Electronics > Repair
TDS3014 adventures
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james_s:
So I scored this dead TDS3014 for free, initial investigation was not promising, all the voltages looked good but there were no signs of life other than the backlight. Then upon further investigation I noticed that one of the two oscillator cans didn't seem to have any output. Could it really be that simple? I poked around and measured both and found identical conditions on the other pins but this one had no output. I thought perhaps something was shorting the output but I'd expect to see *something* on the output in that case but this was flatlined.

Ok so I start digging through my stash of scrap boards I found a similar looking 28MHz oscillator, nowhere near the 75.75MHz of the original but I thought it might at least invoke some small sign of life and tell me if I was on the right track. So I popped that in and imagine my surprise and elation when I hit the power switch and the scope burst to life with the splash screen immediately appearing on the screen. Woohoo!! Then it proceeded to boot up fully and display a trace, although not too surprisingly it feels very sluggish. Still, progress! Seeing formerly dead equipment spring back to life is the sort of thing that makes life worth living  :D

Sooo now I need a 75.75MHz oscillator, I have to assume that Tek had a good reason for using an odd value like that. I have not had much luck with the usual suppliers but maybe I've missed something. The original part is a Fox 401 series with a 5x7mm package, powered by 3.3V. I'm not opposed to adapting something in a different package if I need to. I'm open to buying a used part if anyone has one, maybe one of you has a scrap  TDS mainboard with bad ASICs or something?

I need some knobs too, oddly a bunch of them are crumbling to bits, those are readily available on ebay though. I'd also love to find the optional comm module but first things first.
texaspyro:
There are a lot of programmable oscillator chips out there these days.   Most distributors of these devices offer programming services.
DaJMasta:
If it'll run on a 28MHz oscillator, you may be able to get something in the same ballpark (70-80MHz) that will work fine.  It would be worth checking timekeeping functions and stuff, but if it's got an RTC, the sampling should be controlled by its own clock, so it may not actually be that critical to nail the frequency.  There's definitely the chance it's important, as you mention, but it's worth trying.

It's an odd number for a signal generator, but if you can generate the frequency (or maybe like a 25.25MHz squarewave and filter out the fundamental) you can probably test to see if it actually is the thing that needs the fix.

Another potential replacement option is a VCO or DDS generator chip capable of generating your frequency, but it's probably worth looking around for a crystal first, as it would be a bunch of extra stuff to do.


Is it possible that it's not actually a 75.75MHz oscillator and that's some other package marking?
james_s:
I actually was just coming here to say I ordered a programmable oscillator from digikey that theoretically ought to be a drop-in replacement. It was only about 8 bucks so worth a shot at least.

It boots with the 28MHz oscillator but it doesn't really work properly. It will display the compensator waveform and the menu works for the most part but a lot of things feel weird. I'm going to wait until the new part comes before I bother to mess with it further.

In the meantime I've been doing a deep cleaning. The plastic on this thing is crazy brittle and I've had to glue a few clips and tabs. Almost every one of the knobs has cracked into pieces too, maybe the high temperature from being in Hawaii? I bought a set of new knobs that will fix that right up. The screen also has a bit of weirdness that turns out to be some delamination of the rear polarizer film. Since there is also one stuck pixel I may splurge and spend the 80 bucks or so on a replacement but that can also wait until it's fully working.

Is there a way to hack it into the 500MHz model without the comm module? Looks like people want a few hundred bucks for that option module and I have no real use for it beyond unlocking features.
james_s:
So I was messing around at my bench today and the oscillator from the scope was sitting there still so I decided to solder some wires to it and play around. In the process I cleaned the rust off and noticed that it actually says 75.757 on it which I'm fairly sure is the frequency as there is another identical part in a different area that says 48.000 which is a frequency I've seen elsewhere.

Anyway I hooked it up to power and yep, still stone dead. I tried turning the voltage up as high as about 6V with no signs of life, then I turned the voltage down and was surprised to note that when I got down to around 2.2V it woke up, with an output of about 25MHz. I could turn the voltage up and the waveform would start to get ugly and then abruptly around 2.8V it would suddenly go dead, turn the voltage down and it would wake up again. I experimented, heating it up to the point where the wires desoldered from it and up until then the characteristics changed very little except it would run at a slightly higher voltage. I then used freeze spray to make it very cold and again surprisingly little difference.

So I'm really curious at this point what is going on with this thing. Is it likely that it is itself a programmable oscillator with a 25MHz source clocking an internal PLL that has failed? It's academic at this point since it's obviously defective but this is not a failure mode I've ever encountered or expected.
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