Cleaning a CRT oscilloscope - Page 1
Forum section: electronics / beginners

author: nibo
author_rank: Newbie
timestamp: November 19, 2018, 03:38:25 pm
content: Hi, I'm brand new to the forum, and mostly clueless about electronics so please be patient with me.I recently bought an old CRT oscilloscope from the 80s from eBay. It seems to work quite well (and I love the vintage look) but being 30+ years old needs a bit of cleaning.I'm currently cleaning the exterior (faceplate, buttons, replacing screws) and, once I'm done, I would like to move to the inside. I've been trying to find some info on how to clean the inside of the instrument and I've become very confused. Most stuff I found relating to CRTs says that they're extremely dangerous, use thousands of volts, could explode, and should only be handled by certified professionals otherwise death is certain (I may be exaggerating a little bit but that's the general tone).I just want to blow off the accumulated dust from the case and PCB, while the scope is unplugged and left turned off for a day. Can I just blow canned air over the PCBs (one of which has a sticker with a high voltage warning), tube and transformer?Thanks!

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: November 19, 2018, 07:11:55 pm
content: Yes , you can use air can or compressor to blow off dust , just be careful not to damage any component with high pressure blast.CRT-s are only dangerous if you mess around with their HV power supply.

author: spec
author_rank: Frequent Contributor
timestamp: November 19, 2018, 08:52:42 pm
content: Hi nibo,Sounds like an interesting project- I am a bit of a scope addict and have messed with all kinds of scopes over the years. What model scope do you have? Any chance of a picture?You are wise to be cautious, but you seem to have been exposed to some pretty exaggerated warnings.The first thing, whatever you are doing, is to wear safety glasses- your eyes are easily damaged. I even wear safety glasses while mowing the lawn. The next general precaution is never put your face near to batteries, electrolytic capacitors and the like- just like you would never look down the barrel of a gun.At last to the scope:You will need a stiff, medium-sized art brush, a small container (soup bowl if the wife isn't looking), and a bottle of 95%, or purer, isopropyl alcohol (IPA).With a dry brush clean all surface dust away, especially in any nooks and crannies. A vacuum cleaner nozzle may also be used. While you are cleaning make sure you do not disturb any components. You can also blow the dust out if you wish but go easy with the pressure.There are two areas that require special cleaning and inspection, especially on a scope as old as yours: CRT, HT/EHT power supply. Over the years both areas tend to become greasy and the grease absorbs dust. The end result is that you get an unpleasant foul-smelling deposit which can be conductive and affect the operation of both areas.The CRT will be mounted in a mumetal screen which covers the whole CRT, except the viewing screen and the pins at the rear that connect to a valve holder. At the front on the neck, the mumetal screen will also have an aperture for the EHT connector, which fits into a well in the glass.There are two types of EHT connector: push fit, wire spring. The push fit type simply pulls out, but be very careful. With the spring type, squeeze the rubber boot of the connector so that the spring clip under the rubber boot is compressed. This will allow the connector to be lifted out of the well in the CRT neck. Be careful and do not use much force. The connector will pull out of the glass well quite easily if you have compressed the spring correctly. Some times the EHT connector gets stuck in the well and the boot gets stuck to the CRT neck. If this is the case apply some IPA to the area and gently push/rock the connector until it frees. Be careful though.Next, go to the tail of the CRT and gently pull off the valve holder.Clean all the areas of the CRT now exposed with the brush dipped in IPA. Pay particular attention to the area of the CRT where the EHT connector well is. Do not fill the well with IPA.Keep cleaning until every thing is squeaky clean. Do the same with the EHT connector and its rubber boot. Also clean the EHT lead in the same way. Inspect the EHT lead for cracks and if there are any insulating stand-off washers on the EHT lead also inspect them. In general, the EHT lead should be routed so that it does not get close to other parts of the scope. Stand-off insulating washers are sometimes used for this purpose.With fine wet and dry sandpaper (not woodworking sandpaper) lightly burnish the pins on the CRT base, ensuring not to use excessive force. With a fine round needle file, or better still welding pipe nozzle cleaner (cheap on ebay), lightly burnish the sockets in the valve holder. You will probably find that the two heater pins will be heavily tarnished, so pay particular attention to them. Finally, clean the valve holder and all its components with IPA.The valve holder may have a PCB connected directly to it. If this is the case, inspect all components, the PCB traces, and the solder joints, which crystallize with heat and age. This is a common fault, especially the two CRT heater joints.Give the HT/EHT PSU a similar treatment, especially checking for PCB trace cracks and crystallized joints. If the EHT PSU uses valves, burnish the valve pins and valve base sockets, as previously described. If the HT/EHT PSU uses transistors, pay particular attention to the power transistor that drives the EHT transformer. It is best to remove the power transistor and give it a good clean. Also give the mating surface of the heatsink a good clean. Then, using good thermal grease and a new mica washer (not plastic or foam), remount the power transistor using new insulating washers on the bolt(s). You do all this because often, with time and heat, the thermal grease hardens and cracks, which results in poor heat conduction. The power transistor then runs very hot and the thermal stress can cause poor performance or even complete failure.Now to the difficult part: the EHT transformer. EHT transformers are very delicate and have fine wires which easily break. But it is essential that the windings and flying leads, if present, are cleaned so be very careful. Hopefully your EHT transformer will not have flying leads.Unless you have a high end scope: Tektronix, hp etc, the EHT PSU is likely to be badly designed. Very often the high voltage terminals are so close to the EHT PSU metalwork that they ark, especially if the metal work is bent or misaligned. So check this and if it appears to be a problem modify the EHT PSU to eliminate the problem.In general be suspicious of any areas the involve high voltage/high power. This includes the x and y amplifier output transistors, which may need the same treatment as the HT/EHT transformer driver transistor.WARNINGBefore you use IPA on any parts check in a small area that the IPA does not dissolve any plastic/wax and that it does not remove any legend. I have never had a problem with scopes, but some computer parts are not compatible with IPA.UPDATE (2018_11_20) Best not to use IPA on the outside of the scope, including knobs (thanks to TERRAoperative: reply#7)And that is it- good luck

