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An Oscilloscope and a Cell Tower Walk Into a Shop...
Here's the situation: Sometime last year, we got a scope after some
begging by a tech. He then said that the cell tower out back (somewhere between 50-100 feet away) was causing interference. He quit, and the other 'tech' working in the shop was fired. Now that I'm in there, the 'boss' wants the scope out and working, but all I hear is, "Oh, that doesn't work" from a couple of people. Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...pes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? |
wrote in message oups.com... Here's the situation: Sometime last year, we got a scope after some begging by a tech. He then said that the cell tower out back (somewhere between 50-100 feet away) was causing interference. He quit, and the other 'tech' working in the shop was fired. Now that I'm in there, the 'boss' wants the scope out and working, but all I hear is, "Oh, that doesn't work" from a couple of people. Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...pes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? Your post is hard to follow, but I would check the grounding very thoroughly and also make sure the case is missing no screws. What sort of interference? What happens if you take the scope elsewhere? N |
Nobody knows what sort of interference, except for the guys that no
longer work there. I'll try clearing it up: Tektronix 475(?), sitting on plywood, ontop of a metal table. Scope is plugged into a power strip that is shared by another power strip, a computer, monitor, and hub. That power strip gets it's power from another strip (I believe they are all NCR surge protectors). These strips plug into the same quadplex outlet, which is full. The table is not grounded, and sets on a concrete floor. The shop has walls made of cinder blocks, and the roof is a fairly plain metal type, with steel lattice work. The cell tower is on the same property, about 100 feet away, and I suppose 200ft tall, but I don't know for sure. Tomorrow, if I'm not too busy, I'll go over the scope with a grounded fine tooth comb. |
"NSM" writes:
wrote in message oups.com... Here's the situation: Sometime last year, we got a scope after some begging by a tech. He then said that the cell tower out back (somewhere between 50-100 feet away) was causing interference. He quit, and the other 'tech' working in the shop was fired. Now that I'm in there, the 'boss' wants the scope out and working, but all I hear is, "Oh, that doesn't work" from a couple of people. Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...pes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? Your post is hard to follow, but I would check the grounding very thoroughly and also make sure the case is missing no screws. What sort of interference? What happens if you take the scope elsewhere? But what's the interference look like? Do take the scope off-site and see if it clears up. --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
Don't know..., yet. We just put it back, out from under the table late
Friday, with nothing to probe. *snicker. But, I have a gut feeling that the interference will be minimal, and the tech that begged for the scope never knew what he was doing. |
Comes down to this, the EMI from the tower is well outside the range of
the scope's bandwidth. Therefore one of two things could be happening. One could be that it is picking it up through the mains ground terminal. Even if seperate, the tower could be causing a ground gradient. You could isolate the ground terminal and see if this helps. If so, all you need is a resistor with some Zeners to cure it. You can't leave the scope floated, it may cause a breach between hot and cold grounds. Another possibility is that it's the probe. Do you get the interference when you switch the channel off, or by using the REF on the probe ? I sure hope you ain't trying to use a seperate ground for the DUT, if so this is the problem. That hasn't worked in decades. YOU MUST use the ground provided near the tip of the probe, the one that usually connects with a fork type thingy (luv those technical terms). You can't use a ground connected to a binding post on the scope unless you are dealing with a large signal amplitude. If you go below 5V/Div use the proper ground. The one at the tip. Also, just what type of probe are you using ? JURB |
wrote in message oups.com... Nobody knows what sort of interference, except for the guys that no longer work there. .... Tomorrow, if I'm not too busy, I'll go over the scope with a grounded fine tooth comb. I'd assume it's OK until you see different. Then I'd try using it off site. N |
wrote in message oups.com... Here's the situation: Sometime last year, we got a scope after some begging by a tech. He then said that the cell tower out back (somewhere between 50-100 feet away) was causing interference. He quit, and the other 'tech' working in the shop was fired. Now that I'm in there, the 'boss' wants the scope out and working, but all I hear is, "Oh, that doesn't work" from a couple of people. Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...pes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? What are the symptoms and what are you trying to test with the scope? I've never had a problem with outside interference. |
