| Electronics > Repair |
| TDS3014 adventures |
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| james_s:
Does each channel have its own ADC? There is so little documentation available for these scopes that I actually know very little about the architecture. IIRC they use some sort of CCD technology for the capture memory but I don't know offhand where the separate channels converge. |
| giosif:
Well, that is my novice level of understanding & of describing the scope functionality. Whether those ADG361D ICs are really ADCs or something else, I actually don't know, but those are the ICs I was referring to. So, since each of the other IC types nearby are in a quantity of less than 4 (to match the number of channels), I suspect the issue might be with one of those, rather than the ADG361D on channel 4 or anything below that ("below" = towards the input terminal). But this is just me trying to make sense of the behavior I'm seeing, without a solid understanding of scope functionality or inner workings of this particular model. |
| giosif:
Today, I had a look at the thermal image inside the scope while it was on and I could see the "ADC" IC for channel 4 (aka U400) stays cold, while all others are getting hot. So, something is definitely wrong on that channel. I want to check if U400 at least gets power, but let's see if I can find the right pins (a datasheet would be good now). I will also ask on the forum for anyone with a spare ADG361D they would like to sell. |
| james_s:
Can you find any decoupling capacitors in the vicinity? That's usually a good place to start for finding the voltage to an IC. I'm more inclined to suspect that the chip is not getting enabled rather than not getting power though. Since you're getting data from another channel instead that implies that the same DAC is getting enabled instead of the one you want. It's been a while now since I've poked around inside one of these but look for something like a 74LS138. |
| giosif:
There are several caps on the other side of the board (see picture), and only one tantalum on the same side (and doesn't appear to be related to the power rails for the "ADC" ICs). In terms of ICs in that area of the board other than the ADGxxx ones and processor & memory, I can only see an Elantec 4581CS "sync separator" (quoting from the data sheet, as I don't really know what that means) and an MC33178 op-amp. As a side note, while investigating the board, I noticed resistor R555 (inside the red rectangle in the attached picture) doesn't have any markings and looks kind of suspicious to me. Measuring it in-circuit gives me a reading of approx 39 kOhm. It's a long shot, but anyone here happening to have a similar board at hand that could check the value of that resistor, please? |
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