author: vk6zgo
author_rank: Super Contributor
timestamp: November 19, 2018, 11:02:31 pm
content: Quote from: nibo on November 19, 2018, 03:38:25 pmMost crud you find on the Internet is written by Computer nuts who wouldn't have the first clue about Oscilloscopes.(1) Thousands of volts?Yes, they do, but after they have been turned off for a while, that is no longer present, or has reduced to a barely detectable level.To be completely sure, leave it off for a day---- not what I would do, but I have many years experience, so am quite happy with a shorter period of time.(2) Explode?--- well, actually "implode" is the correct term, as they contain a vacuum, so if the envelope breaks, atmospheric pressure tries to fill that vacuum with air.That said, imploding was something which could happen with very early TV tubes, before they were designed to make that very unlikely.The propensity to implode is directly related to the screen size, so the CRTs used in analog 'scopes are extremely unlikely to implode violently.In my old job, we disposed of many "TV" style CRTs, which had failed in a manner in which "regunning" could not be done.As a safety precaution, we, after throwing them into the dumpster, would get a very long piece of "star picket" & bash the neck of the tube.This did not cause any visible or audible fuss with the more modern (post early 1960s) tubes, & even some really "vintage" ones just let out a bit of a "sigh".I think the horror stories about "imploding" originated in the large Radar CRTs which were widely available Surplus in the 1950s----they really could go "bang", & needed careful handling.Safety glasses?Yes, not for injuries from "implosion", but if you are using compressed air, you might kick up something nasty & get it into your eyes.

author: Ian.M
author_rank: Super Contributor
timestamp: November 19, 2018, 11:57:53 pm
content: The implosion hazard was highest in the early days of thin face plate CRT TVs. If subjected to a mechanical shock, the neck could break where it met the bowl, and get violently sucked in by the vacuum then crash through the thin faceplate. With increasing final anode (EHT) voltages, and the increasing recognition of the dangers of low-level X-ray exposure, the industry moved towards a heavy leaded glass faceplate maintained under compression by a tensioned steel rim-band so that any cracks would not propagate catastrophically. This virtually eliminated the risk of an implosion resulting in high velocity fragments leaving the tube. Handling such a tube isn't particularly hazardous, though as its a large, heavy and still fairly fragile glass object, heavy gauntlet style gloves, stout jacket and trousers, safety workboots and safety goggles are advisable. Probably the highest risk is dropping one on your feet if it hasn't been properly discharged and you get a shock off its EHT connection point (and yes, I've seen that happen)!Back to scope cleaning:I'd be *very* cautious about using an air-duster or compressed air anywhere near any pots, switches or enclosed presets. You absolutely do not want to blow dust and debris into controls that have been shielded from it that will be very difficult to clean out and re-lubricate.A 1/2" natural bristle paintbrush with an unpainted wooden handle is the ideal tool for such cleanup work, together with a small crevice cleaning nozzle on a short, relatively thin flexible extension hose that attaches to the end of a regular vacuum cleaner hose. Any size on the bristles should be washed out of the brush 24H before use with warm water and a drop of dish detergent, then dry it thoroughly on paper towel, and allow to air-dry at a natural humidity level. If you are working on something highly ESD sensitive, the brush can be treated with a static dissipative spray and a conductive nozzle on the vacuum cleaner grounded via a 1Meg resistor is advisable. A small artist's brush is convenient for accessing crevices the 1/2" brush is too big for and the airline or air-duster is only really needed for the most inaccessible corners and crevices.Spec makes excellent points about cleaning high voltage sections of the scope. If you find evidence of dielectric grease in and round connectors, you will probably have to clean it off then reapply new as it will be heavily dust contaminated and can become conductive if disturbed.If a previous owner was a heavy smoker, you are probably in for a whole world of grief - if there is a heavy lair of brown tarry gritty crud over mos or all of the PCBs with more near lower vents where the airflow brought contaminates into the case, take a photo, post it and ask what to do about it.

