quick question-oscilliscope repair.

[jrz126]

I'm working on repairing/diagnosing my oscilloscope, since Im dealing with the CRT power supply, the service manual recommends that I use an isolation transformer. I dont have a plain transformer, but I do have a 0-140V varac.
Is it ok to use this? and there is no ground pin on the varac itself, can I operate the scope without the ground connected?
I was thinking about just running a wire from the ground on the scope to the ground on the wall outlet. I figure it's better than running it without a ground, but I dont know if that will unisolate the scope.

I'm working on it as I'm typing this, so please respond ASAP if you can help.

Thanks.

 

[Nigel Goodwin]

I'm working on repairing/diagnosing my oscilloscope, since Im dealing with the CRT power supply, the service manual recommends that I use an isolation transformer. I dont have a plain transformer, but I do have a 0-140V varac.
Is it ok to use this? and there is no ground pin on the varac itself, can I operate the scope without the ground connected?
I was thinking about just running a wire from the ground on the scope to the ground on the wall outlet. I figure it's better than running it without a ground, but I dont know if that will unisolate the scope.

I'm working on it as I'm typing this, so please respond ASAP if you can help.

If you're still alive? :lol:

A variac provides NO ISOLATION AT ALL, it's only a variable tapped auto-transformer. So it would be totally popintless using it!.

Working on non-isolated mains just requires a little care, a good idea if you're inexperienced is to keep one hand in your pocket - but this applies with an isolation transformer as well.

The most important thing is probably to understand what you are doing, and why, this will lead you to safe practices.

There are basically two schemes of thought, one is to use isolation transformers and everything earthed - the other is to not use a transformer, but work in a completely earth free environment. They are probably both as safe as each other.

Personally I'm quite happy (and confident) to work in a live mains unit with no special precautions - but I've been doing this professionally since 1971!.

One thing I ALWAYS DO, and it's a point of some contention!, is NEVER use an earthed scope - all my scopes have the earth leads removed, and are clearly marked as such. I consider an earthed scope too dangerous to use!.

I don't suggest YOU do this, unless you are fully aware of the reasons behind it - but it's commonplace in the TV service trade, and always has been.

 

[jrz126]

Yes I am still alive...actually I saw that you were currently on the board, so I was waiting until you got to my message :lol:

I guess I dont reall understand what an isolation transformer does exactly.

So is it better to unearth my scope if I dont have an isolation transformer?

EDIT> Oh yeah, and I already know about the "one hand in the pocket" practice

 

[jrz126]

Well the verdict is in...it's a shattered CRT! :cry: I'm so bummed.
Anyone know how universial the CRT's are? can I get one from another scope or does it have to be a 2340A?

 

[Dean Huster]

Tektronix has had no "universal" CRTs since the mid 1960s. Tek makes their own CRTs and always has, so you have to get one from them ($$$$) or find a "hangar queen" that has a good CRT and massive problems elsewhere. Sorry. No substitution here. The good side is this: Tek scopes are nearly always worth repairing because they are of the highest quality and performance. You just have to shop around on ebay and try for a cheap "junker" ... just be sure it has a good jug.

The reason Tek insists on the isolation transformer is that the scope uses a switching supply. You don't need the isolation transformer if you work on the scope on the secondary side of the switching power transformer (which includes all of the LV supplies and the HV supply), because it's isolated from the line. But you do if you have to do work on the primary side and want to connect ground-referenced test equipment into the live circuitry.

Dean

 

[jrz126]

Tektronix has had no "universal" CRTs since the mid 1960s. Tek makes their own CRTs and always has, so you have to get one from them ($$$$) or find a "hangar queen" that has a good CRT and massive problems elsewhere. Sorry. No substitution here. The good side is this: Tek scopes are nearly always worth repairing because they are of the highest quality and performance. You just have to shop around on ebay and try for a cheap "junker" ... just be sure it has a good jug.

....
Dean

Thanks for the info Dean, I highly doubt I'm gonna find a cheap scope on ebay. Maybe I'll look for sellers that have a bunch of different scopes for sale, maybe they have some really busted up ones.

 

[Nigel Goodwin]

Yes I am still alive...actually I saw that you were currently on the board, so I was waiting until you got to my message :lol:

I guess I dont reall understand what an isolation transformer does exactly.

The incoming mains supply has the neutral wire (blue in the UK) connected to ground at the substation - this means you can get a shock by touching just the one live wire (brown in the UK), with the return through your body to ground. If you don't complete a circuit, you can't get a shock!.

The isolation transformer takes the live and neutral wires in the primary of a transformer, and outputs the same voltage on the secondary. This gives you mains voltage on two wires, neither of which is live or neutral, just floating mains!.

This means you can't get a shock from one wire to ground any more, that's really the ONLY effect of an isolation transformer - but you can still get exactly the same shock from one wire to the other. It also means you can connect earthed scopes (or other test equipment) to the live mains side of the unit under test - for the same reasons.

I use isolation transformers at work all the time, the benches are all fitted with them - but I still don't have my scope earthed :lol: I see no advantage in it.

So is it better to unearth my scope if I dont have an isolation transformer?

Not if you've got to ask :!:

 

[spuffock]

Get two biggish mains transformers that are the same, and will handle a couple hundred watts. ( About big enough to smash your foot if you're careless!).
If you connect the secondaries of these together, you can feed mains into the primary of one, and get isolated mains from the (primary) of the other.

 

[zevon8]

If it turns out you need a need a new tube, a really good site for parts is Sphere Research Corp. They have one of the largest selections of Tek ( and many other parts ) around.

https://www.sphere.bc.ca/test/tek-crts.html

They check all their parts, many are "new old stock" or are as suggested above, are "hanger queens."

They are a great source for hard to find bits, from custom Tek IC's to case stickers and knobs.

Hope you get yours fixed!

 

[jrz126]

unless the tubes are repairable, which they arent....then yes I do need a new one. They do have one for a 2340A, but they are asking $225 for it :shock:

 

[zevon8]

I doubt the tube is repairable, since you can really only do mechanical damage to them ( outside burning a coil or filiament .) And yes, unfortunately the tube market is supply and demand, so unless there are alot of spares or toasted scopes about, the tubes get expensive.

I would make a "favorites" search on Ebay and check regularily, Tek scopes, and often Tek parts donors come up alot, you never know.

 

[Dean Huster]

Jeff, I forgot to mention the Yahoo! Tektronix forum. It has a lot of good folks there, many of them former Tek employees (as I am). It's a good source for troubleshooting information as well as a source for getting parts. It's at https://groups.yahoo.com/group/TekScopes

Dean

 

[jrz126]

So is it better to unearth my scope if I dont have an isolation transformer?

Not if you've got to ask :!:

Well, now I'm curious as to why you would/wouldnt want to unearth something? could you please explain?

Also, I emailed tektronics, they gave me a list of scopes that used that particular CRT: 2445B, 2465B, 2455B, 2430A, 2431L, 2432A, 2432M, 2439, 2440.
So now I just have to track one down. hopefully I can find another junk room special .

 

[Nigel Goodwin]

So is it better to unearth my scope if I dont have an isolation transformer?

Not if you've got to ask :!:

Well, now I'm curious as to why you would/wouldnt want to unearth something? could you please explain?

One hand on earthed scope, other hand on live chassis - OUCH! :cry:

Earthing equipment is only benifical in certain circumstances, in other circumstances it can be far more dangerous if an earth wire is connected.

But if you're not confident about the reasons for doing so, and don't fully understand them, it's best not to change things 8)