DSO REPAIR - Page 1
Forum section: electronics / repair

author: VU2CMV
author_rank: Contributor
timestamp: May 11, 2022, 04:38:38 pm
content: I have FALCON 100MHZ DSO. Looks similar to RIGOL or UNI-T scopes. From the Attached picture you can see that the scope is not showing same waveform in both the channels for same applied signal. Ch1 waveform is getting clipped. I have tried with different probes, checked all settings but the problem still is same. Please suggest some corrective measure. If anyone can share schematic of such DSOs , it round be of great help. Thanks and regards,ARNAB ( VU2CMV)

author: Fraser
author_rank: Super Contributor
timestamp: May 11, 2022, 05:27:10 pm
content: In your position I would use another oscilloscope, or even the working channel of your faulty DSO to look at the signal path in the faulty channel. Check at its output that feeds the ADC and see if a correct waveform is present there. If not, work your way back towards the BNC input connector checking where the signal is failing to make it through a component correctly. I suspect a power rail issue as you appear to be losing half of the waveform, but there are other causes of such a fault.With two channels available on a DSO and only one that is showing a fault, there are options to use another oscilloscope to compare signals and voltages so the lack of a schematic diagram is not so serious a problem. You will be well served to dismantle the DSO, photograph the PCB with decent resolution and identify the IC’s used in the channel input stages. The data sheets for the IC’s will help you to choose suitable points to monitor for the injected test signal. If you upload the PCB pictures here, members of this forum may be able to assist with suggestions for test points etc.If the signal coming out of the ‘faulty’ Channel and feeding into the ADC stage is correct, you will need to look at the power supplies to the ADC and hope that it is something simple like that.Fraser

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2022, 05:31:03 pm
content: I'd say the problem is almost certainly in the analog frontend, you're going to have to get in there with a scope or multimeter and start making comparisons between the two channels. Since you have two channels that should be identical you should be able to compare between them until you find out what's wrong. Was it malfunctioning when you acquired it or did you do something that caused it to fail?

author: strawberry
author_rank: Super Contributor
timestamp: May 11, 2022, 05:47:25 pm
content: DS1052Ehttp://rigol.codenaschen.de/index.php/Schematics

author: VU2CMV
author_rank: Contributor
timestamp: May 12, 2022, 09:08:02 am
content: Thanks Fraser for your helpful reply. I have attached a picture of front-end of the DSO. Hope it would be a good photo to analyze the board/circuit

author: VU2CMV
author_rank: Contributor
timestamp: May 12, 2022, 09:13:54 am
content: This is a photo of DSO MAIN Board

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 09:34:10 am
content: 1U1 and 2U1 appear to have been reworked at some point in time. Part swapping as part of a diagnostic process maybe ?Did this unit fail whilst in your use or was it purchased faulty ?Fraser

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 10:44:01 am
content: As a start to working out the signal path through the Channel. The components follow this identification coding ……Q = TransistorIC = Integrated CircuitU = Relay or Switch (either mechanical or semiconductor)EDIT - may also be a semiconductor ?R = ResistorC = CapacitorL = InductorPrefix number is the channel and suffix number is component numberAD8510 is an Op-Amp so an obvious point to check for signal accuracy.Note that inductors will commonly be associated with power supply rails so they can often be good points fir checking power supply rails to the channel input module. Power can be lost or incorrect elsewhere within the channel electronics however.The fact that 1U1 and 1U2 have been reworked would make me check around that component first. There should be a correct input and a correct output. You can compare the signals on the various pins between the working and faulty channels. If one is not producing the correct output, check all other pins for anomalies and then consider the cause of the issue. If the component has failed, efforts will need to go into selecting a suitable replacement or identifying the original part identification. Very possibly the same as used in other DSO’s such as the Rigol etc.