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2018, 01:36:11 am
content: I have a 1950s 8" radar CRT and the thing is actually a bit terrifying, it's made of much thinner glass than more modern tubes and I think about that nearly 15 pounds pushing in on every square inch.I actually had an early 80s 19" tube implode near me years ago. It was a worn out tube from an arcade monitor and for some reason I was lazy and didn't crack the pip off the neck prior to tossing it in the dumpster. I lobbed it and saw it was falling short and I spun around just as the bell hit the hinge of the dumpster lid. There was a deafening boom and glass sprayed all around me with about a 10' circumference.Anyway, that said, smaller tubes are relatively safe and even the big ones almost never implode when installed in the device. The most vulnerable area is the bell.

author: spec
author_rank: Frequent Contributor
timestamp: November 20, 2018, 06:04:16 am
content: Quote from: james_s on November 20, 2018, 01:36:11 amBack in the days of being a low-paid engineer with a young family, I used to collect TVs for the parts. At one time, I had so many TVs around the house that my wife said she would leave if I brought another TV home. But when I did bring another TV home she didn'tAfter stripping the TVs, the tubes were put in sacks and stuck in the shed. When four or five tubes had accumulated we had a tube smashing party. The sacks with the tubes in were put in a row and our kids and the neighbor's kids would throw rocks at the sacks. They thought it was great when a tube went off.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 20, 2018, 07:12:49 am
content: Just be careful you don't clean off any markings on the knobs.I know from experience that the black numbers on the dials on my Tektronics 2253 scope are soluble in iso-alcohol.....

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2018, 11:54:54 pm
content: Quote from: spec on November 20, 2018, 06:04:16 amThese days the CRT is one of the only parts I'm interested in from junk TVs, they're no longer made anymore and it will soon become very hard to keep the classic arcade games and vintage TVs going. B&W tubes are already getting difficult to find, the 23" tubes used in some of the early Atari games are pure unobtanium.

author: spec
author_rank: Frequent Contributor
timestamp: November 21, 2018, 06:09:46 am
content: Quote from: james_s on November 20, 2018, 11:54:54 pmI didn't know that!. At one time you could get reasonably priced re-gunned CRTs of all types from a number of sources.Are arcade games a hobby or a job?

author: vk6zgo
author_rank: Super Contributor
timestamp: November 21, 2018, 06:26:17 am
content: Quote from: spec on November 21, 2018, 06:09:46 amWith the disappearance of CRT using TV sets, the "bread & butter" work for these necessarily specialisedcompanies has disappeared, so that they had the choices of increasing their prices radically for the few remaining users of CRT based equipment, finding something else to do, or worst case, close.

author: spec
author_rank: Frequent Contributor
timestamp: November 21, 2018, 06:31:54 am
content: Quote from: james_s on November 20, 2018, 11:54:54 pmNot sure what you are saying here and why.I made no mention of imploding CRTs in my post.You question the use of safety glasses and then say they protect your eyes from dustJust for the record, I now advise wearing safety glasses for pretty much any operation, from mowing the lawn, to electronics, to motorbike/car maintenance. And the reason is that your eyes are very delicate and easily damaged. I have had serious eye damage from doing the simplest things. While mowing the lawn a small splinter of wood flew off the cutter and entered my eye. And once while messing with a valve scope I got some very fine acidic dust in my eye that was as painful as hell for about a day.And other people have got solder splashes in their eyes, one of the wiremen at work had a major incident with solder in is eye.Just the other day a neighbor got a steel splinter in his eye while sawing a piece of iron by hand.All of these injuries could have been avoided by simply wearing a pair of £1.50UK safety glasses.But in my younger days I would have laughed if someone suggested wearing safety glasses, in fact safety anything

