REAL Noobie, Probably taking on too big of a project - Page 1
Forum section: electronics / beginners

author: That_Young_Merican_Bloke
author_rank: Newbie
timestamp: February 16, 2017, 10:55:27 pm
content: Hello All, I'm one of those few younguns interested in EE, I decided to attempt to diagnose and repair something to get a little bit of a head start. Looking back I probably should have done a little project, like restoring an all american 5 or something small like that. Instead I decided to take on the reparation of a British made, TelequipmenT S51E Oscilloscope. I don't now much about it, and a bit of digging on reddit got me two steps closer, but my fountain of knowledge stopped helping me and now I'm up the creak. as far as I can tell, the scope works a little bit, for maybe five minutes, and then something of starts to smell a bit like sulfur, there's a long rolled up package, that I believe is the issue, but I don't know what it is, or how to go about fixing it. from what I gather it's like the Couplates in Predicta TVs, but don't know exactly what is issue,of course, I have no idea what I'm doing, besides minor experience with CRTs, and fear seriously damaging the machine. all components on the inside are all factory OEM, and I want to replace them, but don't know color codes on resistors, or anything to be honest. I've put it off for so long because I'm somewhat intimidated by the large transformer and the CRT, nothing more scary than 120V AC running through my body, I don't even know the basics, of than what everything is, I don't know what they do, I need edumatication, but cannot find a good source, (College out of picture). now for my real main question, should I try to continue on this oscilloscope, or just get an old, dodgy 5 Tube AM Radio to fix up?

author: neil t
author_rank: Regular Contributor
timestamp: February 16, 2017, 11:21:49 pm
content: Can't hurt as long as you don't do something irreversible, i've always followed the learn what you need to know when you need to know it.With that said high voltage diligence is a pre-requisite and if you don't know what I mean don't stick your fingers (or tools) where they don't belong,death can be fatal and quite spectacular.best regards neil

author: tautech
author_rank: Super Contributor
timestamp: February 17, 2017, 12:24:36 am
content: There's a heap of info online including manuals, use a Google search for Telequipment S51E manualGet a manual from one of those and link us to the same.Some little history of Telequipment, they were a UK company that was acquired by Tektronix and some units were continued to be made under the Telequipment name that was listed as a division of Tek.Another good doc to give guidance:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdfGet a notebook and document everything you do.Google: principle of operation of cathode ray oscilloscopeOh, and welcome to the forum.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 04:00:41 am
content: Yes... welcome to the forum.I know it's been said above - but it is worth repeating:Working on a CRT (cathode ray tube) scope is something to be careful with. There are some sections that have thousands of volts floating around and others have hundreds. You have to respect that... or else.Taking notes is a very good thing - and I'll also add this suggestion: Photos are priceless! ...especially ones takenbeforeyou start pulling things apart.Make sure they are well lit, and in focus. These can show you how things were connected and laid out so if something gets disconnected that you didn't mark or take note of, you will have an idea as to how it goes back together. I'd prefer doing this in conjunction with a schematic, but if one is not available, at least you have a chance.Also, if you want to ask for help here, photos are often really helpful - especially if you are not sure of what a particular component may be.One word of warning about posting images here. If you upload them to the EEVblog server (as Attachments) then there will be some size limitations enforced - but if you upload very large (file size) images to an external image hosting service, try not to include them inline using the tags. Just include the image link and maybe use a smaller version of the image.The reason is simple: If you put, say, 10 5MB images inline, that means that whenever a member views that page of a thread, they will have 50MB of image data being downloaded. For many members, this is just a minor issue - but for some who have data caps and/or slow internet, it can be a major problem.

author: drussell
author_rank: Super Contributor
timestamp: February 17, 2017, 05:56:59 am
content: Sulphur burning smell sounds suspiciously like a selenium rectifier....They really stink when they go bad.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: February 17, 2017, 07:30:52 am
content: Have you tried watching MrCarlsons lab or shago066 on youtube? MrCarlson does an excelent job explaining thing but I really didn't get it until I had watched all of his videos. Tubes are a totally different animal then solid state.Posting pictures helps people understand what you are talking about and more likely to post a response.Oh and the caps on those thing have almost certainly gone bad and dried out by now. They are the rolled up tubes with axial leads and are quite large. If you don't have a way to test them just replace them. You are not going to find the exact values so you are going to have to find something close; it might have 50uf caps and that will be replaced by 47uf. MrCarlsonslab has a hole video on this.Another thing to keep in mind is that many old electronic had hot chassis. They had the neutral of the AC connected to the ground. Or there were caps connecting the AC to the chassis which wasn't a problem until they go bad making innocent metal parts LIVE wires. Its also good to find ut which parts can kill you and which will just give you cancer.

