Tek MDO3024 Attenuator Fault - Page 1
Forum section: electronics / repair

author: WD40
author_rank: Contributor
timestamp: March 13, 2025, 04:33:17 pm
content: Hi,Does this strike you as an analogue or digital fault?I unexpectedly won an Ebay auction for a Tek MDO3024. The seller mentioned in the listing that Ch3 didn't register a waveform or value when below 1V, so I bid accordingly. The rest of the scope seems ok, but on Ch3, if you turn attn below 1V p/Div (10:1 probe, or 0.1V with 1:1 probe) the trace disappears completely - there's no blank line, no frequency readout - and all that remains is the right-hand purple arrow cursor to show vertical position. When ATT is rotated from 1V to 0.5V, the trace doesn't disappear like it would if the sig gen were turned off, it 'fades' over about a second - a sine goes from a continuous line to a dotted line. It kind of 'fades' off.My initial feeling was that it's an analogue fault, where a CMOS switch (shift register output?) or voltage divider resistor has a dry joint or similar, but now I am not so sure. If an attn went open circuit, you'd get a blank line, as if the I/P had been disconnected, right?You can get a replacement attn PCB on Ebay ($800 USD...), but the part is secondhand and the listing warns you that your scope will display an error log when switched on, due to the serial code of the new PCB not matching the scope. If the fault isn't in the attn PCB, that would be a very expensive mistake.Weirdly, the scope appears to have been made in 2017, but hasn't been registered with Tek... It doesn't have the DVM option...if anyone has a link where you can get a code to switch the DVM on, that would be great, as I have registered with Tek to update the firmware (is currently 1.26), but I am still waiting for 'approval'...and am wondering if they will make me jump through hoops / get me to call them so they can attempt to sell me other items before I get anything.Thanks in advance for any tips!nb - It could be a false memory as I was fumbling, getting used to the scope, but C3 seemed to be ok for a few seconds when I switched scope on, but am not 100% sure. It was a cold morning here...am wondering if it worked when cold, having been on postman's cart... Guess I'd have to open it up and apply freezer spray to test this theory...

author: squadchannel
author_rank: Frequent Contributor
timestamp: March 13, 2025, 04:49:24 pm
content: It is too modern to tell, but if the attenuator is not working, then the trace itself should be displayed, although it is not responding to the input signal or is wrong.My guess is that it is the digital part that is going wrong, not the analog attenuator.Have you run a self-test?No serial number registered on the Tek.com? That's odd. prototype?

author: squadchannel
author_rank: Frequent Contributor
timestamp: March 13, 2025, 04:51:49 pm
content: Firmware downloaded. Ver. 1.30https://mega.nz/file/qbow0IgD#HTLeUmzWfxBblZNk4dcxNuLbGvpE3uony3EQWKBiQcI

author: WD40
author_rank: Contributor
timestamp: March 13, 2025, 05:20:17 pm
content: Thank you very much!It says 'all tests passed' when it boots.edit - it's currently installing firmware now. Have fingers crossed!Will I be able to get the DVM feature without waiting for Tek to reply to me?

author: WD40
author_rank: Contributor
timestamp: March 13, 2025, 05:48:52 pm
content: It's installed V1.30 - but the problem is exactly the same as before...Any other ideas gratefully received!NB - Tek have emailed me the DVM code. Whoopee! I now have a function that every other scope I've used in the last 35 years gave me on switch on!

author: WD40
author_rank: Contributor
timestamp: March 13, 2025, 06:00:59 pm
content: I spoke too soon about the DVM... The code is 'invalid'. Does anyone know of a way around it if the code I got supplied with doesn't work? ThanksAnother Update:If I store a waveform, it stays on screen below 1V / 0.1V - i.e. attenuator issue is only a problem when looking at a live waveform.

author: WD40
author_rank: Contributor
timestamp: March 14, 2025, 01:56:18 pm
content: Update: got the DVM working now (have no idea why it did't work before - found the whole code input process slow and annoying, just to get something every other Siglent / Rigol / Owon gives you on a hundred quid scope...).V1.3 installed and running well.Ch3 attenuator problem is still present. Updating the machine hasn't made any difference.I find it curious that the trace line completely disappears when I turn attn to below 0.1V... The encoder must work ok, as I can zoom in as much as I want with a stored waveform. If the issue is in analogue stage, why would the whole trace disappear? Surely it'd act as if a probe had been unplugged, right?

