Input Termination Overheat??!! Tek TDS3014

[TechCB]

Hi!

Does anyone know how to solve the system error "Input Termination Overheat" (see image) in a Tek TDS3014? 

This error appears as soon as I turn ON the oscilloscope even without probes connected.

Thanks

[james_s]

Looks like somebody damaged the 50R terminator by exceeding 5VRMS while in 50 ohm mode. It's inside the attenuator block.

[TechCB]

Looks like somebody damaged the 50R terminator by exceeding 5VRMS while in 50 ohm mode. It's inside the attenuator block.

Thanks for the info, I'll look in to it to see if can replace the 50R terminator.

[TechCB]

Looks like somebody damaged the 50R terminator by exceeding 5VRMS while in 50 ohm mode. It's inside the attenuator block.

I checked the attenuator block and I couldn't find the 50ohm termination resistor. I notice that a relay activates when I select High impedance or 50ohm, so I should be able to trace the termination resistors to replace them, but each channel has 3 relays and the traces aren't visible... So if you have any suggestions, I'll appreciate it.

[james_s]

I've never been inside the attenuator on a TDS3000 so I can't say what it uses but I replaced all the relays in my TDS784C and in that scope the resistors are printed on ceramic hybrids.

[shakalnokturn]

Disclaimer: I have no experience with this model.

I would start by measuring resistance at BNC when 50ohm coupling/termination is active.
The most likely is that someone has abused it at one point.
If you don't measure 50ohm at the BNC well... You have a problem to solve.

If you measure 50ohm, good news you have a different problem to solve.
RTFM to find out if the scope stores a warning message on this kind of abuse that needs to be cleared manually.
Another option is that the power measurement (thermistor on hybrid?) or related amplification/switching has failed.

[Smoky]

I read an article a couple of years ago about the replacement of a Thick-Film 50 Ohm SMD resistor in a Tektronix TDS model having the same issue.

It was located on the back of the ceramic attenuator hybrid and in line with the main trace running out from the input jack.

It is hidden under a layer of conformal coating too.

You first have to remove the ceramic hybrid from the attenuator PC board to gain access.

Maybe it is similar to your model?

Ok, I need to make a comment about this. I'm a novice, so what are the odds that I would make a mistake and apply too much voltage to an input? I'd say, pretty damn good. Why would Tektronix build a board this way and knowing it would require a major effort to repair that resistor? Heck, I think I count at least 50 pins on that hybrid that would need to be desoldered! But first, you'd need to know where to find the resistor! Jeez 

I recall that the article was written by a professor by the name of Mathew at MIT. He was fixing his classroom's scopes.

[Bud]

It is not Tektronix's problem when people exceed device specification. 

[james_s]

50 ohm inputs are somewhat specialized, they were only on high end scopes, the sort of instrument that cost as much as a nice new car, it's expected that the person using one knows what they're doing.

Adding additional protection or using an easily replaced part would compromise performance, despite what the person who repaired that one says, I guarantee the scope does not perform as well as it did with the original attenuator, that resistor is almost certainly going to be more inductive. It's probably still fine for hobby use and it's certainly better than having a burned out terminator but it's not as good as new.

It says right next to the input <5VRMS when 50 Ohm is enabled, if you exceed the maximum rating you will do damage, oscilloscopes are not protected as well as multimeters, it's the cost of being able to take accurate measurements of signals that are hundreds of MHz and/or very fast rise and fall times. Read the manual, obey the maximums or you'll end up with very expensive damage.  It's even easier to fry a $50k spectrum analyzer, they have even less input protection than a scope because they have to be very sensitive and work up into microwave frequencies. Lots of other gear out there is easily destroyed if one does not know how to properly use it, it was never intended to be used by untrained hobbyists.

[Smoky]

Thanks for the explanation James. I didn't realize that resistor was such a critical part.

So it got me thinking I have never chose "50 Ohm" inputs on my TDS420 and I imagine it is used when the input is something other than a probe. So I own one 36" coax cable terminated with BNC connectors. The cable is stamped Belden 8259 RG58 A/U. I want to test the four channels of the scope individually to see if the attenuator resistors are still good.

I set the Siglent 1032X to 50 Ohm output and to 7.562 kHz at 255.1 mVrms for the following tests:



In the first test, I set the scope to "50 Ohm" inputs and found that all four channels hit the frequency but the amplitude is slightly low:

     

For the second test, I switched the oscilloscope to "1 mOhm" inputs and attached a 50 Ohm terminator to the scope's inputs and all four channels hit the frequency but the amplitude turned out to be more accurate:

     

I imagine when using the 50 Ohm input setting that the type of cable, and its length, needs to be matched to the scope?



*Update* Check this out. So I sent the signal from the Siglent 1032X to channel 2 of an HP 35660A. The DSA seems to back-up what the Tektronix scope read for amplitude. The HP measured 254.38 mVrms and the Tek measured and rounded to 254.4 mVrms!



 This sort of tells me that the TDS420 50 Ohm attenuator input resistors are fine and, maybe, that the cable and/or the terminator needs slight adjustment.

*Update* I checked the resistance of the BNC output jacks of the Siglent SDG-1032X with two different multimeters today while the Siglent was set to "50 Ohm" output.

Both multimeters read 50.4 Ohms for channel one and 50.5 Ohms for channel two of the Siglent.

As for the 50 Ohm terminator, it is made by Amp and it measures spot-on at 50 Ohms.

[james_s]

Yes that's exactly what it's for, connecting directly to a 50 ohm terminated source through a 50 ohm coax. The length doesn't matter, only the characteristic impedance of the cable and the impedance of the source.

If yours is off it's possible that the terminator was damaged by someone overheating it or it could be dirty contacts in the relay that enables it. You might try measuring across the input with a multimeter to see if it's actually 50 ohms.


Those relays in the attenuators are problematic in the TDS500/600/700 scopes, I replaced all 20 in mine because SPC was failing. I don't know if the TDS400 series has the same issue but they look harder to replace.

[Mechatrommer]

i recently got about the same message on my TLA7E1 module installedf in TLA714 LA (turning off 50 ohm path due to overheat). the unit has been idled for about a year. last time when i installed and checked the module, there is no such error and i havent subject it to anything near lethal voltage... but i'll open up and analyze it later as i have many tasks in queue atm..