Are these things I should consider bad - Page 4 - Videokarma.org TV - Video

jr_tech · #1 10-02-2013, 01:20 AM

Quote:

Originally Posted by TinCanAlley

Can I use the scope to see if it reads the same AC voltage as the meter? Would also be interesting to see how noisy the AC line is.

I would *NOT* try that...I believe that the scope chassis and therefore the probe "ground" clip are indeed connected to earth ground through the power cord... if you happen to connect the probe ground clip to the "hot" side of the line sparks will fly !

jr

TinCanAlley · #2 10-02-2013, 01:37 PM

Quote:

Originally Posted by jr_tech

I would *NOT* try that...I believe that the scope chassis and therefore the probe "ground" clip are indeed connected to earth ground through the power cord... if you happen to connect the probe ground clip to the "hot" side of the line sparks will fly !

jr

I was just going to touch the tip of the probe to the hot side and leave the ground clip disconnected.

jr_tech · #3 10-02-2013, 01:58 PM

Quote:

Originally Posted by TinCanAlley

I was just going to touch the tip of the probe to the hot side and leave the ground clip disconnected.

Good! I just thought that I should pass on a lesson that I learned the hard way.

Early 'scopes that I used (Heathkits mostly) were not grounded through the power cord, so I had quite a surprise when I connected my first *grounded* Tek scope to a hot chassis radio!

You likely introduce a bit of error by single tip probing, because the scope earth ground may be a slightly different voltage than the "ground" side of the line. The *best* measurement would be obtained by the 2 tip/2 channel "differential" method, described by dtvmcdonald in a post above. Accurate, and safe for the 'scope

jr

dtvmcdonald · #4 10-02-2013, 09:55 AM

As to AC line voltage: if its a two-channel scope, you get two identical probes
and use one of them on the hot side, one on the cold (not ground wire) and
use the difference setting. You don't attach the ground lead of either probe
to anything ... the scope case is connected to the power line ground already.

Username1 · #5 10-02-2013, 11:16 AM

You really need to be careful anytime you connect any plug in test equipment to ground on anything you are working on. You should always use a battery volt meter and check ground on test equipment to ground on your test subject item, If there is a wiring inconsistency either in your bench items, or house wiring, you will see 120v across the two grounds, if you then connect them, yes you will have your own semi-private fireworks show.

TinCanAlley · #6 10-02-2013, 11:45 AM

Okay, there seems to be an issue with the "Trig view" button. When I measure AC in the 50V / Div setting, it covers 7 divisions. I know that's not correct. So when I push the "Trig View" I get the proper waveform and the right number of divisions (2 1/2 divisions). That same button, if you pull it, is the bandwidth limiter. If I play with the button, it eventually shows the proper waveform height. If I touch it again, it gives the expanded waveform again. It's like it's dropping from 50V/div to the next lower setting.

It could be a dirty contact in the switch. I'm going to contact the company I got it from to see if it's okay to give it a shot of Deoxit. If I can't get it going I'm going to have to send it back for repair.

andy · #7 10-02-2013, 01:30 PM

...

TinCanAlley · #8 10-02-2013, 01:36 PM

Quote:

Originally Posted by andy

Actually, that is correct. The wall outlet is supposed to be about 120v RMS. You're looking at peak to peak voltage on the scope. Divide the p-p voltage by 2 to get peak voltage (350v p-p is 175v peak). Divide the peak voltage by the square root of 2 to get RMS (123.7v RMS in your case).

Also, you need to know that most AC volt meters are designed to read AC voltages as RMS, but that it only works for a pure sine wave like the AC power line. For anything else (like a square wave), it will be wrong unless it's a "true RMS" meter.

Well then I feel better. Now I need to get back into the set to recheck the horizontal sweep as the PP I got put it at 160V and the Sams says it should be 120V. Since the board is driven by the 125V B+, I don't see how it could be putting out a 160V waveform.

Fun stuff, fun stuff.

TinCanAlley · #9 10-02-2013, 11:47 AM

Quote:

Originally Posted by Username1

You really need to be careful anytime you connect any plug in test equipment to ground on anything you are working on. You should always use a battery volt meter and check ground on test equipment to ground on your test subject item, If there is a wiring inconsistency either in your bench items, or house wiring, you will see 120v across the two grounds, if you then connect them, yes you will have your own semi-private fireworks show.

So I should put the meter between the ground of the chassis and ground of the probe and look for voltage? If I see voltage I should reverse the plug to see if it makes it better? How much is acceptable?

Username1 · #10 10-02-2013, 02:10 PM

""
Well then I feel better. Now I need to get back into the set to recheck the horizontal sweep as the PP I got put it at 160V and the Sams says it should be 120V. Since the board is driven by the 125V B+, I don't see how it could be putting out a 160V waveform.

""

Is the B+ correct ? are all the B+'s however many there are correct ?
How about B Boost over by the HOT ?

TinCanAlley · #11 10-02-2013, 02:25 PM

Quote:

Originally Posted by Username1

""
Well then I feel better. Now I need to get back into the set to recheck the horizontal sweep as the PP I got put it at 160V and the Sams says it should be 120V. Since the board is driven by the 125V B+, I don't see how it could be putting out a 160V waveform.

""

Is the B+ correct ? are all the B+'s however many there are correct ?
How about B Boost over by the HOT ?

The B+ is at 129V and the B+ Boost is 247 and the same voltages at all their connections points. I'll have to check the other B+s that are in the 20V range. I believe there are two of them (one around 23V and the other around 29V).

Username1 · #12 10-02-2013, 02:37 PM

as long as those larger ones are under control, don't worry too much about the smaller ones right now.... Stick to the ones you are getting that look too high. tp5.

TinCanAlley · #13 10-02-2013, 03:17 PM

Quote:

Originally Posted by Username1

as long as those larger ones are under control, don't worry too much about the smaller ones right now.... Stick to the ones you are getting that look too high. tp5.

Okay, went after the TP5 again. My DMM has it at 134VAC, but my scope has a PP of 170V. Am I missing something? I guess I'll find some other TPs and see if their voltage matches the waveforms in Sams. If they, then something is wrong with TP5. If they don't, then it's my scope and/or probe or Sams is not accurate.

TinCanAlley · #14 10-02-2013, 03:34 PM

With the scope on 50V/Div and putting the probe near the FB, I get this waveform. If you notice, the bottom of the form is ringing the number of jail bars I see on the screen. Is this normal to see this when probing w/out actually making electrical connection to the FB? I get the same form when placing the probe on the collector of the HOT (plastic tip on probe). It's even worse if I go anywhere near the focus wire.

TinCanAlley · #15 10-02-2013, 03:37 PM

Quote:

Originally Posted by Username1

as long as those larger ones are under control, don't worry too much about the smaller ones right now.... Stick to the ones you are getting that look too high. tp5.

Okay, just checked the horizontal pulse which should be 50VPP. I set my scope to 50V/Div and got the noted waveform in Sams as well as a PP of one division. So that one is correct, so the scope seems to be working properly. I've found another TP I'm going to try. It goes to the convergence board and is 170VPP. That should tell me if my scope and/or probe is handling higher voltages correctly.