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 11:03:29 am
content: Hmmm. I am rethinkinking my interpretation of the PCB markings ….. the ‘U’ marling may not be ‘just’ a relay or switch. Could it also be a semiconductor? I believe so.I am wondering if 1U1 and 1U2 are a high bandwidth differential amplifier such as found in the Rigol input stages. The Rigol uses the LMH6552https://www.ti.com/lit/ds/symlink/lmh6552.pdf?ts=1652353142113&ref_url=https%253A%252F%252Fwww.google.com%252FA failure in that amplifier or it’s supply rails would certainly damage the signal that passes through it.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 11:10:54 am
content: Nicely documented Rigol DS1052 Schematics…… Redrawn by forum member Singapura using Visio.https://www.eevblog.com/forum/repair/rigol-ds1052ds1102-schematic-diagrams/Fraser

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 11:21:43 am
content: This page is a useful reference as well. It lists the links to DSO front end images ….https://gr33nonline.wordpress.com/2019/06/18/scope-front-ends/Fraser

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 11:37:50 am
content: This TI application note provides some insight into typical DSO front end topology…..https://www.ti.com/lit/ug/tiduba4/tiduba4.pdfThis will be different in terms of components used, but it shows to building blocks used to create a DSO input channel and which blocks do what in the signal path.

author: VU2CMV
author_rank: Contributor
timestamp: May 12, 2022, 12:45:54 pm
content: No . It was working fine. Bought used one . It happened after one month of unuse

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 12:49:47 pm
content: It would be worth using Isopropyl Alcohol around the 1U1 and 2U1 components to remove the flux residue. I am not saying that this is the fault, but moisture uptake into flux residue can cause unusual issues around amplifiers.I can say with certainty that the input channels of your DSO have had rework carried out on them, either at the factory, or after original sale. Some of the rework soldering is of poor quality. This DSO may have had an intermittent fault that has now become permanent.Fraser

author: VU2CMV
author_rank: Contributor
timestamp: May 12, 2022, 12:59:00 pm
content: While looking into the DSO , I noticed another thing that ch1 waveform is faulty at positive side . Negetive side display and measurement is fine

author: Fraser
author_rank: Super Contributor
timestamp: May 12, 2022, 03:22:09 pm
content: Hence my comment about a faulty power rail as the amplifiers use symetrical power to cope with positive and negative signal swings around the 0V point. A failure in an amplifier output chain can also cause loss of the positive or negative swing of the output.Fraser

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 12, 2022, 09:04:19 pm
content: I can only agree on previous advice mainly given by Fraser.It should actually be a fast enough diagnosis with a DMM and one working channel on the DSO.Comparing behavior on both channels with different inputs DC, LF, HF and playing with offset could also point to which side of the FE to look at.Check how close the shielding cans are to the PCB, poor designs may actually have them touching the PCB above other conductors than ground plane (i.e: above PS rails) once they've worn their way through the solder mask you have trouble...Edit: Could it be a UNI-T UTD2102CE rebadge?

author: VU2CMV
author_rank: Contributor
timestamp: May 13, 2022, 02:17:41 am
content: Thanks for your reply. From the looks it's semm to be rebranded UNI-T UTD2102CE DSO. Please help me found a service manual or schematic of main board. Thanks and regards

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 13, 2022, 08:15:59 am
content: While having a schematic makes things a lot easier, sometimes you have to do without. I think you'll spend more time seeking the schematic for nothing than it would take getting down to it and following the advice you've been given.Most of the time the analog front ends have two signal paths, one for DC/LF, the other for HF is internally AC coupled, hence my previous coment on testing with different signal inputs.

author: Fraser
author_rank: Super Contributor
timestamp: May 13, 2022, 09:10:59 am
content: I have searched for Uni-T DSO schematics in the past and failed to find any. As has been stated, you are better served to spend time working through the faulty channel comparing readings with the known good channel. As has been shown in my previous posts, there is a well defined signal path through the channel electronics and once the area of failure has been located, the actual cause may be determined. In some cases it is necessary to reverse engineer the area of the circuit that contains the fault. In your case I believe it will come down to a failed amplifier or the loss of correct power supplies to an amplifier.Fraser

author: VU2CMV
author_rank: Contributor
timestamp: May 13, 2022, 09:43:12 am
content: Thanks Fraser. I will follow your advice and let the forum know the result. RegardsARNAB, VU2CMV