Scope "wobble"

[N3WWL]

I've been using a Hitachi 20 meg dual trace scope with triggered sweep for audio monitoring for years.  No matter where it's placed around my operating position, I get a wobbly line.  I'm assuming it's interacting with any number of other devices that are running.  Not a big deal, but I'd like to see the trace line flat.  I can speed up the time, but then the waveform isn't proper (too fast).  Any ideas???

[WQ9E]

Jay,

Is the wobble only when transmitting or do you get it all of the time?  What happens when you switch the vertical inputs to ground?  By wobble, do you mean the line is shifting up and down around the center point or does it appear to be hum or interference on the trace?  What happens when you disconnect the probes?

Rodger WQ9E

[N3WWL]

Rodger, the wobble is all the time, unless most of the shack devices are turned off....I will check later if it's still there with the probe disconnected...I believe it is.  No change when switching to ground.  I still think I'm getting interaction from AC.

[Steve - WB3HUZ]

If you are getting the wobble when the channel is grounded, it sounds like you may have a power supply problem. It the wobble at 60 or 120 Hz? Set the trigger to line and see if the wobble syncs up. If it does, it may be filter caps in your power supply or a very bad ground loop.

[W3SLK]

Steve said:

If you are getting the wobble when the channel is grounded, it sounds like you may have a power supply problem. It the wobble at 60 or 120 Hz? Set the trigger to line and see if the wobble syncs up. If it does, it may be filter caps in your power supply or a very bad ground loop.

I concur with the HUZman. Check that out for line sync and then check those PS caps for leakage.

[WQ9E]

Jay,

If it happens with the probes disconnected it may be coming in over the power supply although I am surprised given the filtering and regulation of modern scopes.  Basically, nothing should be riding in from the input AC line and appearing on any of the regulated outputs.  

The mu metal shielding around the CRT should take care of any normal magnetic fields in your radio room but there could be an issue with yours.  I vaguely recall reading that the properties of those are destroyed if subjected to a sharp impact; maybe your shield got dropped on the floor during production.  To test this theory, plug one of the transformer "wall warts" into an extension cord and hold the transformer part fairly close to the side of your scope to see how quickly it begins to impact the trace.

How is the wobble affected by the vertical volts per division setting?

In a case like this, it would be very useful to borrow another scope from a friend to see if it is similarly affected and to trace where the undesirable signal is getting into yours.  Most parts of a scope cannot be effectively checked using itself as the test instrument.

Sorry I cannot be of more specific help but all of my scopes are Tektronix except for a couple of Heath and Yaesu monitor scopes.

Rodger WQ9E

[K1JJ]

Jay,

If the previous suggestons don't shed the answer, try some more:


Run a fat braid from your normal station ground bus and touch different spots on the metal case of the scope to see if the "hum"  drops off.  (gnd loop)

Also plug the scope into a different outlet and also try turning the plug around as a test.  (60 hz AC imbalance)

Try removing the scope ground strap and connecting it elsewhere to another spot in the shack ground bus.  (ground loops)

Move the scope around the shack to see if any particular area is better. (magnetic coupling)

T

[N3WWL]

OK.....I discovered that my linear amplifier is causing the concern.  When everything else is on in the shack there are no problems...only when the amp is on do I expereince this.  The scope is situated next to the amplifier by around 5 inches.  Is there such a thing as proximity effect?  Unfortunatley, the operating desk is very cramped for space and have no choice but to leave it where it is.  Am I out of luck, or is there still a potential remedy?  Thanks for all the replies thus far.

[W1RKW]

proximity effect (magnetic), yes.  big power transformers can influence the electron beam. Try a copper sheet between the two and ground the sheet to the scope. Been there, done that

[WQ9E]

Jay,

You could try putting your scope on the other side of the linear if that puts it further from the amplifier power transformer.  Move the scope around and find the spot that minimizes the hum; also try shielding as noted earlier.

[KA8WTK]

  I had a friend call about a scope problem he was having. The scope just would not adjust the way it used to. He had just rearranged his shack. I said I'd take a look and drove 45 minutes to his house.
  When I got there he showed me the scope and demonstrated the problem. I removed the 2 meter mag mount antenna from on top of the scope and the problem went away. Imagine that.