author: james_s
author_rank: Super Contributor
timestamp: November 21, 2018, 07:21:18 am
content: Quote from: spec on November 21, 2018, 06:09:46 amIt's just a hobby for me (and most others) although there are a few people who restore and sell stuff as a job. Hawkeye was the last CRT rebuilder in the USA and they shut down several years ago when the owner retired. As far as I know, RACS in France was the last rebuilder in the world and they also shut down a few years later. The equipment from Hawkeye as well as a few items from RACS are now in possession of the Early Television Foundation museum in Ohio where they have been working to get it set up and some volunteers have been learning the art of CRT rebuilding. I haven't checked the status of all that recently but given the very small volume and the fact that electron guns are going to have to be either NOS or painstakingly crafted by hand, getting a tube rebuilt is going to be very expensive and only cost effective for valuable antique sets.I forgot to mention there's another tube that is virtually impossible to find, the 19VLUP22 which is an oddball inline color tube with a 100 degree deflection angle and an unusual neck pinout, it was used in most of the Atari and all of the Sega color vector games. A complete monitor typically goes for at least $500 and the supply of replacement tubes dried up ~15 years ago. No other display technology can match the dynamic range of a vector scanned CRT, especially the B&W tubes. Modern flat panel displays look very out of place in the classic games too, it's like seeing a modern MP3 player in the dash of a 1950s car, it just looks weird and wrong.

author: nibo
author_rank: Newbie
timestamp: November 21, 2018, 08:08:16 am
content: Hi all,Sorry for the late reply. Lots of great information posted, I definitely didn't expect this much detail! Many thanks!Quote from: spec on November 19, 2018, 08:52:42 pmI'm afraid its nothing special, just a very basic Philips PM3208 (https://www.radiomuseum.org/r/philips_oscilloscope_pm_320800.html). Released in 1989 according to the linked page, so it's just about to turn 30 after all. Unfortunately, I'm not able to take any pictures of the instrument at the moment, since I'm away from my "lab" for a few days. I'll have to get back to you on that.Your guide seems to be amazing! To tell you the truth, I don't yet feel comfortable doing the kind of work you describe, so I'll definitely have to store the info and wait for my skills to build up. However, a question I have is that, unless I missed it, there's no mention of discharging the CRT or the electrolytic capacitors before doing any work. Is that because you think it is obvious, or because after leaving the scope unplugged for a day there won't be any dangerous charge left? (vk6zgo also mentions that in his message). If there's a need for discharging them, could please describe how you would do that?Lot's of great info on CRTs in the replies also! So just looking at them won't kill you after all.Quote from: TERRA Operative on November 20, 2018, 07:12:49 amOops, too late for that for meThankfully, the faceplate has survived my clumsy attacks with IPA!Thanks again to everyone who took the time to answer.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 21, 2018, 01:49:21 pm
content: Quote from: spec on November 21, 2018, 06:31:54 amIndeed, I said,""Safety glasses?Yes, not for injuries from "implosion", but if you are using compressed air, you might kick up something nasty & get it into your eyes."I was not "having a go" at you, it was more that the OP had specifically mentioned imploding, so I meant to point out that was not the reason you referred to them.Sorry, it was clumsily phrased.QuoteMy then work became very much into safety glasses at one time, but the problem with the cheap glasses supplied was that their optical qualities were appalling, making soldering a lot of small connections incredibly tiring.After a quite short time, they became scratched beyond usability, so had to be replaced.They also soiled easily, so work fitted a glasses cleaning station with a fixed spraycan of glasses cleaner facing outwardat eye level!The cleaning solution was some sort of pungent smelling "devils brew".We techs could see the obvious possibilities, so moved the cleaning station to chest height.In later years, I began to need reading glasses, so normally wore them, instead.I have found that, in situations where I am using power tools & the like, where more formal protection is necessary, that a full face visor is more comfortable, & it allows me to use my prescription glasses as well, if needed, without the possibility of damage to them.I hate real goggles with a passion, but, of course, must wear welding goggles for oxy-acetylene work.

