EEVblog #564 - Tektronix TDS3054 Oscilloscope Teardown Repair

[Maxlor]

One other thing just came to mind: it might be interesting to see a sinus signal in dots mode. I'm thinking that if the erroneous dots are still all in the same region at the top, it might be a problem in the analog section, i.e. something causes the signal to run into the maximum. If it mirrors the actual signal, a bit error in the digital signal seems more likely.

[calexanian]

Well, if google has anything to say:

http://www.seekchip.com/icstock-m/MM9576-VJG.html

Implies those aren't ADC chips but some kind of fancy fifo/ram controller.

My 'guess' is that the maxtek chips would have the adc (or equivalent) - I'd assume one would keep all the analog stuff on the hybrid module versus trying to feed it down to the 'digital' board.

The MM9577 again, if you believe google:

http://www.seekchip.com/icstock-%EF%BC%AD/MM9577-ACE.html

is an ADC (not very fast).

Good luck

cheers,
george.

Fi/Fo chips could be just streaming data straight to ram and the main chip figures out what to do with the data later. It lends credence to the problem actually lying in ram. I wonder. Does each channel have a separate ram chip? Could just be a faulty Dram.

[EEVblog]

Fi/Fo chips could be just streaming data straight to ram and the main chip figures out what to do with the data later. It lends credence to the problem actually lying in ram. I wonder. Does each channel have a separate ram chip? Could just be a faulty Dram.

There are 3 58LC128K18B4 memory chips. I can't find any info on them.

[georges80]

There are 3 58LC128K18B4 memory chips. I can't find any info on them.

Your search mojo must be defective

This datasheet should be close enough for a 1st stab:

http://www.datasheetarchive.com/dl/Scans-061/DSA2IH0096628.pdf

cheers,
george.

[EEVblog]

Yep, that's the one.
128K x 18bit which was obvious form the part number. Third chip is likely intensity data?
It matches the 9bit ADC claim, needing two memory chips for the 4 channels.
Obviously not fast enough for the 5GS/s, so there must be a pipeline in the ASIC.

[calexanian]

And we have no forum members at Tek who can dust off somebody to give their thoughts?

[djsb]

Why not try asking on this yahoo group

http://groups.yahoo.com/neo/groups/TekScopes/info

There are a lot of knowledgeable people in this group who could possibly help.

David.

[peter.mitchell]

Just shot a Part 2 video on this.
No surprises for guessing the outcome...

I'm guessing you mean BER?
I hope that isnt the case.

[TiN]

If you have access to high-bw scope with differential probe, maybe worth check that faulty channel diff-pair output and compare it to working channels.
Set same settings on both channels, feed same signal and outputs from front end should be very close

In that case you can save time fixing what is not broken (front end voodoo board), if signals to ADC chip do look properly. From scope display it seems like
it's digital logic issue (with memory or acquisition), not analog stuff.

Now, I could have not uploaded the video, and kept it until I had finished the repair or done some more stuff, and that's how some bloggers do repair videos.

Don't listen hates, just upload everything you have   If one have better idea to do repair videos - well, making own blog today is easy as sending two bits to web

[Rubi]

The term we use for that is shotgun debugging.

LOL!!!
As an prof. software engineer I love that term.

Sometimes you are so desperate because nothing works as expected that you fall back in this kind of amateur behaviour and guess what unfortunately it never helps finding the problem,...

[mikeselectricstuff]

It might be instructive to look at a ramp waveform, as if the issue is something like a dodgy bit, you may see some regular patterns at certain timebase settings from the monotonic progression of input value - probably most useful in single-shot mode at a high speed to avoid the effects of any filtering, antialiasing etc.

Also, have you thoroughly checked the other channels at a range of timebase an memory settings - dodgy memory may manifest itself on other channels depending on how the memory is allocated in various modes.

[jgorsk]

Hi Dave,

It may be a good idea to look for help at this Tek forum

http://www1.tek.com/forum/viewforum.php?f=5

If I remember correctly, simillar problems with the TDS3000 series were sometimes
solved by running some kind of calibration.

What's in the error log of that scope?

[AndyC_772]

+1 for the idea that it's a digital problem, not an analogue one.

