Tektronics TDS220 offset problem  - Page 1
Forum section: electronics / repair

author: joew777
author_rank: Newbie
timestamp: February 17, 2025, 08:34:31 pm
content: Hi, I'm trying to fix an old Tek TDS220 scope I've saved from the trash and need help with schematics or ideas. Looking for schematics, input circuit diagram or any info about channel offset in the front-end (ADG 321C). I've searched the forum but did not find a similar issue for this scope.The issue is Channel 1 is showing an offset added to any input signal. The same offset shows in DC and AC coupling and varies depending on the Volts/div scale. In the 5V, 2, 1 and 500mV scale the offset is ~3.5V, and goes down to 250-200mV in the 200, 100 and 50mV scales. If I apply any signal, it shows the signal perfectly but added to the offset. If the signal goes offscreen because of the offset, I can pull the position down until I can see it fine. Changing coupling to GND works ok.As a consequence, I guess, it fails the Self Cal, but no errors are logged, no reason informed. Also, it passes all POST tests.Other than that, looks like everything else is working, Channel 2 works perfectly, trigger, memory, measurements, the scope is clean and seems only lightly used.What I have done already (disclaimer: problem was happening already before I touched it:- Scope came with high noise at both CH1 and CH2 so I replaced all elect. capacitors in the power supply. Both channels show low noise now.- Replaced both channel's BNCs which were quite loose.- I've unsoldered the shields above and below the input circuit and measured resistors and capacitors comparing CH1 with CH2. This is somewhat easy even without schematics for the components that are placed symmetrically for CH1 and CH2. Found no significant differences in resistance or capacitance for the symmetric ones.- Did not measure it but I don't think the reference bias is an issue because CH2 works fine.What can I still do (and any available info will be much appreciated):- capture the input protection circuit from the PCB for both channels until the input pins in the front-end (ADG321) and then do a better check on both channels.- understand ADG321 configuration, specially how the offset adjustment is done for each channel.- upgrade the firmware (this is the older board model), currently at FW 1.0 and check if latest has better error logging.- can the issue be due to corrupted or incorrect calibration data? affecting the offset only? Trying to postpone this to avoid having to collect reference equipment and perform more than 50 steps. I will do that before doing any major intervention in the board.Thanks in advance!Joe

author: ondraN
author_rank: Regular Contributor
timestamp: February 18, 2025, 10:01:08 am
content: I would perform the calibration procedure described from page 50 of this service manual. Maybe DC calibration would be enough.https://w140.com/tekwiki/images/5/52/071-0492-03.pdf

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 18, 2025, 10:50:18 am
content: The calibration can be done with relatively basic equipment at a pinch, even referencing another known good scope for amplitude adjustment of input signals.One point of note, you need to do all steps of the calibration correctly and from start to finish otherwise the cal will be aborted and will not be saved (the scope will revert to the previous calibration).If you have a National Instruments GPIB adapter for your PC, you can use this to help automate the calibration process.https://www.hakanh.com/dl/TDS_pv.htmAs for schematics, none have been released. I have been planning to reverse engineer the PCB for a long time, but I just haven't gotten around to it yet.

author: joew777
author_rank: Newbie
timestamp: February 18, 2025, 02:16:42 pm
content: Thank you for your replies !I agree the best way forward is to do a full calibration, even without specialized equipment. I will do it and post results back here. And thank you for the GPIB parameter checking suggestion. The scope came with the CM module and I have a GPIB/USB adapter soI will check that first.Thanks !