Re: New here with a 2465 to clean up
The fan squeals badly. Any advice?
I found why it didnt work. Someone had loosened its plug so it wouldnt make noise anymore. So now I have a fan that runs but it squeals so badly I cant use it. A drop of oil in the end that is accessible did nothing for it. The other bearing requires desoldering the fan from the board. I can do that but I want to fix the bearing while I am in there. Has anyone scaled this mountain before me? Are replacement fans available?
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Show quoted text
--- In TekScopes@..., "Tom Miller" <tmiller11147@...> wrote:
There are no brushes in that motor.
There is an end play adjustment on the rear bearing but it requires removal of the pc board to get to.
Did the fan just pull off the shaft?
T
----- Original Message -----
From: machineguy59
To: TekScopes@...
Sent: Sunday, March 10, 2013 6:14 PM
Subject: [TekScopes] Re: New here with a 2465 to clean up
I will post my experience repairing the fan when I have it completed. But for now, the fan and squirrel cage appear to be intact. The collet is functional although I have not taken the jaws out of the sleeve yet. I was careful removing them and it all looks good to me.
I suspect either the motor windings (not likley I think), the motor "brushes" (more likely), or the drive transistors (what I am hoping for). I will keep my fingers crossed.
--- In TekScopes@..., "Tom Miller" <tmiller11147@> wrote:
>
> We will be interested in how you resolve the fan problem.
>
> it is a well known issue. Most likely the plastic fan collet has broken.
> Hey, I wonder if this part (or something similar) could be made with the 3D printer?
>
>
> Tom
>
> ----- Original Message -----
> From: machineguy59
> To: TekScopes@...
> Sent: Sunday, March 10, 2013 2:05 PM
> Subject: [TekScopes] Re: New here with a 2465 to clean up
>
>
>
> You guys are super. Thanks you again for the warm welcome and helpful advice. I finally got some time to apply some of it and want to give you a status update.
>
> First, I dismantled the front control panel and cleaned everything. There was a minor amount of residue from an old spill (I feared it would be worse) but overall the inside is remarkably clean. I did find that someone had previously glued some knob covers to their inserts and they broke during tear down. I plan to order replacements but I CAN glue them back if necessary.
>
> Second, I built a heat sink for U800 by modifying an old heat sonk from a computer video board. It was messsy to cut notches for adjacent parts but it was worth it. I have a heat sink that fits well and seems to work well. It screws to the same screws that hold the U800 but I was careful to no overtighten. I can post or mail pictures if anyone is interested.
>
> Third, Patrick, your advice to measure voltages on J119 (the test point for regulated low voltage on the mainboard) is spot on. The -8 volt supply is only at -6.4 volts (a 20% error). Also, several voltages have ripple that is far out of spec (I didnt record actual values but will before repairs are started). My review of the schematics shows that -8 supply only feeds the two vertical pre-amps (U100 and U200) and the op amp that sets paramters for VAR gain and DC ballance (U160). My conclusion is that you were right, the -8 volt supply is failing (probably the output filter cap) and this drooping voltage causes the vertical pre-amp gains to be off and variable. Alternatively, one of the preamps or the op amp is overloading the supply but this seems unlikely since other preamp functions are working.
>
> Fourth, I ordered new caps for the entire LVPS (both boards). The spead sheet posted at Tekarc was very helpful and still relevant. All parts are still available at Mouser. I verified this list with the service manual for the 2465 and found I need two additional caps (C1112, and C1050). I also chose to order replacements for the ceramic caps but will probably not change them out since ceramics seem to have a long life expectancy. I apprecaiate any advice on this decision.
>
> Fifth, I found that the case fan is not running. The shaft turns freely and feels to run smoothly but the fan does not turn. If I give it a spin by hand it just stops with no apparent self motivation. So I have a fan to repair. Its one of the "original" designs with a separate transistorized drive board. These were very quiet fans so I will try to restore this one.
>
> So work continues in my spare time. I expect a complete restoration will be possible. I will post updates of my progress along the way. But I wanted to post this thank you for the excellent help I have received from this group. I would not take on this project without your advice.
>
> Mac
>
>
> --- In TekScopes@..., "Patrick Wong" <patwong3@> wrote:
> >
> > Hi Mac,
> >
> > I recommend that you add qservice.tv as a favorite in your browser because you can get lots of useful information about Tektronix scopes there.
> >
> > For example, you will see that the part number for the 2465 low voltage power supply has a slightly different suffix compared to the LVPS for the 2465A and 2465B. Therefore I would not assume that a parts list compiled for a 2465B will work 100% for your unit. It might be 95% correct, or might be further off.
