JLC electric mystery clock leaking volts...

[nickelsilver]

I got asked by a watchmaker friend to have a look at this, he knows zero about electrical stuff and I know a little bit more than that, haha. It runs on a synchronous motor, which had a broken lead. Another friend soldered on new leads for him, but it didn't run; I got it apart and checked everything, back together, and it runs!

 

But, if I check with a voltmeter between one of the leads and the chassis I get around 60 volts, and around 7 on the other lead. Also chassis to ground on the variac (to feed it 120 volts), around 60 volts there too. Checking continuity between the leads and chassis (unplugged) I get nothing, checking the coil out of the piece, nothing from lead to coil case, there should not be any way for these volts to be escaping. Any of you electric gurus have an idea what's happening? I haven't licked my finger and touched the chassis and a water pipe yet but I'm getting tempted...

 

 

 

 

Edited November 6, 2021 by nickelsilver

[Plato]

The clock isn't earthed is it? It's just the two wires going to the stator. Your high impedance meter might just be picking up mains hum. I'm assuming you're on the ac setting on the meter. Connect one end of your meter to earth and the other touching your finger and you'll get a reading even though there's no real voltage.

[JohnR725]

1 hour ago, nickelsilver said:

checking the coil out of the piece, nothing from lead to coil case, there should not be any way for these volts to be escaping.

I assume when you're looking for coil continuity you have the resistance range relatively low? Like if you're using a digital meter on the 2K range and the resistance here is really high you're not going to see it. So put the meter on the highest resistance range and see if it detects anything.

[nickelsilver]

It's an autoranging meter, so nothing I can change there (there's a Simpson 260 around here somewhere but I trust the digital a little more).

5 minutes ago, Plato said:

The clock isn't earthed is it? It's just the two wires going to the stator. Your high impedance meter might just be picking up mains hum. I'm assuming you're on the ac setting on the meter. Connect one end of your meter to earth and the other touching your finger and you'll get a reading even though there's no real voltage.

No earth, just the two leads.

 

Didn't mention it in the first post, but when I started looking it over I hooked it to the variac, on zero volts, then cranked up to 120- with a hand on the chassis- and got whacked! Checking continuity just after and there was definitely a small short from a lead to chassis. I put new heat shrink and potted the small section of wire still exposed right at the coil with some epoxy; after that it's all clean, zero continuity. But I'm a little gun-shy to touch it plugged in now!

[LittleWatchShop]

21 minutes ago, nickelsilver said:

there's a Simpson 260 around here somewhere but I trust the digital a little more

OH NO!  Thou doth besmirch the mighty Simpson 260??  In this case the Simpson which (as I recall) 10k ohms/volt will give a much lower impedance to the leads when measuring a voltage and you will likely not experience what @Platodescribed (essentially you are measuring electrostatic potential).  Also use the Simpson to measure the resistance from each lead to case so you dont have to deal with the autoranging meter bouncing about.  I trust the digital meter...dont get me wrong.

Edited November 6, 2021 by LittleWatchShop

[Plato]

9 minutes ago, nickelsilver said:

It's an autoranging meter, so nothing I can change there (there's a Simpson 260 around here somewhere but I trust the digital a little more).

No earth, just the two leads.

 

Didn't mention it in the first post, but when I started looking it over I hooked it to the variac, on zero volts, then cranked up to 120- with a hand on the chassis- and got whacked! Checking continuity just after and there was definitely a small short from a lead to chassis. I put new heat shrink and potted the small section of wire still exposed right at the coil with some epoxy; after that it's all clean, zero continuity. But I'm a little gun-shy to touch it plugged in now!

Sounds like you found and fixed the original fault. I don't think we've been allowed to use variacs where I work for donkeys years - no isolation, they're regarded as dangerous!

My guess is that you're just picking up noise on your meter. It's picking up induced voltage like a radio antenna... someone will explain it better than me.

[nickelsilver]

Hmm, OK, I'll get out the Simpson and check again. I guess I'll touch it too? It's only 60 volts...

 

Edit- Simpson is at home, I'm at the shop, touched the frame and ground with one hand and nada, not a whiff of anything shocking. Thanks guys!

Edited November 6, 2021 by nickelsilver

[LittleWatchShop]

7 minutes ago, nickelsilver said:

Hmm, OK, I'll get out the Simpson and check again. I guess I'll touch it too? It's only 60 volts...

When I was in high school, I began my pursuit of electronics training.  As a sophomore I took a Basic Electronics class and later took two years of vocational radio and TV repair.  On day in the basics class, the teacher was out having a smoke and we had a contest:  who could hold the two leads of a suicide chord plugged into a variac for the highest voltage.  One kid would grab the leads and another would crank the knob.  This was absolutely stupid.  I am sure we all look back at our teen years and wonder how we survived.

[JohnR725]

1 minute ago, nickelsilver said:

I guess I'll touch it too? It's only 60 volts...

Maybe you can enhance the effect by standing on concrete that's wet with bare feet.

Although I would really prefer you not try the experiment I suspect you might just get one chance to do it and then you leave the group permanently definitely not desirable.

Any idea when the clock was made? Depending upon the age and quality a lot of the wire really isn't the most desirable and the insulation may not be the best.  Also where the wires come out they may be rubbing on things. C can end up with a very high resistance connection to the frame.

 

 

[nickelsilver]

7 minutes ago, LittleWatchShop said:

When I was in high school, I began my pursuit of electronics training.  As a sophomore I took a Basic Electronics class and later took two years of vocational radio and TV repair.  On day in the basics class, the teacher was out having a smoke and we had a contest:  who could hold the two leads of a suicide chord plugged into a variac for the highest voltage.  One kid would grab the leads and another would crank the knob.  This was absolutely stupid.  I am sure we all look back at our teen years and wonder how we survived.

I messed around with some tube gear a few years back, got a good 300v once! Luckily just about 1 second and not across the heart!

 

2 minutes ago, JohnR725 said:

Maybe you can enhance the effect by standing on concrete that's wet with bare feet.

Although I would really prefer you not try the experiment I suspect you might just get one chance to do it and then you leave the group permanently definitely not desirable.

Any idea when the clock was made? Depending upon the age and quality a lot of the wire really isn't the most desirable and the insulation may not be the best.  Also where the wires come out they may be rubbing on things. C can end up with a very high resistance connection to the frame.

 

 

I'm pretty sure this is from the 60s. There's very very little info on them. I know some of the really old clocks that run on motors are quite sketchy with degraded insulation on the coils; there are some sites that explain how to wind new low voltage coils with modern wire, then you run off a transformer.

 

On this one I added extra insulation around the wire that has the heat shrink on it, where it contacts the coil case and movement- you can see that it gets pinched when it's all mounted.

[jdm]

2 hours ago, nickelsilver said:

But, if I check with a voltmeter between one of the leads and the chassis I get around 60 volts, and around 7 on the other lead.

When dealing with leaks, what matter more than voltage is current. Try measuring it if you are curious. For example, one likely won't feel 10 mA, but 30. yes. BTW, your DMM may have a button for manual range selection. Although you would have noticed if that was the case.

 

1 hour ago, Plato said:

Your high impedance meter might just be picking up mains hum.

Correct. Strange things may happen with cheap DMMs as in seeing voltage decrease, while another meter could reveal the true rms voltage.