0.25R resistor in calibrator ground


cmjones01
 

I've just noticed something odd about the calibrator outputs on my
535A scope and 127 plugin power supply. They both have a 0.25R
resistor in series with the calibrator socket's ground connection. The
reason I noticed is that the 127's resistor is open-circuit, so
connecting the calibrator to another piece of equipment left the
ground disconnected, with the expected symptoms.

I couldn't find any information in the manuals as to why this resistor
is there. Why is it? To avoid ground loops?

Chris


keantoken
 

I noticed the same resistor, also open, on my 561B! On Thursday, March 25, 2021, 11:18:00 AM CDT, cmjones01 <chris@stumpie.com> wrote:

I've just noticed something odd about the calibrator outputs on my
535A scope and 127 plugin power supply. They both have a 0.25R
resistor in series with the calibrator socket's ground connection. The
reason I noticed is that the 127's resistor is open-circuit, so
connecting the calibrator to another piece of equipment left the
ground disconnected, with the expected symptoms.

I couldn't find any information in the manuals as to why this resistor
is there. Why is it? To avoid ground loops?

Chris


Dave Wise
 

?Exactly. High-gain plugins (Type B, D, etc) were seeing hum on the calibrator. The resistor made the bottom ranges usable.


In the 535 Mod Summary, it's Mod #1412, effective S/N 5001.


Dave Wise


My "S/N 75" 535-535A prototype needs this mod.

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of keantoken via groups.io <keantoken=yahoo.com@groups.io>
Sent: Thursday, March 25, 2021 9:28 AM
To: tekscopes@groups.io; TekScopes@groups.io
Subject: Re: [TekScopes] 0.25R resistor in calibrator ground

I noticed the same resistor, also open, on my 561B! On Thursday, March 25, 2021, 11:18:00 AM CDT, cmjones01 <chris@stumpie.com> wrote:

I've just noticed something odd about the calibrator outputs on my
535A scope and 127 plugin power supply. They both have a 0.25R
resistor in series with the calibrator socket's ground connection. The
reason I noticed is that the 127's resistor is open-circuit, so
connecting the calibrator to another piece of equipment left the
ground disconnected, with the expected symptoms.

I couldn't find any information in the manuals as to why this resistor
is there. Why is it? To avoid ground loops?

Chris


Don Bitters
 

My guess would be to save the calibrator from a backed signal (feeding an external signal into the calibrator, AC or DC).
Don Bitters


cmjones01
 

Further to this discussion on the 0.25R anti-ground-loop resistor on
tube-era calibrator outputs, I looked further in to why my 127's
calibrator output ground seemed to be open-circuit.

It turned out that there was actually a manufacturing error. There's
an arrangement of plastic bushes and fibre washers to keep the
calibrator socket's outer from being in contact with the front panel.
In my 127, one of the fibre washers was in the wrong place, so the
bottom of the volts/millivolts divider chain was connected to chassis
ground, and both ends of the 0.25R resistor R898 were also connected
to chassis ground. The calibrator socket outer wasn't connected to
anything at all. It was completely floating. The paint seals on the
screws were still intact, so it must have left the factory that way in
1960.

The only reason I noticed is that in my office the power sockets have
no ground connection, so when by chance I powered the 127 from a
different outlet to the scope I was testing it with, they had no
common ground connection via their power cables. Trying to view the
calibrator output it seemed to be imposed on about 100V of mains hum,
which I blamed on a faulty cable. But the cable was blameless. It was
an actual Tek assembly error! I guess nobody had ever noticed due to
never having tried to connect the calibrator to an ungrounded device.

I've moved the offending washer now and the wiring matches the
schematic diagram. It all works as it should now.

Chris

On Thu, Mar 25, 2021 at 6:49 PM Dave Wise <david_wise@phoenix.com> wrote:

?Exactly. High-gain plugins (Type B, D, etc) were seeing hum on the calibrator. The resistor made the bottom ranges usable.


In the 535 Mod Summary, it's Mod #1412, effective S/N 5001.


Dave Wise


My "S/N 75" 535-535A prototype needs this mod.