> >
> > When I worked on the LVPS for my 2465A and 2467 I found small differences between the capacitors. There were a few non-polarized electrolytics in the 2465A power supply. Both scopes were built in mid-1988. Therefore, even within the 2465A/2467 generation, you cannot count on the LV power supplies being identical with regards to capacitor content.
> >
> > I also recommend you replace the two AC X2 line filtering capacitors as you will probably see the yellow transparent plastic body has lots of cracking. This is a potential failure area especially in Europe where the AC line voltage is 220V or more.
> >
> > With regards to the CH1 and CH2 gain problem, I doubt the preamp hybrids are the problem. Assuming the 2465 has similar circuitry as 2465A in this area, I suggest you look at U160, a quad op-amp which controls the gain and DC balance for U100 and U200 (see schematic #4 which shows the vertical attenuators and preamp section.) U160 is powered by the +15V and -8V power supplies (see the power distribution diagram) so a problem with either of those two may help explain the gain problem you noticed.
> >
> > Meanwhile the CH3/CH4 hybrid is powered by the +5V and -5V supplies.
> >
> > Does your A5 board have a lithium battery soldered to one end? If so, that maintains calibration constants.
> >
> > Assuming the battery is original, I suggest you add a replacement battery to your parts ordering form. You will need to maintain voltage with a lab power supply while unsoldering and replacing the battery (using a non-grounded soldering iron.)
> >
> > Good luck,
> >
> > Patrick Wong AK6C
> >
> > --- In TekScopes@..., "machineguy59" <machineguy59@> wrote:
> > >
> > > ...That parts list is for a 2365B but it looks to me that it will also work for a 2465. Does anyone know if there are differences?
> > >
> > > Then, with the heat sink on U800 and the knowledge the glue is cleaned out, I will resume the quest to find why the gain is messed up on Ch 1 and Ch 2. This gain on Ch ! and Ch 2 is my only serious concer for the scope.
> >
>
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Re: Tek 214 running without batteries
--- In TekScopes@..., Scott Harris <scottrharris@...> wrote:
I meant opening the case and hard-wiring in cheap probes or wiring in a cable that terminates to a female BNC so I can use any probe.
It would be a bit of a hatchet job, but would be better than what I have now.
The 211-221 series use non-attenuating probes. With BW of only 500 kHz (5 MHz for 221), the C loading is not really an issue. You could take any divide by 1 probe and mod it into the scope. However, note that most scope probes use a resistive center conductor which is very fine gauge wire that can not be soldered to. So terminating the probe cable properly may be a challenge. Hacking a BNC connector through the case can be dangerous as the floating ground connection would be exposed, causing a shock or shorting hazard. There are electrically insulated BNC connectors available for this purpose. The outer shell and bayonet mount pins are made of plastic, while the back end of the inside of the shell has an electrical contact to connect with the BNC shield. To be safe, you need to use a probe with a similar plastic BNC connector. Again, only use a divide by one probe with the early 200 series scopes. Steve
|
I purchased a pair of these probes for my 475 just because I really couldn't afford anything else. My question is will i ahve any issues with these on the 475? They seem to work reasonably well They pick up good traces from the calibrator and a small 555 based square wave and triangle wave generator and I also have the AVR clock kit and that works well also. But if and when I get into more advanced uses will I need better probes? And what is really the difference between these and say a P6106? Thanks for any info.
|
Re: Tek 214 running without batteries
Qservice may have some:? http://www.qservice.tv/vpasp/shopdisplayproducts.asp?id=220&cat=TEKTRONIX+Probe+Parts+-+Accessories? You need the part number.
I bought some of the tips for the 211 there.
|
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Re: Isolation transformers are not a panacea [was: Variak talk]
An isolation transformer, a Variac, a GFI, or an Isolation Monitor are no
substitute for caution and common sense. They are tools that, when used
properly, can prevent some dangerous conditions from occurring. As Steve
illustrated with his example they can give a false sense of security unless
you understand what you are doing and how to apply them properly.
One small plus to using an isolation transformer that always makes me feel a
little safer than working directly off the 120V power lines is that it is
capable of only delivering a few hundred watts of power if something should
short the power leads together. The energy a 120V power line can deliver
before the circuit breaker trips can be considerably greater.
|
Re: New here with a 2465 to clean up
This link doesnt work and I cant get one that opens directly. So, here is a substitute. This link to the Files at TekscopArc:
Then scroll down and double click on the file named: 2465LVPS_ReCap.xls
Hope this works better.
toggle quoted message
Show quoted text
--- In TekScopes@..., "machineguy59" <machineguy59@...> wrote:
Of course. I should have done that in the first place. Here is the link to a spread sheet sowning parts to re-cap the OVPS of a 2465B:
This is a spread sheet done by Random Walk and is a very complete listing including links to Mouser parts on the web. I verified the list against the service manual for a 2465, identified a few extra I wanted and ordered parts.
