Repair old oscilloscope with a board designated for a later serial number - Page 1
Forum section: electronics / repair

author: buck converter
author_rank: Regular Contributor
timestamp: May 12, 2020, 02:46:54 am
content: Hey everyone,Without school open I have had the time and will to play with electronics again. Unfortunately it seems like my analog oscilloscope, TAS455 is having control issues. It turns on fine with readout, but it is barely responsive to the controls. So its analog setup is basically stuck. I think it is the main CPU board, but I have no way of really knowing. There is a replacement CPU board available on eBayeBay auction: #221675614328but it is for a later model/serial number. According to the service manual, the board that my model uses is 671-2221-01, which I have visibly confirmed. Do you think the later version of the board will still work? One difference I noticed on the eBay listing is that there is a blue wire in the bottom right corner, which mine does not have.

author: james_s
author_rank: Super Contributor
timestamp: May 12, 2020, 04:52:30 am
content: I have no familiarity with the TAS models, they do look quite nice though.I would not be so quick to condemn the CPU board, what troubleshooting have you done? Are all the voltages from the power supply in spec?

author: buck converter
author_rank: Regular Contributor
timestamp: May 12, 2020, 05:20:38 am
content: I have not opened it up yet to troubleshoot, but I probably can after AP exams. After my last post I left it on for a while, and it started to work again. Occasionally I'd have to press a button repeatedly for it to respond. Guess I'll have to turn it on an hour or so before I want to use it (for now). I'm a senior in highschool so it might be a few years before I know how to diagnose the specific problem.

author: james_s
author_rank: Super Contributor
timestamp: May 12, 2020, 07:46:03 am
content: Well don't just throw parts at it, this sort of thing needs proper troubleshooting. If it worked after leaving it on for a while that strongly suggests capacitors. Does this model have those horrid surface mount electrolytics? If it does then every one of those needs to be replaced, and sooner rather than later because the electrolyte leaks out and does severe damage to the boards.

author: kawal
author_rank: Frequent Contributor
timestamp: May 15, 2020, 05:00:10 pm
content: I would clean the keys and the encoders of the front panel. the contact material migrates and the traces get tarnished over time. The buttons dont react as needed. I would try spraying some contact cleaner into the encoders - might have to desolder them and take apart.Dont be afraid to make mistakes - thats how you will learn.Service manual herehttps://www.tek.com/oscilloscope/tas465-manual/tas455-tas465-service-manual

author: buck converter
author_rank: Regular Contributor
timestamp: May 19, 2020, 12:48:08 am
content: yep, the encoders are also a bit buggy, sometimes they jump many steps or no steps at all. I'm not sure if there is a problem with the buttons because when the front panel does respond, either all the controls work (to an extent), or don't work at all. Thank you for the service manual! I'll have to review it some more. One annoying thing is that the text encoding is all messed up, making it hard to search, and copy paste keywords. But I'll manage