author: WD40
author_rank: Contributor
timestamp: March 25, 2025, 11:13:17 am
content: Has anyone managed to fault-find one of these scopes at component level? I haven't seen a service manual for this model, but other Tek SMs are deeply unhelpful, as in " check A, B, C . If it doesn't show 'x' then change whole PCB assembly". I had to buy a broken Tek in order to fix one of my others, as the SM gave no schematic and was simply a sales brochure for their spare parts department...Auto probe calibration does not work for channel 3... Obviously, it cals fine with manual probes, as long as I don't venture below 0.1V (1:1 probe) or 1V (10:1).I found out where the scope came from... Multinational firm. It was bought in 2017 and the most recent PAT sticker says June 2023. Am surprised they used it for that long. I guess it got outed when Ch3 had the issue. But it passes all tests on power up, no fault codes. Unless you use Ch3 you would never know it has a fault.

author: mikehank
author_rank: Regular Contributor
timestamp: March 26, 2025, 02:44:54 pm
content: Check your input impedance on the channel. 1M and 50ohm. Also in the 50 ohm setting there are two paths one for 90mV and below and one for higher voltages. For 50ohms you should be 50+/- 1.5ohms or so.

author: WD40
author_rank: Contributor
timestamp: March 27, 2025, 10:22:08 pm
content: Thanks. I can confirm that the fault occurs regardless of which of the 3 input impedance settings are selected. It seems to occur regardless of input configuration, AC or DC coupled - whatever. The trace completely disappears, giving nothing at all - no blank trace. It fades out like a firework when you select under 0.1V (1:1 probe) - i.e. decays, over what I'd guess to be around a second or maybe less.

author: WD40
author_rank: Contributor
timestamp: March 31, 2025, 10:34:13 am
content: Does anyone have an opinion on whether this is a digital or analogue fault? As the trace is disappearing, I am inclined to think the former...(analogue would be akin to unplugging probe, right? The attn knobs zoom in perfectly on a stored waveform, so am inclined to think it's not a fault on the encoder PCB). Without a block diagram (I wouldn't expect a schematic- that would be sooo last century), I am a little concerned about signal tracing. Assuming that the architecture is mirrored on each channel, then I could put a signal into one channel, set it to the same setting as Ch3 and try to see where the path differs between channels.If it is a digital fault, then it's not likely to be under the screening can - as that will be for delicate analogue signals, right? I do not know if I have the guts to desolder the can TBH, as removing them is always a struggle unless you use a lot of heat. The scope works fine as a 2-channel device (well, 3 channel scope). I rarely need 4 channels. Unless I can get some good pointers on where the fault lurks, I might leave a sleeping dog to lie and think of it as a 3-channel scope.

author: ArcticGeek
author_rank: Regular Contributor
timestamp: March 31, 2025, 03:35:43 pm
content: I don't have any experience debugging the MDO series of scopes, but I have seen similar faults on TDS series of scopes. I'm guessing that this is an "analog" fault with the preamp that is part of the attenuator. This is typically a Tek designed ASIC that does the attenuation and/or pre-amplification. One of the inputs to these preamps is the offset voltage, which allows you to add a DC +/- offset to the signal to change the trace position on the screen. If that part is defective, that offset can add way too much offset and just put the signal offscreen. This can cause the problem you are seeing.That ASIC is responsible for taking that offset voltage and scaling it according to the V/div setting. For example, that offset might range from -2V to +2V to change the position on the screen regardless of what V/div setting is. However, if the part is defective it can misinterpret that and just throw things way out of whack. I've also seen the same issue with the preamp used inside the P6243/P6245 active probes.I highly doubt you will find a replacement preamp/atten IC as it is Tek specific and very hard to find a source.Best,

author: WD40
author_rank: Contributor
timestamp: March 31, 2025, 10:05:15 pm
content: Thanks. When the trace disappears, i.e. you select voltage below 0.1 / 1V, it seems to stay in the same place and fade out. It's as if the pixels disappear in a pattern, i.e. by a quarter / half / three quarter / all.I've installed the DMM. If I assign it to monitor Ch3, I wonder what the voltage will show? Will it continue to monitor voltage and frequency?

author: WD40
author_rank: Contributor
timestamp: April 01, 2025, 05:21:05 pm
content: OK - if I assign the DMM to Ch3 and put the attn to below 0.1 / 1V, in frequency mode it says "Not Yet Computed" and value disappears. In AC RMS mode value disappears and it says "Under Bandwidth".edit - I just noticed that if I put the attn knob into 'fine' mode, it will go below 0.1V. The trace disappears when it is asked to go below 64mV. It's there @ 64mV, but disappears at the next value of 63.5mV