--- In TekScopes@..., Dave C <davec2468@> wrote:
Can you please give the link?
Thanks,
Dave
The spead sheet posted at Tekarc was very helpful and still relevant.
...Mac
|
Re: a note for USA residents buying from Walter at Sphere
However, unless things are very different in Canada, Paypal's fees are nowhere near 7%, so I'm really curious why Walter wants an additional 7% fee from you for this. For the very smallest-volume sellers, Paypal's fee is 2.9%, plus an additional 1% for currency exchange which of course is necessary for any cross-border transactions unless you elect to keep the funds in that foreign currency (which is an option). PayPal also marks up the exchange rate. I don't know if 7% is the right number, but I don't think it's far off. I'm surprised Walter doesn't have a USD account though.
|
Re: Tek 214 running without batteries
I would think a lot depends on how the probe is compensated, and what load is necessary. I'm not familiar with the 214 and it's input circuitry, but you may have to do more than just solder a BNC connector in place.
Mark
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Show quoted text
On 03/12/2013 11:34 AM, Scott Harris wrote: I meant opening the case and hard-wiring in cheap probes or wiring in a cable that terminates to a female BNC so I can use any probe.
It would be a bit of a hatchet job, but would be better than what I have now.
On Mar 12, 2013, at 9:22 AM, Mark Wendt <mark.wendt@...> wrote:
Ah, okay, I see what you are talking about now.
I doubt you are going to find a generic cheap replacement for your scope
the way those are designed.
Mark
On 03/12/2013 11:14 AM, Scott Harris wrote:
The Tek 214 probes are built into the scope and can't be removed without opening the unit.
Take a look at this auction:
I need two of the grey plastic bits. Greece is pretty far and $44 is almost what I paid for the scope.
On Mar 12, 2013, at 9:10 AM, Mark Wendt <mark.wendt@...> wrote:
What model number probes do you have?
Mark
On 03/12/2013 11:01 AM, Scott Harris wrote:
I'm not sure the typical Tek probe tip will fit the 214 probes.
On Mar 12, 2013, at 5:01 AM, Mark Wendt <mark.wendt@...> wrote:
On 03/11/2013 08:34 PM, Scott Harris wrote:
I built some new packs and installed them in my Tex 214: It now works
like champ.
Getting the boards aligned up and stuffed back in that little case
took a few tries, but it's working again.
Now I just need to find some replacement probe tips. Any suggestions?
Should I just buy some cheap probes and replace the ones I have?
-Scott
Scott,
Do a search on Ebay for "tektronix probe tip" - there's tons of them out
there, and most are fairly inexpensive.
Mark ------------------------------------
Yahoo! Groups Links
------------------------------------
Yahoo! Groups Links
------------------------------------
Yahoo! Groups Links
------------------------------------
Yahoo! Groups Links
|
Re: a note for USA residents buying from Walter at Sphere
Paypal charge about 5% for cross currency transactions here in the UK, a bit less for same currency.
D.
|
Re: Isolation transformers are not a panacea [was: Variak talk]
On Tue, 12 Mar 2013 14:31:21 -0000, "Steve" <ditter2@...> wrote: --- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:
An isolation transformer allows you to make measurements safely.? You don't isolate the line powered instruments, but isolate the load.? You don't ever want the case of an instrument to become live.
I don't quite agree with the phrase "An isolation transformer allows you to make measurements safely."
While using an isolation transformer can let you make a floating measurement not referenced to ground, doing so safely requires a keen sense of what you are doing and where the potentials are. A few "newbies" often read this forum for advice, and I would not want them to believe that an isolation transformer magically makes floating measurements safe.
Keeping the measurement instrument (usually a scope) grounded and floating the Device Under Test (DUT) only shifts the place where the user can be exposed to a shock hazard. While it generally will protect the measurement instrument, this practice remains extremely dangerous.