Bill

[WA1GFZ]

60 Hz  H field could radiation off a power supply could be getting into your scope. This will make you think there is an internal power supply problem.
I had the same problem with a monitor in the shack. Try running only the scope in the shack to see if it is stable. If so then move the scope or shield it.
Copper may not work 

[N2DTS]

Its those evil linear amplifiers again!

Brett

[WBear2GCR]

Solution: buy a cheap used and serviceable solid state 100mhz Tektronix scope and wonder why you ever used that old Hitachi!

 

                  _-_-bear

[The Slab Bacon]

proximity effect (magnetic), yes.  big power transformers can influence the electron beam. Try a copper sheet between the two and ground the sheet to the scope. Been there, done that

When I was running my Junkston T-Bolt and a DX-60 I had a similar problem that absolute drove me nutz! I would always pick up a hum in the audio. (you could see it in the scope pattern.) I tried running DC on the VFO filament, the DX-60 filaments, searched for ground loops, etc. It also would not produce the hum when it was on the workbench. I was on the air one night when I found the problem. I turned and slid my chair and watched the hum lessen on the scope waveform.

It was the dynaramic mic picking up magnetic field from the T-bolt's plate transfoma! ! !
I felt like I had been had! a change to a good old D10-4 and it all went away!!
I had been had! ! 

[ke7trp]

I have the same problem with a YO100 scope.  If I flip the Globe Champion on, the Line wobbles.  The Scope is about 4 to 6 inches from the Champions big Power trans.  The only solution I found was to move the scope.


Clark

[Dan N3SMF]

Jay; I will give you the magic apple core..

[w5omr]

OK.....I discovered that my linear amplifier is causing the concern. 

I've got the same problem with my Bogen PA system, modified to act as the speech-amp.  That transformer generates a field, and the two are side-by-side.

http://w5omr.shacknet.nu:81/~w5omr/hamstuff/hamstuff/AM-Stuff/my-shack-06-06-04/looking-right.jpg

I've since relocated things.  The tuner is now up near where the open wire line comes into the shack near the roof, the VFO is down where the Tuner was, the scope is to the right of the Bogen.  Not that moving the scope to the right of the Bogen did any good... the power transformer is on the right side of the speech-amp :-( 

Knowing what it is helps, though.

I sure miss being able to get on the air.  I'm 'stuck' here in this hell hole known as Houston.

[w1vtp]

60 Hz  H field could radiation off a power supply could be getting into your scope. This will make you think there is an internal power supply problem.
I had the same problem with a monitor in the shack. Try running only the scope in the shack to see if it is stable. If so then move the scope or shield it.
Copper may not work 

I've had the exact problem.  Sounds like your linear xfmr magnetic field is doing nasties with your trace.  Line sync would certainly identify it's line related

Al

[w3jn]

Copper or aluminum won't do much to reduce the electromagnetic field that's obviously coupling into the scope from the powerxformer of the amp.  Sheet steel will help, however.

[N3WWL]

I ended up completely redesigning the equipment layout...what a pain in the a$$.  The scope now resides far enough from the power transformer in the amp that the condition has been solved.  Thanks for all the ideas.

[Steve - WB3HUZ]

... or mu-metal which is what most higher end scopes have surrounding the tube.

Copper or aluminum won't do much to reduce the electromagnetic field that's obviously coupling into the scope from the powerxformer of the amp.  Sheet steel will help, however.

[K1JJ]

You could also use Krell metal for shielding, though it may be more difficult to find.



"Jay tries to burn his way through the Krell metal with his disintegration gun, but doesn't even dent it. This is the strongest material man has ever seen."

[ke7trp]

I fixed my scope wobble this morning.   I found that there was to much Stareus on the scopes artifew.  It was a real bear to clean that off. I am not sure what the factory used.

Clark

[WA1GFZ]

Huz,
The Gore-Shield rep just gave me a shield kit of gasket material with nickel plated flexable plastic sheet. I have not had a chance to try it yet to see if it works as well as Mu-metal. Looks interesting