Damaged oscilloscope channel question.  - Page 1
Forum section: electronics / repair

author: Zondar
author_rank: Regular Contributor
timestamp: September 15, 2025, 09:52:28 pm
content: Hello,A channel of my lovely new oscilloscope was damaged by a high voltage electrical shock.The channel works fine in all ways (AC, DC, 50Ohm, Hi-Z, any gain, etc.) except for what you see: A dramatic but slow distortion visible on slower sweep speeds, e.g. in the 1 kHz square wave shown below.No doubt some component in the front-end is fried.Does anyone recognize, or can speculate about, what might cause this symptom?I'm asking before I open it up, since the other option is to send it in for service. I'm skilled enough to repair it if I can identify the damaged part.Thank you.

author: indeterminate
author_rank: Regular Contributor
timestamp: September 15, 2025, 11:04:01 pm
content: With no schematic , photo's or model number its pretty much impossible to make a guessIt wont be to difficult feeding the same signal into two channels and doing a comparison between channels as you walk through the front end.

author: Zondar
author_rank: Regular Contributor
timestamp: September 16, 2025, 12:01:03 am
content: No photo because I haven't opened it yet, no schematic because none are available, and I doubt the model matters. And because I haven't opened it yet, I'm not even sure I can run it with it disasembled enough to probe the front-end.Curiously, the sticker on the scope says that thecalibrationis void if opened, not the warrantee (not that it would likely apply in this case anyway).Oh well, it was a long-shot. I guess I'll open it and see what I can see. I have a feeling there is likely a crater where a part was (zapped with 1,200 volts!).For fun, I'll post a photo if the damage is obvious.

author: ondraN
author_rank: Regular Contributor
timestamp: September 16, 2025, 08:46:16 am
content: It looks like the compensation RC circuits are fried. But since we don't even know the type of oscilloscope, it's really just crystal ball divination.

author: jonpaul
author_rank: Super Contributor
timestamp: September 16, 2025, 09:31:02 am
content: Scope MFG? Model #? Exactly how damaged, most scopes have input protection to 400V.j

author: J-R
author_rank: Super Contributor
timestamp: September 16, 2025, 09:40:40 am
content: What is the reason for hiding the manufacturer and model?From a prior post it is probably a Siglent SDS3034X-HD.

author: 2N3055
author_rank: Super Contributor
timestamp: September 16, 2025, 09:53:29 am
content: Quote from: Zondar on September 16, 2025, 12:01:03 amUh. Shame. It's a new scope. SDS3000xHD if I remember correctly?There might not be crater if energy was limited. In addition to not providing any details on the scope, you did not give any details about what exactly was voltage source that did the damage. If energy was limited maybe only some components in front end are damaged.Also, waveform you show has no meaning because:- we don't know what was applied to the input. Exact waveform and amplitude.- there is no voltage or time scale so we cannot figure out magnitude of signal deformation.- scope has several stages in input circuits. There are relays that switch attenuators. There is VGA that changes amplification. To really figure out what is going on would be trying with a set of signals that would go through through all relevant input settings combinations to figure out at what stage damage is.- Since this is 1 GHz scope design (yes your front end is (was) full 1GHz calibrated), repair need to restore all parameters to factory ones to be good repair. Including parasitics, so choice of components might take some care.- There is a official Service manual that explains in detail how scope can be properly disassembled to reach certain subassembly.- Unfortunately, as you correctly stated, when open scope cannot be run, because there is no fan or metal shield that doubles as air duct.- What you could do is to very carefully disassemble it, carefully pop the cans on the shields on input channels and get a peek at how it looks. Make note that this will mean destructive removal of front label over the channel inputs.If you wish to give it a go, you might. Your scope.If you want the scope to full spec, get a repair quote. If you can handle it, you'll get scope to as new state.If this were my primary scope (and by the way, it is really nice machine) I would send it for repair and kick myself for making stupid mistake.And think about why it is that we keep saying to people all the time not to play with high voltage without high voltage probes and high voltage diff probes..

author: Zondar
author_rank: Regular Contributor
timestamp: September 16, 2025, 03:05:39 pm
content: I was experimenting with a photomultiplier tube. The output should not have exceeded a hundred mV or so max. However, apparently an arc occurred that momentarily dumped charge to the output.Yes, of course I kick myself. As noted, it is a 3000x-HD, if that mattered (and yes, it's nice). I even have a cheap older scope that I could have been using if my brain was engaged at all. But the shiny new scope was on my bench, I wasn't thinking, and of course I wanted to use my new toy.As I said, everything works perfectly well except for that low frequency disturbance (that's the scope's 1kHz calibration output in the photo). I didn't even notice a problem for a whole month, since at a time-base beyond a hundred kHz or, the result is a seemingly perfect match to any other channel. So I'd guess that the digitization is good, the programmable amplifier is good, the relays are good. As a post suggested, something early in the compensation area is likely damaged, whether visible or not.Yes, I should get a quote. I did contact them about sending it in, but I didn't ask for an estimate. (Oddly, they never sent the RMA number they promised.) I don't know if they have a reasonable flat "per incident" repair charge, but the likely outcome is a swap of the whole digitization board, which is most of the value of the scope, so it could be thousands.A service manual! Where can I get this? I don't suppose it has schematics, does it?Anyway, why would I try to repair this myself? Because I'm stubbornly independent beyond the point of rationality. Well, there's another reason, too, but the first rule is to not talk about that!