Here is an example. Assume the user is troubleshooting an old Tek scope, as the original poster mentioned. They want to measure a signal referenced the to the +225 V power supply. The usser keeps the test scope safely grounded, so they are not "floating the scope", and plugs the DUT , a 545A scope, into an isolation transformer. The user connects a probe to the test scope, and connects the probe ground lead to the 545A +225 V power rail. There is no spark, and neither the test scope or the DUT are damaged. However, when the 545A is turned on, its chassis, front panel, and all exposed metal parts are now "floating" to ¨C225 Volts! While the user will not get shocked touching the controls on the test scope, which is safely grounded, touching the aluminum front panel on the 545A they are troubleshooting will deliver a painful shock at the least, or possibly a much more serious injury at the worst. I think the only time I have used an isolation transformers is for primary side power supply measurements where the slightly elevated neutral or the DC side of the input bridge rectifier was a problem. I rather vividly remember the time I had the grabber part of a probe ground lead turn white hot and melt before my eyes. If the user is working on a high power switching power supply, the capacitance of all but the most expensive "medical grade "isolation transformers can cause the supply to explode if the probe ground lead is connected to the wrong location (I have described the mechanism that causes this before ¨C search the archives on isolation transformers, or read the 16 page tutorial I wrote while at Tek ¨C "differential measurements- a primer". Do you mean page 7? Page 16 is the last page and part of the glossary: I always get a thrill chatting with the author of a well known article. :) If the user does not currently own an isolation transformer and is looking at buying one for $50 or more, I would suggest an alternative ¨C a good differential amplifier. If you often have the need to measure signals elevated from ground, or the primary circuits in power supplies, I would suggest you consider a differential amplifier such as a 7A13 with an inexpensive 7K scope such as a 7603. If your main scope is a 54x series, a type W works. A good pair of divide by 10 differential probes to go in front of it is a good addition. Yes, it will cost a bit more than an isolation transformer, but both you and the device under will remain safe. I have an isolation transformer, 7A13 amplifiers, and 7A22 amplifiers but have also gotten good or at least usable results using add and invert mode of a 2 channel oscilloscope while trimming the gain of one channel using the VAR control and sometimes tweaking the probe compensation to trim the common mode rejection.
|
Re: a note for USA residents buying from Walter at Sphere
I really wish Canada and the US would establish better postal ties, with much lower rates to encourage cross-border trade; if any two countries should be very close, it's these two. Instead, the USPS almost doubled rates for many levels of service to Canada about a month ago, in order to get their budget in order since that disastrous bill was passed in 2006 requiring them to pre-fund their workers' retirement plans for 75 years in advance.
However, unless things are very different in Canada, Paypal's fees are nowhere near 7%, so I'm really curious why Walter wants an additional 7% fee from you for this. For the very smallest-volume sellers, Paypal's fee is 2.9%, plus an additional 1% for currency exchange which of course is necessary for any cross-border transactions unless you elect to keep the funds in that foreign currency (which is an option). In fact, calling PP's fees "rapacious" is really quite unwarranted, as they have the absolute lowest fees I've ever seen for a credit card processor. I challenge you to find a CC processor with lower fees; there aren't any. PP is downright cheap for small sellers; they don't charge a monthly fee like most processors. The only place they might not be cheapest is if you're selling in huge volumes, such as more than $100,000 per month. But for small sellers, they can't be beat.
Anyway, the point is, if a seller wants you to pay extra to cover PP's fees, they should only be asking for an additional 2.9% if they're domestic, and 3.9% if they're outside the US. (Of course, I could be mistaken, and Canadian PP users might be getting charged a lot more than us US customers. Considering that cars cost nearly twice as much in Canada as they do in the US, even when they're built in Canada, this wouldn't surprise me too much.)
As for your bad USPS employee, that happens. They're not all highly competent, just like at any business, and worse, they're unionized so it's not that easy to get rid of them. As someone who sends a lot of USPS parcels, my strategy is simple: I go to the same PO most of the time, and I get to know the clerks there, and I know which ones are good and which ones aren't. So if I have a choice between one of my regular highly competent clerks and some stooge who I either know isn't competent, or I haven't dealt with before, I pick the clerk I know. One big issue I have is that many clerks don't know how well how to deal with international parcels, because they don't have to handle them very often since Americans don't generally send much mail internationally. The USPS made a big change to their online customs form over 6 months ago, and I've had many clerks who had never seen it before I came to them.
Dan
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Show quoted text
--- In TekScopes@..., "n6otq" <n6otq@...> wrote:
No matter what some of us may think, Canada still isn't our 51st state.
Recently, Walter at Sphere made me an extremely generous deal on a manual, and asked for either a USPS international money order in payment, or PayPal with a surcharge sufficient to cover PP's rapacious fees.
What I found was -- the current fee for a USPS international money order is USD$4.50, and postage from the states to Canada is $1.10. Walter asked for a 7% PayPal surcharge. In my case, it would have been much cheaper to pay that surcharge, much as I hate PayPal. They're not my pal, friend; they're not my friend, buddy; they're not my buddy, pal. (with apologies to "South Park")
Your mileage may vary, especially as the dollar amount of the transaction rises, but I suggest that you work the numbers before you go to the post office.
|
Re: Isolation transformers are not a panacea [was: Variak talk]
That's because Bender has some key patents.
We used them in huge floating DC battery systems where a true shorting fault
would be catastrophic and extremely dangerous and would shut down and open
contactors everywhere on detection of the first fault. I wouldn't touch the bus
bars but it went from "no arc flash protection will save you" to gloves and HRC
2 satisfied the safety people.
Those units are very pricey but they do show up surplus sometimes and because
almost nobody knows what they are sometimes you can get them for almost nothing.
