suitable ( fairly large) screwdriver,connect a lead with one end on the blade,& the other on the monitor chassis,push the blade under the flexible "Ultor" EHT connection.You will feel the blade touch the metal part of the connector,followed by a loud "zap" as the tube discharges.It is then quite easy to remove the EHT lead.The BIG thing with safety,is unless you need the thingonto adjust something,to always make sure you have turned the poweroff,& unplugged the mains power cord.The "safety freaks" always zoom in on the EHT because it looks scary,but the place you could get seriously zapped is in the switchmodemains power supply.Even when turned off,the capacitors following the input bridge rectifier carry a DC charge which is close to the peak voltage of the incoming mains.For a 240V RMS system,this is around 340v,or for a 120v RMS system,around 170v.These big caps have very low internal resistance,so can source quite serious current.Don't become so terrified that you don't continue to work on these devices---an application of basic commonsense makes such workconsiderably safer than things you do every day without thinking about them--such as crossing a busy road.

author: Skimask
author_rank: Super Contributor
timestamp: January 05, 2013, 04:43:34 am
content: I'm not too worried about the implosion risk. That's more a problem with being clumsy vs. accidentally dropping an o'scope on the neck or something.I've shot at a number of CRTs over the years, either out back on the farm, or out back at somebody else's farm. If I wasn't so worried about the bullet itself being astray, I'd stand right next to a CRT that was about to be shot...although with my back to it...wearing a heavy shirt/coat...with goggles...with gloves...Good times at the old farmstead...I don't plan on working on any CRTs with power on other than to do the basic knob adjustments, the stuff that's easily accessible.Like I said in the opener, light work, basically triage work, do the simple repairs before spending the big $$$ to ship it off to the pro's.Cap's in the PSU - noted. No different than any other SMPS.EHT - I assume you're talking about the flyback xfrmr and the anode cap?What does the E stand for? I assume HT is High Tension.Screwdriver (or other insulated handled probe of some sort), alligator clips, chunk of wire, a low-low value higher wattage resistor...Check... Will build up a couple tonight. One heavier duty one for the initial shock, and one to leave on there afterwards to bleed it down over time.All in all, not really a whole lot to worry about as long as a guy is aware...

author: GeoffS
author_rank: Unknown
timestamp: January 05, 2013, 05:55:09 am
content: Having spent a few years at one time in the arcade game repair business, I can't remember having to fix a dead CRT.They are (or were) pretty rugged devices. Replacements were mostly due to screen burn in.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 05, 2013, 05:57:54 am
content: Yep! that's what I'm referring to!E is for "Extra"--just to let you know there are a lot of "jolts" there!Once the EHT (flyback) transformer is powered down,there are no high volts being produced from that source,so the only EHT left is that which is stored in the(quite small) internal capacitance of the tube.I wouldn't bother with the inline resistor for the tube,as a direct short won't hurt it.A long term bleeder isn't really necessary,either.The residual,or allegedly "built up" voltage might give you a fright if you actually put your finger in the EHT socket,but you are unlikely to do that.I remember when I was a beardless lad,many years back,having to move a bunch of old 23" BW TV tubes,one of which was not packed in a box.I picked it up,& deftly put my thumb in the EHT socket!It was a bit of an "ouch!" moment,but I didn't drop the tube,& all was well.It didn't hurt near as much as when I accidentally put my thumb on a tagstrip with 240V mains on it around the same period.Thatfelt like being hit with a claw hammer!In any case,if you are doing "triage" work,you really will have no need to remove the EHT lead or discharge the tube.Your discharge resistor set up would be best used for the SMPS caps.A note on terminology:-You seem to be using "CRT" to refer to the tube OR the complete monitor.The latter terminology is a sloppy one,used mainly by computer people,which confuses others unfamiliar with its use.Thus, we get stuff on the 'Net,such as"CRTs contain x number of pounds of lead",when they are really referring to the lead content in the solder used on the monitor's PCBs.