When you viewed the waveform in dots mode, the extra random dots you see are clustered together and are well separated from the correct signal. There's a substantial distance in the y-axis between each (faulty) sample point and its neighbours.

If it were an analogue problem, I'd expect to see clipping, or oscillation, or some kind of non-linear distortion - but the resulting signal would be continuous in time and would have constrained bandwidth. What you're seeing on the faulty channel is a signal which appears discontinuous in time and which appears different depending on the ADC sample rate and memory depth in use. That strongly suggests it's a digital problem.

I'd be looking for a memory fault. Maybe try replacing the memory ICs. You can buy some new ones here:

http://www.questcomp.com/questdetails.aspx?pn=MT58LC128K18B4LG-9&utm_source=Findchips&utm_medium=cpc&utm_term=MT58LC128K18B4LG-9&utm_content=PartDetails&utm_campaign=FindchipsWS

Another option in the meantime might be to swap over the ADG361B chips between a working channel and the faulty one. If the fault moves with the chip, at least you know what it is even if a replacement IC isn't readily available.

[Anks]

I would check the diff pair and if that looked good using another channel for reference I would swap chips on the main board.

[EEVblog]

Also, have you thoroughly checked the other channels at a range of timebase an memory settings - dodgy memory may manifest itself on other channels depending on how the memory is allocated in various modes.

The other channels seem to work just fine.
I do need to do more thorough testing and calibration etc though.

[Rubi]

You could remove the ferrites from the differential output on the hybrid and bridge the output from another channel.
So you could see for sure if the hybrid is responsible for the problem.

[tszaboo]

My guess would be that you have one bit flipped on the digital side. This can be at the parallel communication somewhere, and the cause for this could be ground bouncing. I would check the decoupling capacitors if any of those are cracked, or if you have significant noise on the power supply right at the ASIC pins.

[Rufus]

It looks like a digital problem to me. Something like the MSb of the conversion being intermittently hot.

I think more detective work on what was actually being displayed would have been worthwhile. If it is digital and the hybrid is all analog there was no point looking at it.

The disappearing trace at low sweep rates might also give some clues. Is it really disappearing or is the glitch detector always capturing a 'hot' bit at lower rates and you end up with an offset signal above the trigger level? That would indicate the problem is before the trace memory.

If you run with the scope open there are lots of options for comparing signals and heating/cooling/poking things to see if you can make it more or less intermittent.

[PA0PBZ]

The wires heat-shrinked with the AC are most likely mainly for the line trigger.

[jerryuk]

I had exactly the same fault with a TDS3032B (Noise superimposed on the signal)
Initially I contacted Tektronix but they wanted over £1000 just to take a look so I decided to pull it apart and see what I could find.
Same pcb as the 54 but only 2 hybrid modules fitted of course.
Carried out the same tests but could find nothing so reassembled the scope.
I then found that by selecting bandwidth limit of 20Mhz or 150Mhz the fault did not show.
Switching back to Full BW and the fault was still there.
However I switched the bandwidth to 20 MHz and cycled the power and then reselected Full BW
Unbelievably the fault had gone
That was over a year ago and the fault has not come back (I did try some percussive maintenance to see if it was a poor joint etc but could not get the fault to come back).
May be worth a try
NOTE: I cannot remember the exact sequence I carried out but it was something like I describe above but it was definately the Bandwidth setting that I was changing.

Jerry

[vikpc]

Maybe it just relays?

[ResR]

Have you checked the passive components? Maybe some resistor is burned out or capacitor cracked and doesn't filter the noise any longer on that channel?? Sometimes a transistor that is starting to fail gives out noise with the signal also.

[Alexei.Polkhanov]

My guess would be that you have one bit flipped on the digital side. This can be at the parallel communication somewhere, and the cause for this could be ground bouncing. I would check the decoupling capacitors if any of those are cracked, or if you have significant noise on the power supply right at the ASIC pins.

I would +1 to that. Signal on screen does show signs of what you have described.

[atw60444]

Did Dave say he'd uploaded #565, part two? No sign of it here yet

[Jonny]

Did Dave say he'd uploaded #565, part two? No sign of it here yet

There, There.... he only said he shot it. I'm sure we'll see it soon enough