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of keantoken via groups.io <keantoken=yahoo.com@groups.io>
Sent: Thursday, March 25, 2021 9:28 AM
To: tekscopes@groups.io; TekScopes@groups.io
Subject: Re: [TekScopes] 0.25R resistor in calibrator ground

I noticed the same resistor, also open, on my 561B! On Thursday, March 25, 2021, 11:18:00 AM CDT, cmjones01 <chris@stumpie.com> wrote:

I've just noticed something odd about the calibrator outputs on my
535A scope and 127 plugin power supply. They both have a 0.25R
resistor in series with the calibrator socket's ground connection. The
reason I noticed is that the 127's resistor is open-circuit, so
connecting the calibrator to another piece of equipment left the
ground disconnected, with the expected symptoms.

I couldn't find any information in the manuals as to why this resistor
is there. Why is it? To avoid ground loops?

Chris















Albert Otten
 

On Thu, Mar 25, 2021 at 05:17 PM, cmjones01 wrote:


I couldn't find any information in the manuals as to why this resistor
is there. Why is it? To avoid ground loops?
561B (mentioned by keantoken) manual page 3-5 bottom left: "R183, which is about ten times the resistance of the braid of a 42-inch coax cable, cancels any ground loop current that may exist between the CAL OUT connector and some other instrument chassis." I didn't look in other manuals.
Albert


cmjones01
 

On Wed, Apr 7, 2021 at 3:35 PM Albert Otten <aodiversen@concepts.nl> wrote:

On Thu, Mar 25, 2021 at 05:17 PM, cmjones01 wrote:

I couldn't find any information in the manuals as to why this resistor
is there. Why is it? To avoid ground loops?
561B (mentioned by keantoken) manual page 3-5 bottom left: "R183, which is about ten times the resistance of the braid of a 42-inch coax cable, cancels any ground loop current that may exist between the CAL OUT connector and some other instrument chassis." I didn't look in other manuals.
Thank you for confirming this, Albert. My previous message got
somewhat mangled in transit so I'll try to post the text again here:

Further to this discussion on the 0.25R anti-ground-loop resistor on
tube-era calibrator outputs, I looked further in to why my 127's
calibrator output ground seemed to be open-circuit.

It turned out that there was actually a manufacturing error. There's
an arrangement of plastic bushes and fibre washers to keep the
calibrator socket's outer from being in contact with the front panel.
In my 127, one of the fibre washers was in the wrong place, so the
bottom of the volts/millivolts divider chain was connected to chassis
ground, and both ends of the 0.25R resistor R898 were also connected
to chassis ground. The calibrator socket outer wasn't connected to
anything at all. It was completely floating. The paint seals on the
screws were still intact, so it must have left the factory that way in
1960.

The only reason I noticed is that in my office the power sockets have
no ground connection, so when by chance I powered the 127 from a
different outlet to the scope I was testing it with, they had no
common ground connection via their power cables. Trying to view the
calibrator output it seemed to be imposed on about 100V of mains hum,
which I blamed on a faulty cable. But the cable was blameless. It was
an actual Tek assembly error! I guess nobody had ever noticed due to
never having tried to connect the calibrator to an ungrounded device.

I've moved the offending washer now and the wiring matches the
schematic diagram. It all works as it should now.

Chris


Jim Adney
 

On Wed, Apr 7, 2021 at 04:40 AM, cmjones01 wrote:

It turned out that there was actually a manufacturing error. There's
an arrangement of plastic bushes and fibre washers to keep the
calibrator socket's outer from being in contact with the front panel.
In my 127, one of the fibre washers was in the wrong place, so the
bottom of the volts/millivolts divider chain was connected to chassis
ground, and both ends of the 0.25R resistor R898 were also connected
to chassis ground.
Nice catch! ;-)

As fans and long time users of Tek products, I suspect we all tend to assume that all Tek products leave the factory in perfect condition. I certainly used to, but in the past 20 years, I've come across 2 factory assembly errors that surprised me. I was able to fix both, but they came as a shock at the time. Unless someone else beats me to it, I think I'll start a thread on discovered factory assembly errors.


 

On Fri, Apr 9, 2021 at 05:18 PM, Jim Adney wrote:


Unless someone else beats me to it, I think I'll start a thread on discovered
factory assembly errors.
How about a freebie: I once bought a rackmount 7844 (400 MHz double beam) mainframe in unused condition, apart from a few light scratches from the sliding action. Seller didn't know much about its history.
While trying it out, I found the screen info collapsed and the scope produced a loud tick when I touched the horizontal separation knob, at the far top right of the front.
Turned out that the knob controls a pot, soldered into a PCB which is mounted at a distance on a horizontal aluminum chassis plate.
Someone forgot to cut the protruding legs of the pots during manufacture. The endpoints of the pot are connected straight to the 15V supplies....

Raymond