Topics

Isolation???

Mike Harmon
 

I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.

I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
Mike Harmon, WB0LDJ
mharmon at att dot net

 

Mike:

As a suggestion:  Use the 7a26 with two x10 probes.  Set the 7a26 with
"DISPLAY MODE" set to "ADD" and with "CH 2 POLARITY" set to "INVERT". 
Connect the two probe ground leads (or optionally only one probe's
ground lead) to the Motorola power supply chassis (I assume this can be
considered as a viable power ground point) and the tip of the channel A
probe to the +41V test point, and channel B probe to the positive DC
output terminal.  This should allow the 7a26 to perform differential
measurements between the two points.  You may have to play with the AC
vs DC input selections on the 7a26 input channels, and/or adjust the
gain control(s) to "non-calibrated" positions for this measurement (to
get the gains of the two channels to accurately match each other, but
the downside of this is that you lose some measure of vertical
calibration accuracy).

I think this will work.  Although I've never done it myself; hopefully
someone more knowledgable than me will chime in if I am way off base.

Mike Dinolfo N4MWP

On 5/23/19 10:07 PM, Mike Harmon wrote:
I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.

I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
Mike Harmon, WB0LDJ
mharmon at att dot net


 

Hi Mike,
This is where it pays to have a 7A13 Differential Comparator and a P6055 differential probe set if you are going to see some high frequency signals (DC to 110MHz) when you do this.
Alternatively if the signals are under 1MHz the 7A22 differential Amplifier with a P6055 differential probe set.

Both the 7A13 and the 7A22 are must have plugins for any 7000 series scope.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Mike Harmon
Sent: Thursday, May 23, 2019 7:07 PM
Subject: [TekScopes] Isolation???

I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.

I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
Mike Harmon, WB0LDJ
mharmon at att dot net




--
Dennis Tillman W7PF
TekScopes Moderator

Dale H. Cook
 

On 5/23/2019 10:07 PM, Mike Harmon wrote:

Does anyone have a better (safer) way to measure the differential waveform?
The rule of thumb when isolation is required is "Don't isolate the instrument - isolate the DUT." Plug the Motorola supply into an isolation transformer - a true isolation transformer like my RCA WP-26A Isotap which has a 400VA secondary. You want a real isolation transformer that isolates everything from hot, neutral and ground. What you don't want is something labeled an "isolation transformer" intended for EMI filtering that does not provide full isolation.
--
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcity/radios/

Harvey White
 

On 5/23/2019 10:07 PM, Mike Harmon wrote:
I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.
What's immediately jumping out at me is that you have a 7904A with a 7A26.  You can put a scope probe on channel 1 to the positive DC output, and the scope probe from channel 2 on the 41v test point.  Set channel 2 to invert, then the display mode on the plugin to "ADD".

The 41 volts should be within the common mode range of the 7A26, but a quick look at the manual will tell you for what ranges.

Very standard way of measuring a differential voltage.

A 7A22 or a 7A13 has a built in differential voltage capability, so you wouldn't necessarily need the DC voltage, but with the 7A26, you should be just fine.

Harvey


I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
See above....


Mike Harmon, WB0LDJ
mharmon at att dot net



dnmeeks
 

Using the two-probe method that Mike suggested is the usual way of making a
differential measurement (when expensive diff probes are not available, or
when the voltages are too high for them).

One thing I wanted to add, is you can simply connect the probes' grounds
together WITHOUT connecting them to chassis ground or anything else. That is
also a common technique.

Good luck -

Dan

 

Hi Harvey,
Regarding Mike Harmon's need for a way to measure a signal common mode without a true differential amplifier:

The devil is in the details. Amplifier design involves a variety of compromises which the end user would normally not be aware provided they used the amplifier as it was designed to be used. One of the things to be aware of with the vertical amplifiers Tek designs is their limited off-screen range. As long as the signals being displayed are on the screen or within a few divisions off screen they are in their linear range. But when you exceed that range the amplifier will saturate. You will not see this since it will be off screen. But the effect of this saturation is to slow down the recovery time of the signal considerably.

Tek lists the "Overdrive Recovery Time" in its specifications for each vertical amplifier. For instance the Overdrive Recovery Time for the 7A26 as 0.1mSec or less. Depending on what you are measuring that can be significant. As a general rule as long as the trace is on screen you don't have to worry about overdriving slowing or distorting the signal.

How successful you will be using one of Tek's general purpose dual trace plugins (7A12, 7A18, 7A18A, 7A24, 7A26) as a "poor-man's" differential amplifier will depend on attention to these details:
1) Both probe's must have exactly the same attenuation. The probes should be as closely matched as possible and be adjusted so their attenuation is the same.
2) The gain of each channel should be adjusted to be the same.
3) The signal coming to each channel must be attenuated the same amount by setting each Volts/Div knob the same if possible.
4) Each signal (by itself) should not go off screen at any point
5) The combined Add of CH1 to CH2 Inverted should not go off screen at any point.
6) The probe ground leads should be left floating. This may result in picking up a lot of 60Hz AC.

How much of a DC offset the vertical amplifier can handle between channels is not specified. The maximum each channel can take is for a 7A26 is 250VDC but that may only be under certain conditions.

Dennis Tillman W7PF

-----Original Message-----
From: Harvey White, Sent: Thursday, May 23, 2019 7:47 PM

On 5/23/2019 10:07 PM, Mike Harmon wrote:
I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.
What's immediately jumping out at me is that you have a 7904A with a 7A26. You can put a scope probe on channel 1 to the positive DC output, and the scope probe from channel 2 on the 41v test point. Set channel 2 to invert, then the display mode on the plugin to "ADD".

The 41 volts should be within the common mode range of the 7A26, but a quick look at the manual will tell you for what ranges.

Very standard way of measuring a differential voltage.

A 7A22 or a 7A13 has a built in differential voltage capability, so you wouldn't necessarily need the DC voltage, but with the 7A26, you should be just fine.

Harvey



--
Dennis Tillman W7PF
TekScopes Moderator