Using O-Scope with AC Mains Transformer Question

[Ian MacGregor]

Hi
I'm currently helping a friend tech some issues with his Neumann cutting lathe. We are in the US, so the lathe is running off a 110 V to 220 V mains transformer. I need to look at some signal voltages inside the lathe and I'm a little unsure of the proper way to reference the scope ground probe. The scope would be plugged into the regular wall 110V socket. What is the best way to go about this? Am I fine just attaching the scope probe's ground lead to the lathe's ground?

Thanks for your help!

 

[walter]

[quote author="Ian MacGregor"] Am I fine just attaching the scope probe's ground lead to the lathe's ground?

Thanks for your help![/quote]
I would think so. Are you running a 220vAC lathe on 110vAC stepped up with a transformer? I know 110 is single phase, and 220v is two phase, two 110v 180 degrees out of phase. I'm curious about the motor. Is it designed to run off of two phase or is single phase o.k.?

 

[shabtek]

I would trial and error it...if its an isolating trans (not autoformer) i'd link scope ground/house ground to recorders chassis. --220v is not 2 phases 180 deg apart.(if it were they would "cancel")

 

[Gold]

In the lathe nothing is floating DC voltage wise. You can ground to chassis/safety ground. In the SAL/VG74 the -5v and -7v use an "isolated ground" . You should reference that if looking at those circuits.

What's the problem?

 

[Ian MacGregor]

Ok,
You guys were right, I just hooked the scope's ground probe to the lathe circuit ground and everything was fine. The issue was in the depth control; test cuts revealed a pretty big 120/240Hz hum that disappeared when the depth control was disabled/bypassed. I suspected the power supply filter caps and sure enough, after replacing the caps, test cuts showed much, much less hum.

Thanks for the help!

 

[PRR]

> I know 110 is single phase, and 220v is two phase, two 110v 180 degrees out of phase.

Only in the US and related places.

And primarily for Residential (or similar) work. In industry you sometimes find 240V single (and a lot of 208 single).

In the UK and most of Europe, standard wall power is 230V hot to ground. It means better insulation than Tommy Edison had when he picked "safe" 100V power, but it means a lot less copper.

If he's stepping 120V to 240V and applying it to the hot terminals of his lathe wall-plug, he's got it right. Depending on connections it could be one-side-grounded, un-grounded, or even center-tapped; makes little difference.

After that is is as Gold says: reference most things to safety ground.

I'd fret about 50Hz motor turning 40RPM at 60Hz, but obviously his motor has its own reference. Shows that the last disk-cutter I used had a handle. ("Portable" was not the right word, but they did move.)

 

[Ian MacGregor]

I'm pretty sure that he was stepping up the 120 to 240. The lathe has been used at his location for years without issue; he just had some recent test pressings come back with low level hum issues. It was pretty nerve-wracking to have to wait for him to cut a test disc, and then listen to play back to hear if the hum was fixed or not!


Maybe this is a good time for me to ask a side question: When is it NOT ok to connect a scope's ground probe to a circuit under test??

Ian

 

[Gold]

[quote author="Ian MacGregor"] When is it NOT ok to connect a scope's ground probe to a circuit under test??
[/quote]

I don't have a general answer but the the -5v and -7v circuits related to the circuit breaker in the SAL74 rack are floating. Grounding the probe wouldn't give reliable results.

Out of curiosity was the hum vertical or reverse polarity? Because the depth control is of the difference signal.

 

[JohnRoberts]

[quote author="Ian MacGregor"]


Maybe this is a good time for me to ask a side question: When is it NOT ok to connect a scope's ground probe to a circuit under test??

Ian[/quote]

Some power amp outputs are actually driven differentially, so a scope ground could actually short circuit the amp output if it is also referenced to ground.

Sometimes a scope ground can actually introduce a ground loop making measurements of noise with a scope a little difficult. Well designed gear should not have a problem with scope grounds connected to an output ground.

JR

 

[Gold]

I'm interested in knowing more about this problem as I haven't run into this before. When you say you disabled the depth control what do you mean? Without any depth current the cutterhead wouldn't touch the disk surface with a normal calibration. This is like pressing the automatic cutterhead drop button which removes the depth current.

 

[Ian MacGregor]

[quote author="Gold"]I'm interested in knowing more about this problem as I haven't run into this before. When you say you disabled the depth control what do you mean? Without any depth current the cutterhead wouldn't touch the disk surface with a normal calibration. This is like pressing the automatic cutterhead drop button which removes the depth current.[/quote]

To be perfectly honest, I really don't know all that much about the lathe "inner workings." My friend indicated that he could disable the depth control (maybe by turning it from "auto" to "manual"?).

Looking at the schematic for the depth amplifier (looked like a couple of rectifiers and power amp output), I could see that any issues with the power supply filter caps could be the cause of the 240Hz hum he was seeing. After replacing the filter caps, the hum did indeed go away.

He explained how the lathe operated a couple timesl but I'm still a bit lost!!

Ian

 

[CJ]

Use a good isolation transformer on the scope.

Or rip out the center prong like I do, but we will not post that because it is not really the right way to go about it.

I have seen one dramatic cable cookage, and one storage scope blown up via the "ground probe" time bomb.

 

[Gold]

Was this a tube amplifier? If so it can indeed be disabled. I assumed the lathe was a VMS66/70. There is nothing that would be labeled depth amplifier on a 66/70. I used to have a VMS62 special with the pitch/depth/integration amplifiers. It is now being used as a turntable by a rich Korean.

In the AM132 and VMS62 Special lathes there is manually adjusted depth current in the lathe. There is one of those five blade connectors that goes into a normalling socket at the rear left corner to insert the depth amplifier. There is also a through switch on the amplifier its self. Those normalling contacts can go wonky too.

 

[Ian MacGregor]

Ah yes... it was a tube amplifier. As far as I know, this is a super old lathe that was made by Neumann, but modified by RCA.

Ian

 

[Gold]

The AM132 was made starting in the late 1950's. 1957 or 58. The VMS62 special was basically the same thing with some niceties. The VMS62 came along in 1962 and added an LPI (lines per inch) meter to the drive box. The pitch/depth amplifiers were a little different cosmetically but I think the circuit was the same. You could buy them either with or without the automatic pitch/depth amplifiers. Neumann was the first to have this capability. It was very fancy and very expensive.

I spent five years cutting on one and it was a good machine. I use a fancy Zuma 6800 bus computer with a VMS66 now. I wouldn't want to go back but it's entirely possible to cut good sides on one. It does take more work and it would be impossible to squeeze that last dB of level out of a long side but these days no one is doing shootouts.

 

[Gold]

The VMS66/70 are functionally identical. They use an analog computer for control. On the underside of the turntable there are alternating white/black quadrants. A light is projected on the underside and a light dependent resistor goes high/low. This is the clock. There is a card for each quadrant that stores the DC values describing the audio in capacitors. Then there is a shift register. These drive the pitch motor and the depth solenoid.

 

[CJ]

Here is the PFB 150 if anybody is interested:

http://vacuumbr.ipower.com//The_Lab/Fairchild/Rein_Narma/gothamp.gif