author: SeanB
author_rank: Super Contributor
timestamp: January 05, 2013, 06:00:21 am
content: ETH is normally in the range of 15 to 30kV, with a tube capacitance of around 2nF. Bleeder will be a thin probe to get under the cap, a well insulated wire and a series 10k or so 10W wirewound resistor. Other end is a small croc clip to clip to the CRT braided earthing strap and you are done.Common faults are dying caps ( often bulging or having shrunken sleeves) most commonly in the power supply or across the LOPT derived supplies. You have multiple supplies, often around 3-4 from the PSU, 5V, 12V 150V and possibly 30V or so, then a few others depending on the monitor derived from the LOPT. Often a standby PSU is there as well, to provide 5V standby. Cooked resistors are common, along with dry joints on the LOPT (just resolder them as a matter of course when you are in the area, if not dry now they will be soon) and the driver transistor and the efficiency diode. Dry joints there can send off semiconductors in a big way. Hot inductors are very common, just check them for dry joints, but they often just run hot.

author: Skimask
author_rank: Super Contributor
timestamp: January 05, 2013, 06:34:22 am
content: @GeoffSDone 4 so far, and have only been at it for 2 days (they've got a big backlog). Every one of them has a bad case of burn in, and so far, no dead CRTs. Just dead circuitry surrounding it.@vk6zgoA direct short might not hurt it, but a bit of resistance would probably cut back on the spark itself thereby reducing the possibility of me jumping any farther than I have toLong term bleeder isn't necessary? Is it a bad idea?I'm not beardless at the moment. Retired from the USAF Oct 2011, still growing everything out for a change after 24+ years of keeping everything nice and short."In any case,if you are doing "triage" work,you really will have no need to remove the EHT lead or discharge the tube."True, but I've also got a 4 year old that wanders around the shop once in awhile.Terminology - Yes, you are correct of course. CRT is easier to spell than monitor@SeanBBleeder - Noted...Common faults - also noted, not much different than anything else out there.

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: January 05, 2013, 11:07:50 am
content: Having done work on CRT's, the most I ever really had to do was replace vertical hold IC's and fuses... as scary as they seem on the inside, as soon as you discharge the tube and the caps on the board, it should be as safe as any other piece of electronics.This site has some good info too...http://www.overclockers.com.au/wiki/TV_/_Monitor_/_Fault_Diagnosischeers

author: G7PSK
author_rank: Super Contributor
timestamp: January 05, 2013, 05:14:23 pm
content: The only way I know to make a CRT implode is to break it on the front or the side if you break the neck you only get a slight hiss or a pop at the most, to break the front takes extreme force a sledge hammer or a bullet will do it bricks tend to bounce off.

author: Skimask
author_rank: Super Contributor
timestamp: January 06, 2013, 07:29:18 am
content: Built up a couple CRT discharging probe tool things. Work like a champ. Not afraid of discharging CRTs anymore, but not being stupid about it either.Did another 3 monitors today. Doing pretty good. Cleaned and refurb'd 7 monitors in 3 days. Swapped out dead/suspicious caps, cleaned up rotten solder joints, noisy pots, 2 vertical amp SIPs.I'm 7 for 8...or maybe 7 1/2 for 8.The 8th monitor is doing some waving around...I've looked in my books, surfed the Google, can't find anything...likely due to using the wrong search terms...(monitor waving, monitor wavy, monitor lower 2/3 waves, etc).Anybody got any better search terms so I can look this up for myself? (or heck, if you've got a definitive answer/fix, I'm all ears)

author: justanothercanuck
author_rank: Frequent Contributor
timestamp: January 06, 2013, 01:34:42 pm
content: I've seen that before, but I can't seem to remember the fix for that, it's definitely in the TV though...You could try replacing the vertical, but that might not be it though, because it doesn't affect the whole screen... but I'd check around it for capacitors too. Hope you find it.

author: SeanB
author_rank: Super Contributor
timestamp: January 06, 2013, 01:49:41 pm
content: Power supply capacitors are low in value, leading to 100Hz ripple on the power rails. Check the input capacitor, caps on power lines from the PSU for low value. If it was a colour monitor I would also check the degaussing posistor ( 2 or 3 lead white or black/blue device by the power inlet) that it was not broken ( rattling noise is a good giveaway when removed) or leaking, though now these often have a relay that disconnects them when not used to save 5W of power. Check also for open circuit mains rectifier diodes.