author: Ian.M
author_rank: Super Contributor
timestamp: November 21, 2018, 02:37:08 pm
content: If you get prescription glasses with toughened glass lenses, without anti-glare coatings, they will stand up to a lot of abuse without scratching. Get the safety frames with side shields and your eyes will be adequately protected against most light projectiles without compromising comfort or your ability to do fine work.You'll still need the plastic safety goggles over the top (or even a full face visor), if there is going to be dust and/or fine debris flying everywhere, large projectiles or large quantities of hazardous liquids splashing around, but you wont be spending as much time in the cheap goggles so comfort and durability wont be as much of an issue.Back to the scope cleaning:Spec suggested cleaning the tube base socket pin holes with a "fine needle file". *DO* *NOT* do that unless the fine needle file is thinner than the tube pins or you'll distort the contacts and they will become very unreliable. Depending on the pin thickness you may be able to use a wooden toothpick dipped in contact cleaner (or even abrasive metal polish if the contacts are really badly tarnished, but it needs to be totally cleaned off with contact cleaner afterwards), but if they are too thin for a toothpick, you may need to grind the end off a small jeweller's screwdriver and grind a flat on one side of its shaft to make a D shaped reamer you can use to scrape their contact surface. The Wet & Dry paper used for cleaning the pins should be very fine - anything coarser than 400 grit will leave too many scratches. Personally, I'd get 1000 grit for that sort of job - a sheet will last a very long time if you use big cheap scissors to cut off a narrow strip to use as you need it.On discharging capacitors:After leaving it 24H, If in doubt short them directly across their pins. Yes its bad for them if there's still charge and you get a 'bang' out of it, but its much better than you finding that charge with your finger and knocking the job off the bench!The tube is a special case - you need a discharger that can be slipped under the EHT final anode connector cap till it reaches the spring contact. A strip of brass with a V notch in the end mounted inline to an insulating handle, with about 18" of grounding lead with a croc-clip on the end of it soldered to the brass strip where it joins to the handle is what you need. Round off all corners and edges with fine sandpaper so it doesn't scrape anything or leave possibly conductive scratch marks on the glass when you push it under the EHT connector rubber cap.The ground clip *MUST* be clipped to the chassis wherever the tube mounts to, not any other ground in the circuit as the surge current can blow sensitive ICs if it passes through the ground trace on a circuit board. If you are ever working on CRT monitors or TVs, there's usually a spring (or spring tensioned braid strap) stretched across one corner of the tube bowl to make contact with the external aquadag coating - clip your EHT discharger ground lead to one end of the strap.To actually discharge the tube and EHT system, *AFTER* checking the ground lead is securely corrected and in the right place, if you have difficulty inserting the brass strip under the EHT connector rubber cap, peel up the edge of the cap with a non-conductive pry tool (e.g. a plastic chopstick with the end ground to a sharp 30 deg bevel), and slip the V end of the brass strip under the cap keeping it flat to the tube surface till you feel it engage with the contact spring. Hold both tools as far away from the working end as its possible to still grip them securely - a insulating handle doesn't do any good if your fat fingers are at the wrong end of it!Ideally, your discharger would include a high voltage resistor in its ground lead to limit the surge current, but if you do that its handle needs to be designed to provide 50KV of insulation (worst case EH is up around 30KV, and its difficult to find HV resistors that wont flash over at such a high voltage. Also the part of the ground lead between the brass strip and the resistor would need to be EHT cable.Anyone unfamiliar with CRTs should read the safety brief at Silicon Sam's Repair FAQ. The main site (repairfaq.org) seems to be down currently, but here's a link to the CRT safety brief at a mirror:http://lasersam.org/sam/crtfaq.htm#crtcsa

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2018, 03:45:06 pm
content: With some care and a minimum of preparation, the inside may be washed down with water.https://www.radiomuseum.org/forum/tektronix_washing_your_instrument.html

author: 6PTsocket
author_rank: Regular Contributor
timestamp: November 21, 2018, 06:54:31 pm
content: Quote from: spec on November 19, 2018, 08:52:42 pmI have become a big fan of Endust for Electronics. It is great for exterior clean up. It works great on plastic, metal and glass. "Oil free, wax free, ammonia free" I use it on laptops, eyeglasses and even to get rid of fingerprints on stainless kitchen appliances. It has never removed anything it is not supposed to.As for vacuuming, there is a generic adapter set that everybody sells for a few bucks. It adapts a household vacuum hose to a few feet of minature hose and then to an assortment of minature brush and crevice tools. It is great for getting into tight places. Harbor Freight sells it and even they do not put a brand name on it.Sent from my SM-G900V using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: November 21, 2018, 11:40:31 pm
content: One of my favorite cleaners is some stuff called Oil Eater. It works great on that dark grime that builds up on control panels and such, I suspect because the grime is mostly skin oils. A little squirt of oil eater on an old toothbrush and it comes right off. Just wear gloves because while non-toxic, it will suck the oil right out of your skin.