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TDS540 CH3 dead

Hello all,

I have a TDS540A that was failing the acquisition board on it's power-on self test. This unit had the problem where the small electrolytics were leaking all over the board. After re-capping and cleaning the entire unit, it now passes the self test, but CH3 is completely dead. I probably should have done a preliminary functional check first, ah well... Anyway, probing the input on the (unknown) PLCC device U1200 shows that the input of the chip is permanently biased at about -1VDC no matter what the input to CH3 is. Swapping the coax output from the attenuator board from another channel yields the same result, so I'm pretty sure that U1200 is the culprit. Unfortunately, I cannot determine what this part is - it has a heatsink epoxied to the top of it, and I'm afraid of doing more damage by trying to break the thing off. I know that Tek never released schematics for this unit, but am hoping someone might know what the chip is. Thanks for any help!

Reply to
JW
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It's extremely difficult for people to help you if you don't include a valid email address.

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Reply to
mike

Hello,

Why? All I'm looking for is what the part # of a chip is. Follow-ups to Usenet posts should be on Usenet, not email. I very rarely use a valid email address, as I hate spam.

Reply to
JW

The chip part number is 155-0378-xx. My bad. I thought you actually wanted help fixing the scope.

As all you need is the part number, I'll go back to sleep. Sorry for the mistake. I'll endeavor to abide by the usenet posting rules. mike

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Reply to
mike

mike wrote in news:BZTzj.11258$C1.7871@trnddc07:

FYI,a 155-xxxx-xx is a TEK-made IC,out of production and no longer stocked by TEK.

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Jim Yanik
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Reply to
Jim Yanik

Well, yeah, but I just wanted to know if it was Tek custom or an off the shelf part. Given the number above, I guess that answers my question. :) I'm 99% sure that that's the defective chip. Anyway, I'll have to look for a donor scope to repair this one. Would you happen to know if the TDS520A uses the same chip?

I hope I didn't come off as being rude, that certainly wasn't my intent. I appreciate the help, Mike. Thanks!

Reply to
JW

JW wrote in news: snipped-for-privacy@4ax.com:

I believe so,since they essentially are the same scope. SW is a large part of the difference in them. you might even be able to use the whole board in the TDS540.

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Jim Yanik
jyanik
at
kua.net
Reply to
Jim Yanik

yes they're the same chip.

You come off as stubborn!!

If I understand what you posted here, that can't happen. Stated another way, my interpretation of you experiments is that the results are inconsistent. Stated a third way, there are some other things you should try before you go replacing the chip.

You've got an old board that's been compromised by corrosion. Even if you have state-of-the-art SMT desoldering equipment, you're gonna be fighting the corroded contacts and stressing a weakened board. And I bet you 23-cents that changing the chip won't fix it. And I predict you'll end up with additional problems.

And if you decide to replace the board, where do you think you're gonna find a board that doesn't have the same problem? You aren't, outside a working instrument...and even those will have the problem soon. Nobody's gonna part out a working time bomb at a price you want to pay.

My favorite repair axim, "If it ain't broke, don't fix it," has a corollary, "If it's not broke too bad, don't fix it." A three-channel TDS540 is a powerful instrument. A TDS540 with an acquisition board you busted trying to fix, isn't.

All that aside, I've offered, on multiple occasions, to take this off- group and help you fix it. You're too smart for your own good... and STUBBORN!!! And I support your right to shoot yourself in the foot as often as you see fit.

I'm gonna TRY to make this my last post on this topic... but I'm weak and easily agitated.

mike

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Reply to
mike

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