Peter
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Show quoted text
On 3/12/2013 11:27 AM, Stefan Trethan wrote: Yes, it's called an isolation monitor. Bender makes most of them:
<>
They are mostly used in hospitals and such where the isolation is used
to provide redundancy against faults, not additional safety.
It gives them time to find the first fault while the power stays on.
Wouldn't hurt to add such a thing to a test bench, but a lot of the
time I intentionally, and carefully, ground the system with the scope
ground.
If I was working on metal chassis equipment a lot I would equip the
isolation transformer with a grounded outlet (earth conductor
(chassis) connected to earth ground). This is against code (isolation
transformers are supposed to have two prong outlets) but it's just no
good to have a big metal chassis possibly floating around at voltage.
ST
On Tue, Mar 12, 2013 at 4:15 PM, Peter Gottlieb <hpnpilot@...> wrote:
There is a type of GF detector which works on an isolated system and will trip
on the first fault. The way it works is to inject a tiny current wave onto the
circuit and observe the voltage change and then calculate the resistance to
ground. There are compensation settings for capacitance to ground and you can
set warning and trip points. One of these on an isolation transformer would
protect you against some errors but not the -225 volt one mentioned before!
Peter
On 3/12/2013 10:57 AM, Stefan Trethan wrote:
Well said, Steve!
While power by isolation transformer itself is a very safe thing,
people usually defeat the safety mechanism instantly by producing the
first fault with the scope probe ground.
Isolated power is dangerous in a way, because while it is tolerant
towards the first fault there is no indication this first fault has
occurred and the second fault may be deadly.
This is why isolation monitors exist.....
Many other protection systems (GFI, earthing, ...) show you in some
way that a fault has occurred (usually by clanging noise, followed by
darkness, followed by swearing).
ST
On Tue, Mar 12, 2013 at 3:31 PM, Steve <ditter2@...> wrote:
--- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:
An isolation transformer allows you to make measurements safely. You don't isolate the line powered instruments, but isolate the load. You don't ever want the case of an instrument to become live. I don't quite agree with the phrase "An isolation transformer allows you to make measurements safely."
While using an isolation transformer can let you make a floating measurement not referenced to ground, doing so safely requires a keen sense of what you are doing and where the potentials are. A few "newbies" often read this forum for advice, and I would not want them to believe that an isolation transformer magically makes floating measurements safe.
Keeping the measurement instrument (usually a scope) grounded and floating the Device Under Test (DUT) only shifts the place where the user can be exposed to a shock hazard. While it generally will protect the measurement instrument, this practice remains extremely dangerous.
Here is an example. Assume the user is troubleshooting an old Tek scope, as the original poster mentioned. They want to measure a signal referenced the to the +225 V power supply. The usser keeps the test scope safely grounded, so they are not "floating the scope", and plugs the DUT , a 545A scope, into an isolation transformer. The user connects a probe to the test scope, and connects the probe ground lead to the 545A +225 V power rail. There is no spark, and neither the test scope or the DUT are damaged. However, when the 545A is turned on, its chassis, front panel, and all exposed metal parts are now "floating" to ¨C225 Volts! While the user will not get shocked touching the controls on the test scope, which is safely grounded, touching the aluminum front panel on the 545A they are troubleshooting will deliver a painful shock at the least, or possibly a much more serious injury at the worst.
If the user is working on a high power switching power supply, the capacitance of all but the most expensive "medical grade "isolation transformers can cause the supply to explode if the probe ground lead is connected to the wrong location (I have described the mechanism that causes this before ¨C search the archives on isolation transformers, or read the 16 page tutorial I wrote while at Tek ¨C "differential measurements- a primer".
If the user does not currently own an isolation transformer and is looking at buying one for $50 or more, I would suggest an alternative ¨C a good differential amplifier. If you often have the need to measure signals elevated from ground, or the primary circuits in power supplies, I would suggest you consider a differential amplifier such as a 7A13 with an inexpensive 7K scope such as a 7603. If your main scope is a 54x series, a type W works. A good pair of divide by 10 differential probes to go in front of it is a good addition. Yes, it will cost a bit more than an isolation transformer, but both you and the device under will remain safe.
Steve
------------------------------------
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|
Re: Tek 214 running without batteries
I meant opening the case and hard-wiring in cheap probes or wiring in a cable that terminates to a female BNC so I can use any probe.
It would be a bit of a hatchet job, but would be better than what I have now.
toggle quoted message
Show quoted text
On Mar 12, 2013, at 9:22 AM, Mark Wendt <mark.wendt@...> wrote: Ah, okay, I see what you are talking about now.
I doubt you are going to find a generic cheap replacement for your scope
the way those are designed.
Mark
On 03/12/2013 11:14 AM, Scott Harris wrote:
The Tek 214 probes are built into the scope and can't be removed without opening the unit.
