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TEK Scope Fixes Itself

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tekochip
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Platinum
Reverse Polarity?
tekochip   5/27/2013 9:18:41 AM
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The only other time I've seen LEDs short is from reverse bias.  It doesn't take much current to take them down, either.

a.saji
User Rank
Silver
Re: Reverse Polarity?
a.saji   5/27/2013 11:13:55 AM
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@tekochip: Is it ? I thought it consumes more power in such scenarios.                            

Nancy Golden
User Rank
Platinum
Brought Back Memories
Nancy Golden   5/27/2013 1:05:20 PM
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Interesting article and nice fix. I enjoyed this article specifically because it brought back great memories. As a student, I had my own home-lab set up in my bedroom (my room- mate at the time had two girls so I needed to have it out of reach of curious fingers) and I had the same setup. I was fortunate enough to buy one of the Tektronix scopes that had been used in the labs at school when they remodeled one of the labs and did some upgrades that made a few become available at a price a poor student could afford. Being a faculty assistant at the time allowed me to call dibs on one of them. It was only 20MHz but I was so proud of it and it was my workhorse. When I entered industry the o'scopes that were available (digital storage scopes - wow!) blew me away. My function generator was bought at the sidewalk sale in Dallas - it was a prebuilt trainer that was easy to work on - just pull the top off when something quit working - pretty easy to troubleshoot. It also had AC and DC  power supplies. Love those home labs - they are so much fun!

Nancy Golden
User Rank
Platinum
Re: Reverse Polarity?
Nancy Golden   5/27/2013 1:23:49 PM
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Me too, tekochip - if the LED is forward biased, I haven't seen one go out as long as the LED is rated for the current level they are running at. Current limiting resistors usually prevent such issues. A friend of mine said he has seen LEDs put right across the secondary of a 16 volt transformer with a series resistor but no diode to block the reverse voltage - the company told the manufacturer that we don't recommend that and they said they have been doing it for years with no problems. Hard to believe!

tekochip
User Rank
Platinum
Re: Reverse Polarity?
tekochip   5/27/2013 7:35:15 PM
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Every diode is different, of course, but the reverse breakdown voltage is about 5V, and the diode can be damaged by currents in the microamp range.  So, if you have a 470 Ohm current limiting resistor in series with the LED and then you reverse the the current, the diode will breakdown and be damaged by the 10mA current.

I shorted one this way, and it did rather surprise me.

OLD_CURMUDGEON
User Rank
Platinum
TEK Scopes
OLD_CURMUDGEON   5/28/2013 12:07:38 PM
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The older TEK scopes were workhorses & for the most part could be repaired should some component fail.  However, in recent times, TEK has taken a more selfish road in my opinion.  The last scope we had in our area was a 2465B, 4 channel, 400 MHz unit.  It was a great scope, having the ability to save waveforms in memory & do some other advanced functions.  However, in a recent event, we were analyzing the outputs of some bench power supplies for steady-state ripple.  On day one of this investigation, all went well.  The following day we powered up the 2465B, and it would NOT complete its POST.  In desparation we called a local office (national) repair facility.  They analyzed the scope, and claimed it was unfeasible to repair since the repair would be as high as purchasing a reconditioned one.  So, knowing about the TEKTRONIX museum on the west coast, we sent it to them for analysis & repair.  They traced it to a PROM chip, but could not repair it due to the propriety of the code.  TEKTRONIX no longer supports this model, so we are now scopeless.  The museum's terms are such that IF they cannot repair a scope, you can donate it to them as a parts source, and that's what we elected to do.

Steve L
User Rank
Iron
Re: TEK Scopes
Steve L   5/28/2013 7:53:54 PM
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I would not characterize Tek as selfish, it's just that the complexity inside the scopes is getting to the point where component level repair is almost impossible. BGA chips, fine pitch PQFP. It isn't like the good old 16 pin dip days. Even at Tek, for the most part, PCB replacement is the normal repair route.

As for 2465 ROMS, the Tek scopes Yahoo groop would have been a good resource.
Steve Lindberg

 

 

Steve L
User Rank
Iron
Re: Reverse Polarity?
Steve L   5/28/2013 7:56:23 PM
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I think this was a case of random component failure. The replacements have worked for almost 20 years now.

OLD_CURMUDGEON
User Rank
Platinum
Re: TEK Scopes
OLD_CURMUDGEON   5/29/2013 8:26:18 AM
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Well, I'm sorry to disagree with you, but I believe that they ARE selfish in that they discontinue product support too early for a piece of capital equipment such as this scope.  We're NOT talking about a consumer product which may have a limited life by its very design.  When I began my career in electronics 50 years ago, we had some of the most advanced test equipment of its day in our laboratory group, and ALL the equipment had Owner's manuals & Service manuals which were kept in the Engineering Library under strict control.

While I'm NOT denying that modern-day test equipment IS far more complex than the equipment I "cut my teeth" on, nevertheless, it seems to me that there SHOULD BE more direct factory resource for replacement parts over a longer period of time.  Furthermore, my comment about TEK was not of my own design.  I've read many blogs over the years, and TEK has gained a reputation for abandoning many of their former products.  Additionally, when I sent this scope to the repair facility for diagnosis, the manager of the office called me with the result.  It was his words on the phone that added more credence to the negative comments that I've read in other places.

Finally, while the site that you referenced may be a source for certain items, IF the fellows who run the TEK museum in Oregon, and who WERE long-time TEK employees were NOT able to repair this model, then I would think that I exhausted all the viable avenues.  I'm sure they're aware of every source for TEK replacement parts. 

Thank you.

p.s.  For your information, here's a copy of the e-mail that I received from the fellow who runs the TEK museum.....  I highlighted the one important sentence in his response to my intial e-mail.

Dear Customer,

 We repair customer's scopes for donations to the museum, most local customers, but those that wish to pay shipping in both directions for their "special" vintage instruments. We will go thru the instrument and give you a quotation for repairs (if it can be repaired). Some of the newer stuff have proprietary integrated circuits which are Tek made and no longer available.

 Ed Sinclair

 Edward B. Sinclair

Board Chairman

4620A SW Beaverton Hillsdale Hwy

Portland, OR 97221

503-644-0161 Museum

503-209-5894 Cell

eds@vintagetek.org

www.vintagetek.org

notarboca
User Rank
Gold
Re: Brought Back Memories
notarboca   5/31/2013 4:42:02 PM
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Ah, memories! The first scope I ever used on the test bench was a TEK 535/545 on a rolling cart.  Ran at the amazing speed of 18 Mhz!  It was also good as a small space heater, but solved many an analog problem for me!

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