Agilent DSO3102A - Page 1
Forum section: electronics / repair

author: mwbarth36
author_rank: Regular Contributor
timestamp: August 15, 2019, 02:03:47 am
content: Does anyone know if it is possible to obtain schematics for the Agilent DSO3102A scope. Both channels of a scope I have behave as if they are AC coupled with a very small capacitor. Sine waves appear, just not proper amplitude. As the input frequency increases, the display is closer and closer to the correct amplitude. A square wave as input appears as a spike with a decay back to zero, just as one would expect from a small capacitance in series with the input. I am sure I could at least locate the problem if I had access to schematics, may be more difficult to fix as this scope is highly integrated. The board is loaded with test points and would be very easy to trace signals if a schematic were available. Can anyone help?

author: DaJMasta
author_rank: Super Contributor
timestamp: August 15, 2019, 02:42:16 am
content: This happens even in DC coupling mode? Also I assume it won't display a DC signal at all?Sounds like an electrical break between the connector and the frontend. I'd take something with a bit of magnification and look on each, but you can probably buzz them out between the BNC center pin and the pad where it lands on the PCB - there probably isn't continuity (broken center pin in the connector), or applying pressure to the pin/pad could restore it when measuring the center pin and the pad/trace(cold/broken solder joint).

author: mwbarth36
author_rank: Regular Contributor
timestamp: August 15, 2019, 01:27:28 pm
content: Exactly, happens even in DC coupling mode, and no hope of displaying a DC signal. I fixed a LeCroy scope recently that had the problem you describe, a bad solder joint. However, this is both channels, for this to happen to both channels at the same time would be strange. Anyway, thanks for the suggestion, it sure sounds like a possibility. I do have the main board out now and will check.

author: Jwalling
author_rank: Unknown
timestamp: August 15, 2019, 03:18:02 pm
content: Maybe a few bad relays? Odd that it's on both channels. Measure the resistance at the BNCs with a DMM, it should be about 1M.Is the fault on all V/div settings?

author: DaJMasta
author_rank: Super Contributor
timestamp: August 15, 2019, 09:12:39 pm
content: A bash on the front could do multiple channels at once, granted you'd expect to see some scuffs or something, at least.If the connectors are good, it's also something you can test by feeding a signal into the bad inputs and then using a different scope to probe along the input signal path through the front end.I don't know the internal architecture, but if there's one ADC especially, maybe trace backwards from there and see if there's some visible damage on the board? It's probably differential at that point, so getting the same kind of break in four signal lines seems unlikely, but if there's a cut on the PCB or something across it, that could act similarly. Probably worth close inspection of the PCB/parts between the input and the ADC if none of the previous options work.

author: mwbarth36
author_rank: Regular Contributor
timestamp: August 16, 2019, 10:11:47 pm
content: Correct, fault is on all settings. I just took the shield off channel one input circuits and probed (with Ohm meter). Pretty difficult to operate this scope when disassembled. Then I took shield off channel two, and a problem became obvious, several components around the input BNC were charred with damage to board. So this explains channel 2, as there is obviously an open circuit (DC wise) close to the input. Now the question becomes why does channel one act the same. All components look normal. Only thing I can figure is that someone 'overvoltaged' both inputs, and some component acted like a fuse for channel one, but channel two was less forgiving. If only I had a schematic I know I could fix this now, even a schematic of simply the front end. Now I can't identify the open component in channel one. Components values are not marked, at least with numbers indicating a value, so I am pretty well stuck without a partial schematic at least. Any suggestions are greatly appreciated!

author: SilverSolder
author_rank: Super Contributor
timestamp: August 17, 2019, 10:53:52 am
content: Schematics are quite rare for newer generations of equipment, sadly.Unless a lot of components are burned, it may be doable to carefully remove the corresponding components from the good channel and measure them accurately with an LCR meter?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 17, 2019, 11:06:03 am
content: Quote from: SilverSolder on August 17, 2019, 10:53:52 amIf only he had a good channel...I'd try to draw a diagram from channel 1, leaving the unknown components without value. Then it should be more or less obvious where the signal is supposed to go and that way you should be able to at least find the broken component.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 17, 2019, 11:26:42 am
content: Fair point @PA0PBZPerhaps try to inject a small signal further and further upstream of the front end, getting ever closer to the ADC, and see if it is possible to get a normal looking signal at some point... to help confirm that the ADC itself actually does work, and get a more specific location of where things stop working.Also, let's not forget the holy commandments... particularly, "Thou shalt measure power supply voltages". - could the overload on the input have burned out a voltage regulator or other power supply component, so the vertical amps simply don't have the right supply voltage and therefore don't work properly?

author: mwbarth36
author_rank: Regular Contributor
timestamp: August 18, 2019, 10:47:58 pm
content: After removing the BNC input connectors it was fairly easy to make some measurements. My former analysis apparently was correct. The two circled resistors are open >20Meg resistance in channel one. The upper resistor is the one that caused the most damage in channel two. Actually the lower resistor in channel two still measures 164k Ohms. As burned as it is i don't trust that to be the original value. There appears to be nothing else that has suffered. I do believe all the damage was confined to the front end, remember the edges of a square wave gets through fine, and a sine waves amplitude is proportional to frequency as one would expect if the input were ac coupled with a small capacitor. Anyway, I feel I could fix this beast if..........I only knew the value of the one (upper) resistor. It would be nice to verify the lower resistor value also, since the one in channel two has been stressed badly.I am assuming there is no hope of obtaining a schematic for this board (made in China), just wondering if anyone out there has had to open one of these scopes, and when trying to troubleshoot has measured the value of this resistor? My only other option is to, someday, obtain another one of these scopes, tear it down and measure the resistor value. Not a pleasant thought, as this scope was not made to disassemble for sure.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 19, 2019, 08:17:33 am
content: This DSO3102A seems to be a rebadged Rigol, maybe you can find the model (5000 series?) and some internal pictures? The X39 resistor should be 39 Ohm, see here:http://www.marsport.org.uk/smd/res.htm

author: vk2cow
author_rank: Newbie
timestamp: December 03, 2019, 07:44:21 pm
content: Hi mwbarth36,have you managed to repair this scope? I have rescued two units from the bin and they both suffer from exactly the same problem. The good news is that one of them seems to have a channel working, so perhaps I can try to verify component values on that channels...73, Dimitris

author: vk2cow
author_rank: Newbie
timestamp: December 03, 2019, 08:50:28 pm
content: Hi all,I opened up one of the DSO3102A scopes with the faulty front end and was able to (more or less) confirm that the resistor (39 ohm?) identified by mwbarth36 is the main problem. I did that by supplying a known amplitude 100 kHz signal from my signal generator and short-circuiting the blown up resistor with my tweezers - the CRO showed the correct amplitude signal straight away. Obviously, short circuiting is not the solution as the resistor is there for a reason. If it's in series with a 1 Mohm input impedance, I wonder if its purpose in life is to explode, of course (pardon the pun). Either way, it appears that the scopes can be brought back to life with a bit of effort. Only problem for me is that I have no equipment to deal with the tiny SMD components.On a side note, the scope is really cheap and nasty inside. I don't like it at all. The main board has "Rigol" silkscreened on it. The metal frame is kept together (in a couple of places) with rivets.73, Dimitris