Restoring Old 0-scopes, am i going to die?

quick question.

i recently got back into electronics after a long while. i just found a tek 2246 o-scope dirt cheap.

getting the new scope, reminded me that i had picked up 2 cheap "vintage" scopes back in the day, and recently renewed intrest in them enough to have me dig them out.

the tek has a small screwdriver operated calibration knob on the front to adjust if the trace is not perpendicular to the screen. the 2 older ones dont. i just cleaned up a lab-volt 792 that is a bit off. th pots are still not fully functional/noisy, i plan on just trying to spray in some contact cleaner and work them back and forth a while. the other is a rca wo-33a that is WAAAY off. this one i have to take the blame for, i removed the tube to get at the screen overlay that was crumpled/wrinkled and needed to be straightened out.

so, my question. i know there asr a lot of high voltages going around in there. if i were to start each of them up, outsode of the case, and loosen the clamp on the back of the crt to rotate it to allign the gnd trace with the grid, is that ok, or am i going to end up electrocuting myself? is that the preferred method, or is there something else i should be doing?

one more thing, im actually quite impressed w/ the lab-volt. i really think its quite usefull yet/ not totally obsolete. one thing though, i though it was scrap, the calibration was way off. then i tested it with the cal signal from the tek, and vice versa, and found out that the cal signal on the labvolt was actually off by quite a bit. is there any way to calibrate the calibration? i found a manual for the rca online, but i cant find anything on the lab volt to help me with it at all.

thanks

the inside of it all looks very well constructed, clean and neat, and mostly " modern" components. there are a ton of small trim pots in it, but they are all soldered into position.

ya might get bit a little and say a few things that make yer momma grab the bar of soap ... but , you won't be electrocuted .
well , i should not say won't ... there is a small chance but you would really have to botch things up .

how do you think they align those crts to start with ?

Pretty much depends on what you touch (and what you touch it with)...

If you keep one hand in your pocket (and the rest of your body is NOT grounded), there's little danger of a heart-stopping event.

Hand to hand/leg contact (allowing current through the chest area) is very dangerous.

i know about not shorting out caps and what not, just wondering if the glass tube was enough to insulate the voltage in the tube.

i could wear heavy rubber gloves, but i feel that would be more of a hazard, as i wouldnt really be able to tell what else i was bumping/hitting/potentially shorting out.

would a pair of disposable nitrile gloves actually help any?

Simple tubes, with no connections up the bulb, have all the nasties at the end, in the socket. Steer clear of that and you'll be okay. This type of tube is usually operated with the 2nd anode at or near ground (relatively speaking... give or take 300V say), which is connected to the inside coating. Instead of the anode being at HV, the cathode is at some negative voltage, usually on the order of -2kV.

If you see pins on the side of the tube, those are probably deflection plates at B+ or thereabouts: not jump-through-the-air stuff, but still shocking (200V?).

if you see a heavy rubber cable going up towards the front, ending in a suction cup connector, you have an accelerator type. Tektronix used a lot of these. Negative voltage is still used in the gun, and the deflection plates are around ground (relatively speaking, give or take up to 400V for a tube circuit), but this HV connector is a good bit higher, maybe +4 to +10kV. (I don't think scopes used much more than that, but big TVs took it up to 30kV and more!) In this case, there's usually a coating on the *outside* of the tube, which is (or is supposed to be) grounded. As long as that's grounded, it's okay to touch in theory, but I wouldn't suggest getting very close to anything *else* inside there that might be at high voltages.

Tim

Golden rule is if you don't know what you're doing, don't do it. We would never advocate a total novice delving into the back of an old tube TV - 'scopes would qualify the same way.

Gain experience first - study the device, compare the schematic to the physical parts and layout, identify where is safe and where isn't. Learn about electrocution and how to prevent it and how to assist someone who has suffered it. Don't work alone.

But your question "am I going to die...?", the answer is yes. We all do eventually.

"Pretty much depends on what you touch (and what you touch it with)..."
this is true .