Take a look at this auction:
I need two of the grey plastic bits. Greece is pretty far and $44 is almost what I paid for the scope.
On Mar 12, 2013, at 9:10 AM, Mark Wendt <mark.wendt@...> wrote:
What model number probes do you have?
Mark
On 03/12/2013 11:01 AM, Scott Harris wrote:
I'm not sure the typical Tek probe tip will fit the 214 probes.
On Mar 12, 2013, at 5:01 AM, Mark Wendt <mark.wendt@...> wrote:
On 03/11/2013 08:34 PM, Scott Harris wrote:
I built some new packs and installed them in my Tex 214: It now works
like champ.
Getting the boards aligned up and stuffed back in that little case
took a few tries, but it's working again.
Now I just need to find some replacement probe tips. Any suggestions?
Should I just buy some cheap probes and replace the ones I have?
-Scott
Scott,
Do a search on Ebay for "tektronix probe tip" - there's tons of them out
there, and most are fairly inexpensive.
Mark ------------------------------------
Yahoo! Groups Links
------------------------------------
Yahoo! Groups Links
------------------------------------
Yahoo! Groups Links
|
Re: Isolation transformers are not a panacea [was: Variak talk]
Yes, it's called an isolation monitor. Bender makes most of them:
<>
They are mostly used in hospitals and such where the isolation is used
to provide redundancy against faults, not additional safety.
It gives them time to find the first fault while the power stays on.
Wouldn't hurt to add such a thing to a test bench, but a lot of the
time I intentionally, and carefully, ground the system with the scope
ground.
If I was working on metal chassis equipment a lot I would equip the
isolation transformer with a grounded outlet (earth conductor
(chassis) connected to earth ground). This is against code (isolation
transformers are supposed to have two prong outlets) but it's just no
good to have a big metal chassis possibly floating around at voltage.
ST
toggle quoted message
Show quoted text
On Tue, Mar 12, 2013 at 4:15 PM, Peter Gottlieb <hpnpilot@...> wrote: There is a type of GF detector which works on an isolated system and will trip
on the first fault. The way it works is to inject a tiny current wave onto the
circuit and observe the voltage change and then calculate the resistance to
ground. There are compensation settings for capacitance to ground and you can
set warning and trip points. One of these on an isolation transformer would
protect you against some errors but not the -225 volt one mentioned before!
Peter
On 3/12/2013 10:57 AM, Stefan Trethan wrote:
Well said, Steve!
While power by isolation transformer itself is a very safe thing,
people usually defeat the safety mechanism instantly by producing the
first fault with the scope probe ground.
Isolated power is dangerous in a way, because while it is tolerant
towards the first fault there is no indication this first fault has
occurred and the second fault may be deadly.
This is why isolation monitors exist.....
Many other protection systems (GFI, earthing, ...) show you in some
way that a fault has occurred (usually by clanging noise, followed by
darkness, followed by swearing).
ST
On Tue, Mar 12, 2013 at 3:31 PM, Steve <ditter2@...> wrote:
--- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:
An isolation transformer allows you to make measurements safely. You don't isolate the line powered instruments, but isolate the load. You don't ever want the case of an instrument to become live. I don't quite agree with the phrase "An isolation transformer allows you to make measurements safely."
While using an isolation transformer can let you make a floating measurement not referenced to ground, doing so safely requires a keen sense of what you are doing and where the potentials are. A few "newbies" often read this forum for advice, and I would not want them to believe that an isolation transformer magically makes floating measurements safe.
Keeping the measurement instrument (usually a scope) grounded and floating the Device Under Test (DUT) only shifts the place where the user can be exposed to a shock hazard. While it generally will protect the measurement instrument, this practice remains extremely dangerous.
Here is an example. Assume the user is troubleshooting an old Tek scope, as the original poster mentioned. They want to measure a signal referenced the to the +225 V power supply. The usser keeps the test scope safely grounded, so they are not "floating the scope", and plugs the DUT , a 545A scope, into an isolation transformer. The user connects a probe to the test scope, and connects the probe ground lead to the 545A +225 V power rail. There is no spark, and neither the test scope or the DUT are damaged. However, when the 545A is turned on, its chassis, front panel, and all exposed metal parts are now "floating" to ¨C225 Volts! While the user will not get shocked touching the controls on the test scope, which is safely grounded, touching the aluminum front panel on the 545A they are troubleshooting will deliver a painful shock at the least, or possibly a much more serious injury at the worst.
If the user is working on a high power switching power supply, the capacitance of all but the most expensive "medical grade "isolation transformers can cause the supply to explode if the probe ground lead is connected to the wrong location (I have described the mechanism that causes this before ¨C search the archives on isolation transformers, or read the 16 page tutorial I wrote while at Tek ¨C "differential measurements- a primer".