wrote in message oups.com... Don't know..., yet. We just put it back, out from under the table late Friday, with nothing to probe. *snicker. But, I have a gut feeling that the interference will be minimal, and the tech that begged for the scope never knew what he was doing. As others have suggested, take the thing home and see if it works there. The inexperienced seem to think an oscilloscope is a magical tool that will automatically show you what's wrong with something, while in reality it'll show you what's there but you still have to know what *should* be there for it to be of any use. I suspect either the "tech" simply didn't know how to properly use the scope, or there's a fault in the scope itself. Those old Tek scopes are wonderful but they are getting a bit old, dried up filter caps in the power supply are not uncommon. |
On 20 Feb 2005 18:31:05 -0800, wrote:
Here's the situation: Sometime last year, we got a scope after some begging by a tech. What did you (he) get it _for_? What are you intending to do with it now that you have it? Now, I have very little experience with scopes Start with the basics, and learn a bit about how it's supposed to work. If you don't know what you're doing, or what it's supposed to do, you can't make much of a judgement on whether it's working correctly, or not. Tek stuff is quite well known, so you're in luck there. A few minutes spent with Google should turn up several places that will sell you a copy of the manual for your scope. In there, you'll find instructions for setting it up, using it, and most important here, testing and calibrating it. A few more minutes spent with Google should turn up some tutorials on how a scope works and how to use one. Here's one: http://www.doctronics.co.uk/scope.htm you may want to start with to get familiar with the controls and what they're for. There are others out there too. -- | David Gersic www.zaccaria-pinball.com | | Saddam Hussein still has his job. Do you? | | Email address is munged to avoid spammers. Remove the underscores. | |
NSM wrote:
I'd assume it's OK until you see different. Then I'd try using it off site. In any case, unless the cell site has severe downtilt antennas (not very likely if it is 200' tall) the RF emitted from the cell site is going to be well above your shop. |
"NSM" wrote in news:o6cSd.7056$9a3.4552@edtnps91:
wrote in message oups.com... Here's the situation: Sometime last year, we got a scope after some begging by a tech. He then said that the cell tower out back (somewhere between 50-100 feet away) was causing interference. He quit, and the other 'tech' working in the shop was fired. Now that I'm in there, the 'boss' wants the scope out and working, but all I hear is, "Oh, that doesn't work" from a couple of people. Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...cs/Notes/week1 /oscilloscopes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? Borrow and try another O'scope. Your post is hard to follow, but I would check the grounding very thoroughly and also make sure the case is missing no screws. What sort of interference? What happens if you take the scope elsewhere? N Make sure inside,none of the PCBs are missing screws,or have them loose. -- Jim Yanik jyanik at kua.net |
Jim Adney wrote in
: On 20 Feb 2005 18:31:05 -0800 wrote: Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...cs/Notes/week1 /oscilloscopes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? That's a very nice scope from 30 years ago. It is a 200 MHz scope and may well work perfectly, or may have some problems related to age. It's quite possible that it can pick up some "off the air" noise sufficient to affect it at it's most sensitive ranges, but otherwise it should be fine. I don't think there is anything you can do to improve the scope itself, but putting it in a metal building would help a lot. RCA in Indianapolis used to have Faraday mesh cages for their engineers to work on projects.They now are Thomson-CSF,I believe.The cages are probably still there,too. -- Jim Yanik jyanik at kua.net |
"Jim Yanik" . wrote in message .. . RCA in Indianapolis used to have Faraday mesh cages for their engineers to work on projects.They now are Thomson-CSF,I believe.The cages are probably still there,too. Wouldn't it give you an odd feeling - being kept in a cage? Add bananas and what have you got G? N |
"NSM" wrote in news:ENuSd.13569$0h.4090@clgrps13:
"Jim Yanik" . wrote in message .. . RCA in Indianapolis used to have Faraday mesh cages for their engineers to work on projects.They now are Thomson-CSF,I believe.The cages are probably still there,too. Wouldn't it give you an odd feeling - being kept in a cage? Add bananas and what have you got G? N I suspect their pay was good,more than banannas. 8-) -- Jim Yanik jyanik at kua.net |
Taking it home would be a bad idea..., the guy that was fired had taken
something home. If I can find something to test it with, I'll certainly let you guys know what happens. |
Just to clarify, it's a Tek 465. Short day for me, all I could do was
check the outside case. I noticed it had some warranty stickers on it, so perhaps it came straight from a repair shop. |
I found a manual he http://bama.edebris.com/manuals/ . I'll spend my
paid time looking it over. And as for what we're going to do with it, I really don't know. The 'shop' is in the business of reselling used equipment, and I figure in 100 years, we'll need to go through the piles of broken/bad terminals and printers, find what is really wrong with them, and fix them. |
On 21 Feb 2005 17:23:53 GMT Jim Yanik . wrote:
