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Where's a good calibration lab when you need one?

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Jimelectr
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re: Where's a good calibration lab when you need one?
Jimelectr   9/2/2010 5:16:49 AM
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Thanks for commenting, everyone! Yes, the problem could have been a flaky trimmer cap. It's been close to 20 years ago now, so I don't recall all that much. Anyway, I sold the scope to a fellow EE in 2002. Yes, there are lots of cheap scopes on ebay these days. The last one I bought was a Tek 7904 with 7A26 and 7B85 plugins for $100 in perfect working condition! This was after selling my Iwatsu 250 MHz scope with a broken trigger mode switch (stuck in single-shot mode, the least often used mode!) for $150. I've seen the Tek 7000 series plugins for as low as $10, and a cherried-out 7A26 will only set you back about $70. Add to that all the USB RF/microwave test equipment out there for less than $1000, and it's a great time to be a garage entrepreneur. Assuming you have the time, unlike me.

Lawrence.Yeatts
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re: Where's a good calibration lab when you need one?
Lawrence.Yeatts   9/1/2010 2:54:28 PM
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Granted that a good calibration procedure would include cleaning all switches and verifying operation in all modes. However it seems the author was fooled by a symptom disappearing as he made a adjustment on his oscilloscope. It could happen to anyone not familiar with the design of the input circuits, but I just wanted to point this out. I am intimately familiar with the input circuits on those oscilloscopes and the HF frequency compensation trimmers for the stepped attenuator would affect high frequency response only. The AC coupling on the input sets a single-pole high pass filter at a subsonic frequency, apparantly this roll-off was occurring at a higher frequency than it should. Based on my experience the issue was more likely a dirty contact on the input coupling mode switch or a bad connection somewhere.

Kinnar
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re: Where's a good calibration lab when you need one?
Kinnar   8/3/2010 11:51:22 AM
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This is a great experience, but the aging effect was more pronounce on Analog Devices, but due to digital electronics the thing are getting changed and it is a tough time of survival for calibration labs.

Haldor
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re: Where's a good calibration lab when you need one?
Haldor   8/2/2010 4:44:07 PM
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ttt3 wrote: "I've always wondered exactly what goes on at these calibration labs." No kidding I was having a problem with my Tek TDS420A scope. The probe sensor circuitry that automatically detected what kind of scope is connected was flaky. I dropped the scope off with the calibration people with a note explaining the problem. They gave it back with a clean bill of health. It immediately failed when I tried it. When I challenged the lab people (they hadn't left yet) they admitted that they didn't use the scope probes when they tested the scope. They just plugged a BNC cable between the scope and the signal generator. That also meant that all of their testing was being done using the 50 ohm input impedence mode on the scope. What a rediculous idea, the probes can have a huge impact on the accuracy of the scope and virtually 100% of the time I am using a 10:1 probe with my scope. I got them to agree to at least function test the probes on my scope during annual calibration. Turned out I needed to replace the BCN connector PCB on my TDS420A.

WKetel
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re: Where's a good calibration lab when you need one?
WKetel   7/30/2010 9:55:35 PM
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What we learn here is that it is not always a requirement to have the whole manual, provided that a copy of the circuit is available and you understand what is supposed to be happening, After that, when you find out what the difference is between what is happening and what is supposed to be happening, often the exact failure becomes obvious. At least, that is the second best approach to fault diagnosis. The first best is to consider what usually fails on an item and check that part, but it only is a useful approach when the failure symptom matches the previous failure symptom. As an example, there is one model of Sony cassette recorder that would develop a hum on playback. No amount of part checking in the power supply area could turn up an out-of-spec part. It seems that for that particular model, the cure for the hum is replacementof the volume control potentiometer. That was it every time, for all of that model that came in for repair, for over a year, about 26 different machines.

Mongo647
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re: Where's a good calibration lab when you need one?
Mongo647   7/30/2010 6:43:44 PM
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My experience with the manuals is that the older and LESS integrated the circuitry, the more adjustments were necessary/possible, and the more likely the adjustment procedure was described. Yes, in the pre-microprocessor instruments from Tek and HP particularly, there is a checklist set up to copy and perform step by step. And a cal lab needs to do more than cal, it needs to retroactively inform if an instrument is found to be out of cal, so that notification is made that measurements made with that instrument may be suspect. Not just in-cal for the future, but affirming measurements made in the past.

David Ashton
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re: Where's a good calibration lab when you need one?
David Ashton   7/30/2010 9:16:21 AM
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It's always good when you can get a manual. Some time ago someone gave me a 30-year old Fluke bench DMM. Fluke still had the service manual on their web site. I also managed to get (not frum Fluke, although they were very helpful) an LCD display for it. Result, a seriously useful DMM for amost nothing. I'm a big fan of Fluke and others who do their best to support their older products.

thebatosai
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re: Where's a good calibration lab when you need one?
thebatosai   7/28/2010 9:28:36 PM
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I love your story. With knowledge and experience comes the confidence to solve problems.

jnissen
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re: Where's a good calibration lab when you need one?
jnissen   7/28/2010 6:16:04 PM
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Who's to say it was the lab? I've had intermittent issues like this crop up from time to time. Just the fact the unit was moved and the vibration induced in the process can often wreck havoc. I'm not defending a lab if it was poor work on their part but to paint them as the source is a bit of a stretch. Jim's own comment "I just barely touched it, and like magic, the frequency response of the scope dropped down to where it should be!" I'd of suspected a connector, oxide in the trimmer, etc... and the mere fact you touched it was enough to overcome the issue.

ttt3
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re: Where's a good calibration lab when you need one?
ttt3   7/28/2010 5:38:15 PM
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I've always wondered exactly what goes on at these calibration labs. I think most of the time they just run a few tests, the instrument passes, they slap a new sticker on and send the equipment back out the door. Exactly how those tests are written (and who deems them acceptable) has always been a bit of a mystery to me. Does Tek have a "calibration manual" for their scopes, or does each lab come up with their own test procedure?

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