author: karoru
author_rank: Regular Contributor
timestamp: February 17, 2017, 12:42:31 pm
content: Quote from: Brumby on February 17, 2017, 04:00:41 amCareful, but not be scared of. Accidents happen when people start getting nervous, forget basic principles and get manually sloppy. Easiest way to hurt oneself is not to get acquainted with accidental electron flow but to hit something near the workbench because you probed something with shaky hands, shorted power line to ground and jumped around because of loud "bang"

author: joeqsmith
author_rank: Super Contributor
timestamp: February 17, 2017, 12:57:09 pm
content: Fun little project. I was working on an analog scope once. It was not the first time I worked with a CRT. I discharged the tube. Some time later, after I had removed the HV connector from the tube, I touched it by accident and my arm flung back. Put a small hole in the tip of my finger. Never underestimate the CRT!Be safe and have fun.

author: karoru
author_rank: Regular Contributor
timestamp: February 17, 2017, 12:58:29 pm
content: You're not a real CRT technician unless you punched yourself by accident

author: tautech
author_rank: Super Contributor
timestamp: February 17, 2017, 07:21:14 pm
content: I've fixed a good few CRO's and not had a bite from the PDA ....yet.Might have something to do with getting a bite from a 700V rectifier tube plate supply when I was a teenager....lessens learnt young are not forgotten.This old Telequipment probably hasn't got PDA but only EHT @ ~2KV, not so bad but still bites.OP seems shy, but shouldn't be....he needs to get on with this fix......the world of electronics awaits him.Probably the old scope had sat for a while without being powered up and should've been bought up slowly to let the caps reform....something a newbie likely wouldn't know.

author: james_s
author_rank: Super Contributor
timestamp: February 17, 2017, 07:32:52 pm
content: Can you take a picture of the "rolled up" object and post it so we can tell you what it is?You need to respect this stuff as it can be dangerous, but don't fear it. Just use a little common sense and be careful. Take care around the CRT, the charge they store is not going to kill you but they can implode explosively if struck and will spray glass shards everywhere. Some of the really old ones are made of thin glass and much more fragile than later types.Man, selenium rectifiers, the worst smell I have smelled in my life was from one of those burning up. I switched on an old power supply I found in my uncle's shop and it started to sizzle and spew colored smoke, seems like it was green but it was a long time ago. I yanked the cord and ran outside gagging, the whole shop reeked for weeks after that. Blech.

author: That_Young_Merican_Bloke
author_rank: Newbie
timestamp: February 18, 2017, 05:29:37 pm
content: Most often I watch Shango066 and BandersenTV for inspiration and entertainment

author: That_Young_Merican_Bloke
author_rank: Newbie
timestamp: February 18, 2017, 05:58:55 pm
content: I will attach a photo of the part later, but it is marked as MR6 on board and is right above C12 on board sketch in manual.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 19, 2017, 12:40:05 am
content: Quote from: raspberrypi on February 17, 2017, 07:30:52 amThis was common with the old "All American Five" radios which the OP mentioned in passing.The usual argument as to why this was done is that it saved the expense, size, & weight of a power transformer.Australian "mantel" radios all had power transformers (except for a few "specials" for remote areas with DC mains).As you could fit, & comfortably hold, the transformer in the palm of your hand, the weight & size argument isn't very valid.Cost?----Providing special series filament tubes, building in extra insulation, & so on, would seem to erode most of that saving.Oscilloscopes didn't use transformerless power supplies-----I'll leave it as an exercise for the reader, as to why such supplies would be a bad idea in 'scopes.QuoteThis is more common with modern equipment--Oscilloscopes almost universally had the chassis hard wired to the protective earth conductor of a 3 conductor power cord.As a British design,the Telequipment is rated from the outset for 240v AC supplies so it will be "loafing along" at 120v.