author: Skimask
author_rank: Super Contributor
timestamp: January 06, 2013, 07:08:48 pm
content: Good call...PSU ripple - Somehow I got to thinking about it when I woke up from a dead sleep last night.The transformer assembly is located in the case near the lower left corner of the screen, so if it was a magnet, it might "pull" on the screen a bit.NTSC is 29.97 interlaced frames/sec.... Double that, 59.94hz. PSU ripple would show up at 60Hz & 120Hz. If you watch the video, you'll see the "wave" occur about every 8+ seconds.1 / .06 = 16.6, divide that by 2, 8.3 seconds. Another look at the video shows about the same time.So next thing is gonna be to pull the transformer assembly out of the way and see what happens......as well as double-double-check the PSU caps and diodes.

author: Skimask
author_rank: Super Contributor
timestamp: January 07, 2013, 07:09:47 am
content: bah...moved the transformer/psu assembly, no change.swapped out all the electrolytics and the big square ceramics (???type???), no change.Wishing I knew my CRTs better...

author: Teneyes
author_rank: Frequent Contributor
timestamp: January 07, 2013, 07:39:25 am
content: Quote from: vk6zgo on January 05, 2013, 05:57:54 am

author: Skimask
author_rank: Super Contributor
timestamp: January 07, 2013, 08:31:02 am
content: @SeanB - Good call! I thought I replaced all the caps in the PSU section earlier tonight. Wrong... Missed one. And that fixed it. Found the schematics. That particular cap looks like it was a smoothing cap across the feedback for the B+ regulator. Took it out. Had good cap's, but really high ESR compared to the one I put in there. Been running for almost an hour now. No wavy-ness happening.

author: SeanB
author_rank: Super Contributor
timestamp: January 07, 2013, 04:57:50 pm
content: Often with these problems look at the small electrolytics as well, they can do really funny things when they are dying. Good to see you are learning from experience. The monitor should now be good for a few more years.I have a whole stash of old CGA mono monitors in a store at work, they are there for spare parts for CCTV cameras, just in case the ones there die, though the dying ones are old colour TV sets. That also has the wave on it, been in and changed the one 10uF 160V cap on the 100V rail with a 22uF 200V unit ( scrap board that had the closest donor cap that was still looking reasonable) which reduced it a lot. Old monitor, very flat CRT and badly burnt colours on it, so not doing more now it shows a picture. Next step will be a drop in of a cheap LCD TV set.

author: Skimask
author_rank: Super Contributor
timestamp: January 08, 2013, 12:05:04 am
content: That's basically what I found on 2 more I fixed today. For cap values, close enough is good enough, for the most part. Higher cap values, higher voltage ratings, ok. The cap's I took out had good values, but were leaking on the ohmmeter.I failed completely on a 3rd one though. Horizontal has all sorts of wavy going on, top to bottom. Swapped out all the caps, including the little ceramic discs. The H-width coil (or is it the H-position coil?) is jacked up where you put in the adjustment tool, and according to my reading, that usually means that the core is mangled. meh...looks like it just turned into a spare parts rig unless I can find another coil.

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 06:15:23 am
content: Just remember with those coils is that they are polarised, and if you get a used coil from a scrapped TV set try it both ways and see where it is most linear. They have a magnet glued to the ferrite that places a bias so that the coil will saturate at a certain current to correct the scan.

author: Skimask
author_rank: Super Contributor
timestamp: January 08, 2013, 07:57:38 am
content: Learned something else right there...and makes sense.This latest CRT has different waves going all down both sides, kinda like a jagged edge of a puzzle, not a smooth sine wave (or multiple thereof), almost random, but it is repeatable between power-ons.I don't have much of a stock of spare parts yet, so I'm giving up on that one anyway. The screen burn is much worse than the others so far. The whole "time vs money" thing.

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 02:56:33 pm
content: Caps again, but the film ones in the horizontal circuit, or dry joints on the LOPT, or a dying lopt all cause this. If the tube is very poor check as well high value resistors on the tube base and the focus circuit ( but be aware that the voltage varies between 600 to 2000v and is at a very high impedance). Also check beam current limiting resistors ( on the lopt low side, a resistor of a few k between a pin and ground, connected to the ground side of the high voltage winding). Often the HT lead is broken inside either the lopt or the tube cap, or the resistor that is often there as a spark limiter is open circuit, but with 25kV it will pass current.You think I have read a lot of Television over the years, and repaired a few. RIP Les Lawry Johns, he always did an interesting and entertaining column each month. With the missus and the various characters he met, along with the opposition Snoddies.