Rigol DS1054Z CH2 failing self-calibration - Page 1
Forum section: electronics / testgear

author: gstorto
author_rank: Contributor
timestamp: April 10, 2025, 09:51:30 pm
content: I bought this oscilloscope in 2015, I have not used it much, and I noticed a long time ago there was a problem with Channel 2.The waveforms it was displaying on Channel 2 didn't match other channels, but it wasn't completely broken. For example, the test square wave was displayed, however the amplitude and the offset were wrong.I tried a FW upgrade, ran the self calibration procedure and it stays forever trying to calibrate channel 2.I noticed the wave that is shown during calibration moves much slower than Channel 1, it also never goes beyond a few mV negative. Channel 1 however scans the whole range.So I disassembled the oscilloscope and I am following Dave's schematic of the front end and measuring resistors/capacitors/diodes/transistors and ICs. I am also comparing them against CH1, and so far so good (red means it has been checked):However, since I don't know how the self-calibration procedure works, I am wondering if this is even a front end problem?I don't see anything on Dave's schematic of the front end that would suggest the oscilloscope can inject a signal to calibrate the channel. So since it fails self-calibration, it isn't even a front end problem?Any suggestions on what to check would be highly appreciated

author: RoGeorge
author_rank: Super Contributor
timestamp: April 11, 2025, 06:42:14 am
content: I wouldn't check each component, but rather the entire front-end at once. Inject on both CH1 and CH2 the same signal, then check the output of both front ends with another oscilloscope (or maybe check using CH3 and CH4 probes connected to CH1 and respectively CH4 front end outputs.If the outputs are close enough, and if they both respond about the same to level/position/vernier adjust, then move on to checking further to the ADC path.About the schematic, I never double check it, but beware that any reverse-engineered schematic might be incomplete, or might contain mistakes. Not saying this schematic is wrong, but might have unwanted mismatches or missing parts.The oscilloscope can certainly inject signals into the front end, and the most probably point to do that is at the "?" mark in the schematic. My guess is the "?" point (the one in the vertical position section of the schematic) is tied to the output of a DAC, to control the DC offset of the trace. During calibration, the DAC is putting different offset values, then the ADC measures the output voltage and calculate the required correction coefficients, such that the display would be correct no matter the range or the vertical position used by the operator.Check that the DAC voltage (that is inserted into the front end) varies while the up/down trace position knob is rotated.Maybe the front end is OK. You'll have to reverse-engineer the schematic further, towards the MUX/ADC.Regarding the self-calibration procedure taking "forever", how long did you let it work? Beware it may take 30 minutes or more. First, let the oscilloscope run normally for at least half an hour (to reach thermal equilibrium), then start a self calibration and let it to run for at least an hour or two, just to be sure.

author: gstorto
author_rank: Contributor
timestamp: April 14, 2025, 09:37:22 pm
content: Quote from: RoGeorge on April 11, 2025, 06:42:14 amNo, they are not.Setup to probe CH1 differential pairs to ADC:Result of probing CH1:Setup to probe CH2 differential pairs to ADC:Result of probing CH2:Single-ended probes for CH2:Here is how the wires were soldered:These two resistors pads go to the ADC in matched length tracks. So what I am probing should be what the ADC sees.Quote from: RoGeorge on April 11, 2025, 06:42:14 amThere are some mistakes, especially the 'FET input amp' part, but in general it is correct and it helps a lot.Quote from: RoGeorge on April 11, 2025, 06:42:14 amOh, I see! I didn't know that. It makes sense! ThanksQuote from: RoGeorge on April 11, 2025, 06:42:14 amIs this really needed now we have the waveforms shown above? It seems the amplitude is already wrong, it doesn't seem to be a DC offset issue (unless it is saturating something?).Quote from: RoGeorge on April 11, 2025, 06:42:14 amYes, it was one of the first things I saw online. I did way more than the 30 minutes warm-up and I left it running for 8 hours. It always gets stuck trying to calibrate CH2.It seems it never goes lower than a few mV negative, and it remains there. I tried to leave it running for a long time (8 hours) and it seems it "retries" but it always gets stuck at these few mV negative.If you are curious, I uploaded a video of what happens:Also, how the reference square wave are displayed by CH1:Now by CH2:Also, I contacted Rigol and they asked me to do a factory reset and a FW reflash, and the result is the same, the calibration always fails on CH2.Their conclusion is it's a HW problem, and they quoted me €322 (mainboard) + €120 (labour) to fix it...What do you think my next step should be? Probe down towards the BNC until the waveform matches?Thanks

author: RoGeorge
author_rank: Super Contributor
timestamp: April 15, 2025, 05:14:52 am
content: Yes, seems like a good idea to keep probing closer and closer towards the BNC inputs, to find out where exactly the 2 front ends start to differ.

author: gstorto
author_rank: Contributor
timestamp: May 07, 2025, 11:49:12 pm
content: Quote from: RoGeorge on April 15, 2025, 05:14:52 amI probed the input of the differential amplifier relative to GND:This is the result (CH1 no bug):CH2 (problematic channel):So there seems to be a big difference of 800 mV on the DC offset, the signal amplitude is similar.The interesting thing is if I modify the vertical position on the problematic channel to have about the same DC offset as on CH1, the square wave amplitude shown on the problematic oscilloscopes approaches the real value:When this happens, the DC offset I measure with the other oscilloscope approaches the value on the good channel (CH1).Does this indicate a problem with the differential amplifier, or something weird with the vertical position circuitry?