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2017, 06:21:07 am
content: Quote from: vk6zgo on February 19, 2017, 12:40:05 amIt didn't. They made those "special" tubes in *HUGE* quantities, the "AA5" design was made by nearly every radio manufacture out there and the radios were pumped out in the millions. Even decades since the last ones were manufactured NOS 50C5 and 35W4 tubes are still readily available. These tubes were very high volume parts, no more "special" than countless other tube types. No extra insulation was required, the 50V filament of the 50C5 was still relatively low voltage. Transformers have always been expensive, they require a lot of labor to assemble and use a relatively large amount of material, the power transformer was one of the most expensive single components in a typical radio at the time. The transformerless design was much cheaper and allowed radios to be made smaller, some surprisingly small. I once had a Japanese radio using miniature 9 pin tubes and the whole radio was only about 6" wide and 3" high. The 5 tube transformerless design was engineered down to a minimal cost and aside from the safety issues it really was impressive. They perform reasonably well, often even when component values have drifted far out of tolerance.

author: boffin
author_rank: Unknown
timestamp: February 19, 2017, 07:01:17 am
content: Well, actually somebody has done something similar in the past here on EEVBLOG:https://www.eevblog.com/forum/repair/repairing-bwd-509b-oscilloscope/for each of the Selenium rectifiers, two 1kv 1N4007 diodes in series + a 20k dropper resistor (you'll probably want a 10W resistor to be safe).

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2017, 07:58:27 pm
content: I dont't want to spoil the fun or demotivate you, but:I think this isnotthe right starter project!As mentioned by others, there is significantly higher voltage than 120V present in cathode ray tube equipment -- thousands of volts. As also mentioned by others: Yes, this can be handled by always applying proper operating procedures.BUT:As a beginner, in my experience youwon'talways apply proper procedures! Youwillforget that you meant to switch off the power before making that adjustment, youwillforget that that big capacitor is still charged, youwilltry to cut corners and use a non-isolated screwdriver (because that's the only one you have around)...It's a complex device and subject matter, things are new and unfamiliar and confusing enough anyway -- you just won't have the bandwidth to always think of safety in parallel. I certainly got in touch with 230V a couple of times when I started in electrics and electronics, and am glad that it was not a higher voltage...If there is another repair project that interests you, please start with that! No CRT equipment, and preferably no other tube equipment either. And if you can avoid mains voltage altogether (working on something that's powered by batteries or an external low voltage supply), that's even better. It's good to see young players starting into electronics, but it would be even better if you end up getting old with this hobby (or profession).

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2017, 08:20:03 pm
content: While the EHT is what most people fear when working with CRT equipment, it's actually considerably less dangerous than the "low" voltages in the range of a few hundred Volts present in other parts of the circuit. The anode supply to a CRT is very low current, I've been zapped a couple of times over the years and it feels like a very strong static discharge like you'd get touching a grounded object on a dry winter day. It doesn't feel good but it's not likely to kill you. The lower voltages on the other hand can deliver very powerful shocks, especially the B+ supply in a lot of monitors which can be 200V or more with a lot of current behind it.

author: ebastler
author_rank: Super Contributor
timestamp: February 19, 2017, 09:37:03 pm
content: Quote from: james_s on February 19, 2017, 08:20:03 pmYes, I know. Hence my closing advice to not chooseanytube equipment for first learning experience.My key point is that, as a beginner, you have too many things on your mind to be able to concentrate onwhereto expectwhathigh voltage, underwhichboundary conditions, while you are doing all this unfamiliar work on an unfamiliar piece of equipment. So, please start gaining experience with something that can't harm you. (Thats not "you" of course, james_s, but you get my drift)

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2017, 10:35:50 pm
content: Oh yeah, I get what you're saying, I was only commenting on the fact that if one does choose to work on a CRT device, they should not get too caught up in worrying about the EHT because there are other more dangerous lower voltages in the circuit. Not just tube equipment either but anything mains powered. A true beginner ought to stick to low voltage devices that run on DC or wall adapters first and poke around in higher voltage equipment once they can identify components at a glance and have a general idea of what not to touch and when.