If the user does not currently own an isolation transformer and is looking at buying one for $50 or more, I would suggest an alternative ¨C a good differential amplifier. If you often have the need to measure signals elevated from ground, or the primary circuits in power supplies, I would suggest you consider a differential amplifier such as a 7A13 with an inexpensive 7K scope such as a 7603. If your main scope is a 54x series, a type W works. A good pair of divide by 10 differential probes to go in front of it is a good addition. Yes, it will cost a bit more than an isolation transformer, but both you and the device under will remain safe.
Steve
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Re: Tek 214 running without batteries
Ah, okay, I see what you are talking about now.
I doubt you are going to find a generic cheap replacement for your scope the way those are designed.
Mark
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Show quoted text
On 03/12/2013 11:14 AM, Scott Harris wrote: The Tek 214 probes are built into the scope and can't be removed without opening the unit.
Take a look at this auction:
I need two of the grey plastic bits. Greece is pretty far and $44 is almost what I paid for the scope.
On Mar 12, 2013, at 9:10 AM, Mark Wendt <mark.wendt@...> wrote:
What model number probes do you have?
Mark
On 03/12/2013 11:01 AM, Scott Harris wrote:
I'm not sure the typical Tek probe tip will fit the 214 probes.
On Mar 12, 2013, at 5:01 AM, Mark Wendt <mark.wendt@...> wrote:
On 03/11/2013 08:34 PM, Scott Harris wrote:
I built some new packs and installed them in my Tex 214: It now works
like champ.
Getting the boards aligned up and stuffed back in that little case
took a few tries, but it's working again.
Now I just need to find some replacement probe tips. Any suggestions?
Should I just buy some cheap probes and replace the ones I have?
-Scott
Scott,
Do a search on Ebay for "tektronix probe tip" - there's tons of them out
there, and most are fairly inexpensive.
Mark ------------------------------------
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Re: Isolation transformers are not a panacea [was: Variak talk]
There is a type of GF detector which works on an isolated system and will trip
on the first fault. The way it works is to inject a tiny current wave onto the
circuit and observe the voltage change and then calculate the resistance to
ground. There are compensation settings for capacitance to ground and you can
set warning and trip points. One of these on an isolation transformer would
protect you against some errors but not the -225 volt one mentioned before!
Peter
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On 3/12/2013 10:57 AM, Stefan Trethan wrote: Well said, Steve!
While power by isolation transformer itself is a very safe thing,
people usually defeat the safety mechanism instantly by producing the
first fault with the scope probe ground.
Isolated power is dangerous in a way, because while it is tolerant
towards the first fault there is no indication this first fault has
occurred and the second fault may be deadly.
This is why isolation monitors exist.....
Many other protection systems (GFI, earthing, ...) show you in some
way that a fault has occurred (usually by clanging noise, followed by
darkness, followed by swearing).
ST
On Tue, Mar 12, 2013 at 3:31 PM, Steve <ditter2@...> wrote:
--- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:
An isolation transformer allows you to make measurements safely. You don't isolate the line powered instruments, but isolate the load. You don't ever want the case of an instrument to become live. I don't quite agree with the phrase "An isolation transformer allows you to make measurements safely."
While using an isolation transformer can let you make a floating measurement not referenced to ground, doing so safely requires a keen sense of what you are doing and where the potentials are. A few "newbies" often read this forum for advice, and I would not want them to believe that an isolation transformer magically makes floating measurements safe.
Keeping the measurement instrument (usually a scope) grounded and floating the Device Under Test (DUT) only shifts the place where the user can be exposed to a shock hazard. While it generally will protect the measurement instrument, this practice remains extremely dangerous.
Here is an example. Assume the user is troubleshooting an old Tek scope, as the original poster mentioned. They want to measure a signal referenced the to the +225 V power supply. The usser keeps the test scope safely grounded, so they are not "floating the scope", and plugs the DUT , a 545A scope, into an isolation transformer. The user connects a probe to the test scope, and connects the probe ground lead to the 545A +225 V power rail. There is no spark, and neither the test scope or the DUT are damaged. However, when the 545A is turned on, its chassis, front panel, and all exposed metal parts are now "floating" to ¨C225 Volts! While the user will not get shocked touching the controls on the test scope, which is safely grounded, touching the aluminum front panel on the 545A they are troubleshooting will deliver a painful shock at the least, or possibly a much more serious injury at the worst.
If the user is working on a high power switching power supply, the capacitance of all but the most expensive "medical grade "isolation transformers can cause the supply to explode if the probe ground lead is connected to the wrong location (I have described the mechanism that causes this before ¨C search the archives on isolation transformers, or read the 16 page tutorial I wrote while at Tek ¨C "differential measurements- a primer".