author: 2N3055
author_rank: Super Contributor
timestamp: September 16, 2025, 04:05:53 pm
content: Quote from: Zondar on September 16, 2025, 03:05:39 pmFirstly, I was not scorning you. Not my place and I am sure you are mad by yourself quite enough.Secondly, I do not presume your level of competence. I simply created an summary.There is service manual on Siglent EU. I presume there is one on Siglent NA as well. If not get one from EU. Unfortunately there are no schematics and there are none outside the factory.But Service manual will let you properly disassemble it at least, because it is not that simple. As I said, front decal over channel inputs must be removed for full MB removal and that sticker cannot be reinstalled. It will deform on removal.I would still ask for a quote. Siglent is quite more reasonable on pricing than Big guys. It won't be 2 burgers either. But much better than famous manufacturers..If you plan to disassemble it anyways, price or just stubbornness for reason, take special care of flat cables especially ones that go to screen.Inputs are in individual RF shield boxes. Front end is dual path so there is DC servo opamp that takes care of low frequencies and DC offset, and high frequency path...If 1MHz (or up) squarewave looks fine, maybe only low frequency path is damaged.Check DC resistance of input on 1 MΩ and 50Ω setting.There will also be some protection networks on input and attenuators.So if you have some frequency where displayed waveform looks the same as on non damaged CH, try that same waveform at different sensitivity levels. If all ranges switch and look Ok then you might have been lucky and damage is localized.What is really problematic is fact that you cannot power on scope when open. Not only thermally but you literally have to disassemble it fully to get where you need to be.Whatever you decide, good luck!Best,Sinisa

author: tooki
author_rank: Super Contributor
timestamp: September 16, 2025, 06:35:43 pm
content: Quote from: Zondar on September 16, 2025, 12:01:03 amWhy would you think this?

author: Zondar
author_rank: Regular Contributor
timestamp: September 16, 2025, 09:02:05 pm
content: Quote from: 2N3055 on September 16, 2025, 04:05:53 pmThe input resistance is 1 MOhm and 50 Ohms, the same as other channels. All ranges switch normally.You say there are high and low frequency paths. Is the transition between them gradual or a step change? It appears to me that the problem exists regardless, but I can only see it, e.g. in a square wave, at low frequencies. By 100 kHz or so, it becomes difficult or impossible to see because the time-constant of the problem is roughly 1 kHz, and the channel starts to look and behave completely normally.I've watched the tear-downs of the 1000HD and 2000HD series. There were no stickers that I could see on the cans. I looked at the circuitry in the close-ups and the protection circuitry is probably the diode or diodes marked T4 (i.e. "DC2" and "DC3" in the 1000HD, and "DA3" in the 2000HD). Those would be prime suspects to look at. But another worry is that I may not be able to identify a correct replacement part, e.g. just "T4" isn't enough, none of the capacitors show values, etc.Perhaps acceptance is called for? Like now I have a 3.5 channel scope? Sigh.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2025, 09:10:15 pm
content: The cross over in the frequency is naturally somewhat smooth, with some 1st or maybe 2nd order filter characteristic. 1 kHz cross over sounds very reasonable. So there is indeed the chance that the low frequency DC part is broken, especially an OP-amp, that may be relatively simple to replace and not even an unusual part.With a bad OP-amp from the low frequency part one would expect things like higher than normal DC drift, maybe a somewhat asymmetric range for the y position adjustment.Accepting a damaged channel may be a real option. It is not pretty, but still a usable scope.

author: tooki
author_rank: Super Contributor
timestamp: September 20, 2025, 12:04:57 pm
content: Quote from: Zondar on September 16, 2025, 09:02:05 pmFor caps you can still measure the ones from a working channel.

author: tautech
author_rank: Super Contributor
timestamp: September 21, 2025, 01:23:57 am
content: Quote from: Zondar on September 16, 2025, 09:02:05 pmMeans little without screenshots.With luck I can get a partial schematic and/or some component values but I won't even go to the trouble without quality screenshots that show the issue very clearly.