Using Osciloscope on Hot Chassis - Videokarma.org TV - Video

Tubejunke · #1 08-28-2015, 05:46 PM

I need to know exactly what method must me followed to do this without blowing up your scope. I already knew that you couldn't just go in there and clip probe's ground lead to chassis and start poking around, but I still didn't know what I am to do.

In this particular case, I am working with a 50s Zenith Transoceanic radio. It has a floating ground, so I connected not to the chassis but to the grounding point. Hot end of the probe was not connected nor was connection to the scope at the input terminal.

As soon as I touched the probe connector to the input terminal of the scope a spark flew. Luckily for me the scope is OK. Possibly because I was working with a Variac on a reduced voltage as I suspected trouble.

What do I need to do here folks? Thanks for any input!

Tube

walterbeers · #2 08-28-2015, 06:15 PM

I have a dual trace scope, with only one good, but well worn probe now, because in the past I did the same thing. You need to use an isolation transformer, as most variacs do not isolate the equipment from the line. I myself have a isolation transformer that also works as a variac with switchable voltage output. If you could get a hold of one something like the once pictured that I have, they are a lifesaver, (possibly literately). Believe me, line voltage across your body is not fun. Yes, it happened one to me when I lived with my parents many years ago, from a faulty waffle iron, next to the kitchen sink. Luckily I was able to jerk myself off of it, as AC current has a tendency to lock your muscles.

Tubejunke · #3 08-28-2015, 06:36 PM

OK, great! I have been trying to find exactly what you have or something similar for a while for a decent price. I see old RCA WP-25A units on Ebay that aren't too high priced, but they aren't variable. Usually anything that is variable and isolates from the line is not cheap. I guess I could buy one of the basic 1 to 1 ratio transformers I see and use it in conjunction with my Variac to save some money.

Am I to isolate the equipment being tested or the test equipment, or both?

walterbeers · #4 08-28-2015, 06:56 PM

As long as one of them is isolated you should be fine. I normally isolate the piece of equipment that I'm working on. I got mine from an RCA distributor many years ago, when they sold VIZ test equipment. I agree they are very hard to find. You could also use a 1 to 1 ratio isolation transformer and then plug your variac into it. It would serve the same purpose, just a little more hassle having to use both pieces of equipment.

Tubejunke · #5 08-28-2015, 08:19 PM

Thank you so much sir for what I feel that I should already know having some years under my belt in the repair and/or restoration of vintage radio and TV. Frankly, I never ran into this problem as I along with others I know that are in the electronics field don't use a scope that much if at all for troubleshooting.

Until I got college training I didn't even know how to use one, but I'm getting to a point in life where I just find waveforms as a troubleshooting method to be an interesting challenge. Back to the isoformer; there is really no excuse to be working on anything with a hot chassis without one. I have been real lucky I guess for a fair amount of years. Years ago I thought that a Variac was one, but thank goodness I know better now.

old_tv_nut · #6 08-28-2015, 08:21 PM

Definitely isolate the equipment you are working on. That makes the chassis be ground potential through the ground lead of your scope. Doing it the other way (isolating the scope) could end up with everything including the scope floating on the hot side - not good.

Electronic M · #7 08-28-2015, 08:21 PM

Just don't connect both the test equipment and the radio to the isolation transformer at the same time....It is just as bad (for the electronics) that way as no transformer at all.

I just have separate a variac and isolation transformer and daisy chain them when I need both functions....I've considered building them into a common box, but decided I need isolation rarely enough to not want to always lug both around.

old_coot88 · #8 08-28-2015, 08:40 PM

If you get an isolation xfmr, be sure its current rating is high enough to handle whatever you may be testing in the future.

dieseljeep · #9 08-28-2015, 09:46 PM

I never ground my scope or my VA62. I keep those grounding adaptors handy.
Even though, I have isolation transformers, I still do it that way.
I'm not worried about getting a shock from my equipment, plus all my circuits have GFI protection.

Tubejunke · #10 08-29-2015, 03:22 AM

This is great. Now I am going the right direction. The GFI is great anywhere in a shop or home, but I currently have none installed. I need to, but this isoformer is a must with all my "American Five" radios and at least one early 50s Motorola 17" television with hot chassis. I can't believe I have gone decades now without an isoformer! Geez!

dieseljeep · #11 08-29-2015, 10:35 AM

Quote:

Originally Posted by Tubejunke

This is great. Now I am going the right direction. The GFI is great anywhere in a shop or home, but I currently have none installed. I need to, but this isoformer is a must with all my "American Five" radios and at least one early 50s Motorola 17" television with hot chassis. I can't believe I have gone decades now without an isoformer! Geez!

All my GFCI's are permanently installed, but I made a few portable ones, using a power cord from a scrap outlet strip and a plastic single gang box, to use my power tools, where there is no GFCI available.
I've been using them ever since they came out.
It also saved a few headaches from accidental damage to a hot chassis set, when I bumped the heat sink with a cable TV connector. They respond that fast.

Electronic M · #12 08-29-2015, 04:58 PM

One thing you can do for a cheap quick isolation transformer is, if you have 2 identical power transformers, connect the secondaries of each together, connect the primary of one transformer to a plug and the primary of the other to an outlet.

zeno · #13 08-29-2015, 07:40 PM

If you are going to do this as a life long hobby I
strongly recommend one of the combo units such as
the Sencore PR57. The reason they cost $$ is they
are still very useful. In fact when working on solid
state TV's its almost essential. Its neat without
wires & clutter in your way. The metering is also
super useful. And you can do easy leakage tests &
some have a soldering iron output ( B&K ). Put one
on your Christmas wish list.

73 Zeno

Findm-Keepm · #14 08-29-2015, 08:32 PM

Quote:

Originally Posted by dieseljeep

I never ground my scope or my VA62. I keep those grounding adaptors handy.
Even though, I have isolation transformers, I still do it that way.
I'm not worried about getting a shock from my equipment, plus all my circuits have GFI protection.

Everyone of my late father's oscopes and meters had the ground pin cut off.

I've got probably 3 or 4 commercial isolation transformers and a Sola power conditioner/stabilizer, but rarely use an isolation transformer. One GFCI, an Arc-fault breaker, and the rarity of hot-chassis stuff on my bench is the real reason.

Dad had one of those metal-raceway power strips across the front of the bench - I got shocked once when my knee made contact with the grounded power strip and my hand was steadying a little 13" hot chassis portable. Note to self - wearing shorts is a no-no around Dad's bench...