If the user does not currently own an isolation transformer and is looking at buying one for $50 or more, I would suggest an alternative ¨C a good differential amplifier. If you often have the need to measure signals elevated from ground, or the primary circuits in power supplies, I would suggest you consider a differential amplifier such as a 7A13 with an inexpensive 7K scope such as a 7603. If your main scope is a 54x series, a type W works. A good pair of divide by 10 differential probes to go in front of it is a good addition. Yes, it will cost a bit more than an isolation transformer, but both you and the device under will remain safe.
Steve
------------------------------------
Yahoo! Groups Links
-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 10.0.1430 / Virus Database: 2641/5666 - Release Date: 03/12/13
|
Re: Tek 214 running without batteries
The Tek 214 probes are built into the scope and can't be removed without opening the unit.
Take a look at this auction:
I need two of the grey plastic bits. Greece is pretty far and $44 is almost what I paid for the scope.
toggle quoted message
Show quoted text
On Mar 12, 2013, at 9:10 AM, Mark Wendt <mark.wendt@...> wrote: What model number probes do you have?
Mark
On 03/12/2013 11:01 AM, Scott Harris wrote:
I'm not sure the typical Tek probe tip will fit the 214 probes.
On Mar 12, 2013, at 5:01 AM, Mark Wendt <mark.wendt@...> wrote:
On 03/11/2013 08:34 PM, Scott Harris wrote:
I built some new packs and installed them in my Tex 214: It now works
like champ.
Getting the boards aligned up and stuffed back in that little case
took a few tries, but it's working again.
Now I just need to find some replacement probe tips. Any suggestions?
Should I just buy some cheap probes and replace the ones I have?
-Scott
Scott,
Do a search on Ebay for "tektronix probe tip" - there's tons of them out
there, and most are fairly inexpensive.
Mark
------------------------------------
Yahoo! Groups Links
|
Re: Tek 214 running without batteries
What model number probes do you have?
Mark
toggle quoted message
Show quoted text
On 03/12/2013 11:01 AM, Scott Harris wrote: I'm not sure the typical Tek probe tip will fit the 214 probes.
On Mar 12, 2013, at 5:01 AM, Mark Wendt <mark.wendt@...> wrote:
On 03/11/2013 08:34 PM, Scott Harris wrote:
I built some new packs and installed them in my Tex 214: It now works
like champ.
Getting the boards aligned up and stuffed back in that little case
took a few tries, but it's working again.
Now I just need to find some replacement probe tips. Any suggestions?
Should I just buy some cheap probes and replace the ones I have?
-Scott
Scott,
Do a search on Ebay for "tektronix probe tip" - there's tons of them out
there, and most are fairly inexpensive.
Mark
|
Re: Isolation transformers are not a panacea [was: Variac talk]
I have a nice isolation-variac unit I built, but hardly ever use. With differential inputs and diff probes there really isn't much need for it. I suppose if you work on those old hot chassis TVs maybe.
Where I used to work they had battery operated LeCroy scopes which they used when they wanted to be floating, but that was more to break ground loops for reducing measurement noise than to have the ground of the scope at some high potential.
You must always take care when working with high voltages... it's not the place to be throwing leads around without thought.
There's nothing inherently wrong or dangerous about floating a measurement instrument... every portable DMM is that way... it is when you have common grounds, exposed metal chassis parts, high voltages or energy that things start requiring thought. Go into what you're doing with a plan!
Peter
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On 3/12/2013 10:57 AM, jerry massengale wrote:
Steve,
Thank you so much. AC connectors with a grounding pins are there for a very good reason. Any isolation should be an absolute last resort. Using an isolation transformer is dangerous and usually unnecessary.
Jerry Massengale
-----Original Message-----
From: Steve <ditter2@...>
To: TekScopes <TekScopes@...>
Sent: Tue, Mar 12, 2013 9:36 am
Subject: [TekScopes] Re: Isolation transformers are not a panacea [was: Variac talk]
Variac should have been spelled with a "c", not Varik. My mistake
Steve
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Checked by AVG - www.avg.com <>
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Re: Isolation transformers are not a panacea [was: Variac talk]
That must be why they are used to power electric shaver outlets in bathrooms ;-)
Just joking, we are in agreement, only that you went a bit over the top maybe.
Isolation transformers can be used safely, allowing one to make measurements almost impossible any other way.
It just requires people to know what they are doing, otherwise evolution will happen.
ST
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On Tue, Mar 12, 2013 at 3:57 PM, jerry massengale <j_massengale@...> wrote:
Steve,
Thank you so much. AC connectors with a grounding pins are there for a very good reason. Any isolation should be an absolute last resort. Using an isolation transformer is dangerous and usually unnecessary.
Jerry Massengale
|
David C. Partridge
David Hess