"safety first" ... something this fellow has no concept of as well as common sense .
hopefully he has lost the ability to procreate .

http://www.youtube.com/watch?v=HGb1flXFh3s

i can assure you, 100%, beyond any doubt, i will not be sticking my d*** in it, so im good there.

The golden rule of putting your fingers into live electronics, is to keep the other hand in the pocket, and keep other bodyparts from touching metal parts. You may get pinched but not electrocuted this way.

The next rule is not to wear any metal on your hand, like rings or a watch. Shorting some heavy current with a ring may well cut off your finger.

TOK

yup, adjusted the labvolt last night, and im still here. i jsut heard that there are several thousand volts in a crt and dint know if the glass was actually a good enough insulator or if touching it was a no-no.

a friend of mine was changing a battery in his car once. he shorted out the wrench on the fender with his wedding ring. burnt his finger prety bad, but no major damage. he still does, however, have a nice keepsake of the event, a 1/2" wrench with a wedding ring permanently welded to one end of it!

This post has been edited by deth502 on October 26, 2012 09:08 pm

Yes, high current circuits pose slightly different hazards, as your mate found out. Instantly melting wires, of bringing jewelry cherry red in a fraction of a second being lessons learned.

HV in consumer stuff is way more likely to injure you as you pull away, drop something, or occasionally punch yourself in the face - rather than actually send you to the afterlife - but it's still to be respected and one handed work wearing typical shoes on the average house floor is the absolute bare minimum of respect to offer.

I suspect whether we admit it or not, many of us who've been bitten more than we've seen real blonde carpet tend to get a wee bit lazy between reminders. I got one last week energizing some neon sign tube with cheap gator clips and a cattle prod .

Some stuff scares - and rightly so - the most hardened of us. Worked on a 3KV 100(?)A panel once that frankly terrified me because I doubted that there wouldn't have been any 'little' reminders if i were to botch up.

BTW, your scope round or square tube? Older and simple round tube units IIRC simply rotated the tube to adjust the trace.

"i jsut heard that there are several thousand volts in a crt ..."

it depends on the crt and circuit it is in .
common o'scopes run at quite lower voltages on the accelerator section(s) and second anode than an old 19" television crt .
the old 25" color crts ran about 20-25KV on the second anode .
some of the projection crts ran around 32-40KV on the second anode .

a google search of the terms above as well as looking up the specs on o'scopes will show the relatively lower voltages used .

obviously the glass is a good insulator ... else wise crts would not work .
the accelerator and second anode connectors are the objects to be avoided .
there is a carbon/graphite based "paint" that is used to provide an external ground on the crt "bell" ... this is commonly referred to as "aquadag" (another google search) .
this will form a capacitor with the glass as the dielectric and the internal coating as the "other side" .
this capacitor (or glorified leyden jar) is quite capable of holding a charge for extended periods of time .
discharging the aquadag to the second anode is a must before handling the crt (obviously with the power turned off) .
even then , i have seen crts that were discharged and after setting for some time "charged back up" due to dielectric absorption .
--- just like a weapon , they are always loaded ---

yup. im not so smart on the electronics, but ive been doing electrician work since i was 12. many times ive been working on live circuits (mostly due to laziness, but some times there is a legitimate reason) and gotten a "reminder". never been bitten inside of a service box, though, as im always exponentially more cautious in there, due to the mere ease of touching something,

they are round tube scopes. ones prob just going to be decoration, one will likely be my "junker".

thats a scary thought. once somethings been unplugged for a while, i tend to not even care.

Ive been bit with 700vac off of a tube radio transformer. while it hurts like hell, i doubt it will kill you.

Just make sure you are not holding the chassis (which is usually 0v potential) with one hand while you prod with another.

Also a higher wattage 1k ohm resistor with clip leads makes good for bleeding out the B+ capacitors before you dig in.

quick follow up on this, i wrote to lab-volt customer service, and they actually supplied me with a pdf copy of the original, long out of production, 792 operators manual.
thumbs up for lab-volt's customer service!!

anyway, yup, it actually says in the manual to loosen the clips and rotate the crt just as i did.

now i just need to calibrate it. seems to be way off.