RCA in Indianapolis used to have Faraday mesh cages for their engineers to work on projects.They now are Thomson-CSF,I believe.The cages are probably still there,too. I bought a used 465B (100 Mhz) scope quite a few years ago. I had it shipped to me at work and checked it out quickly when it came in and everything looked fine. When I took it home and tried it again that night I was surprised to find that there was noticable noise on its most sensitive settings. This bugged me for a few days until I just happened to notice that the noise seemed to be synced to the radio station I was listening to. Turning off the radio did not help, but taking the scope back to work, which happened to be in a metal building, eliminated the problem. The scope was fine, my environment was just noisy. ;-) - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
wrote in message oups.com... Taking it home would be a bad idea..., the guy that was fired had taken something home. I assume you can arrange to sign it out. BTW, NOTHING should leave the shop without being signed out. You'd be amazed how much stuff I have seen vanish from different places over the years because no-one could remember who the hell had borrowed it. If I can find something to test it with, I'll certainly let you guys know what happens. Sure thing. N |
wrote:
Here's the situation: Sometime last year, we got a scope after some begging by a tech. He then said that the cell tower out back (somewhere between 50-100 feet away) was causing interference. He quit, and the other 'tech' working in the shop was fired. Now that I'm in there, the 'boss' wants the scope out and working, but all I hear is, "Oh, that doesn't work" from a couple of people. Now, I have very little experience with scopes, and think that possibly this is a grounding problem, or of course, there is a need for shielding, or knocking down the tower itself. The scope is used, and probably around 20 years old, and looks like this: http://dustbunny.physics.indiana.edu...pes/t475_c.gif , the Tektronix 475. Any suggestions on shielding the interference, or testing to make sure of the existence of local interference? Didn't see any mention of the MAGNITUDE of the interference. I'd been using 50 MHz scopes for decades. Got a 500MHz one and started seeing low level noise. Punch the FFT button and I can see every FM and TV station for miles around. It's coming in the probe. Hooking the ground clip to the tip doesn't help. Shorting the tip directly to the tip ground fixes the problem. Ain't nothing to be done. If your probe is broken, the problem can/will be much worse. The cell tower may not have anything to do with what you're seeing. There may be other stuff besides cellphones on the tower. I'd borrow a spectrum analyzer. mike -- Return address is VALID but some sites block emails with links. Delete this sig when replying. .. Wanted, PCMCIA SCSI Card for HP m820 CDRW. FS 500MHz Tek DSOscilloscope TDS540 Make Offer Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. MAKE THE OBVIOUS CHANGES TO THE LINK htremovethistp://www.geocities.com/SiliconValley/Monitor/4710/ |
On 21 Feb 2005 19:30:11 -0800 wrote:
Just to clarify, it's a Tek 465. Short day for me, all I could do was check the outside case. I noticed it had some warranty stickers on it, so perhaps it came straight from a repair shop. That's a very popular 100 MHz scope made between 1970 and 1780. They were and still are great instruments. This was probably the most popular scope ever made by Tektronix, until it was replaced in '80 by the 465B. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
"Jim Adney" wrote in message ... On 21 Feb 2005 19:30:11 -0800 wrote: Just to clarify, it's a Tek 465. Short day for me, all I could do was check the outside case. I noticed it had some warranty stickers on it, so perhaps it came straight from a repair shop. That's a very popular 100 MHz scope made between 1970 and 1780. They were and still are great instruments. This was probably the most popular scope ever made by Tektronix, until it was replaced in '80 by the 465B. Wow, 1780, I bet they were *really* great instruments for the time :) Seriously though, I have a 465B and it's a wonderful scope, haven't used anything modern yet I'd trade it for though I have been wishing I had one of those 4 channel DSO's to augment it. |
On Thu, 24 Feb 2005 03:35:57 GMT "James Sweet"
wrote: "Jim Adney" wrote in message That's a very popular 100 MHz scope made between 1970 and 1780. They were and still are great instruments. This was probably the most popular scope ever made by Tektronix, until it was replaced in '80 by the 465B. Wow, 1780, I bet they were *really* great instruments for the time :) Yeah, sorry. I didn't notice that typo until I saw it posted here.... Seriously though, I have a 465B and it's a wonderful scope, haven't used anything modern yet I'd trade it for though I have been wishing I had one of those 4 channel DSO's to augment it. I have a 465B, also, and I completely agree with you. It's an exceptionally well engineered and bioengineered scope and I love mine. OTOH, I'm actually starting to like the DSO I have at work. It certainly has some annoying quirks, but once you learn how you are supposed to use it, it works pretty well. I'd still have to say that I'd rather it worked just like my 465B, but with the DSO size. - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
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