Textronix TDS 420 Repair - Page 1
Forum section: electronics / repair

author: 9291Sam
author_rank: Contributor
timestamp: February 19, 2018, 08:14:51 pm
content: I was watching the How to get a $50 oscilloscope video and managed to score a TDS 420 for $20 USD at a local electronics shop with one catch. It does not work. when you press the power button the Channel one Light and the "ARM" Light turn on Picture :http://prntscr.com/ih0rleI have no idea where to start with this any suggestions?

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2018, 08:30:23 pm
content: Welcome to the forum.Let's start with the manuals:http://w140.com/tekwiki/wiki/TDS420Have you used a scope before and have some good idea of how to operate one ?What other test gear do you have to attempt a repair ?This might help too:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: 9291Sam
author_rank: Contributor
timestamp: February 19, 2018, 08:37:40 pm
content: Thanks for the welcome.I have used a scope before (Only for about 5 minutes)I have most of the other basic test equipment. Multimeter, Bench supply, Soldering Iron (Duh). ect.( No ESR meter is the big one)

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2018, 08:47:30 pm
content: Quote from: 9291Sam on February 19, 2018, 08:37:40 pmTypically you need first ensure the PSU meets spec. Do this before anything else. Don't worry about the HV rails initially.QuoteNot so much, you can do a lot with just a capacitance meter and just flag/replace anything that's ~10% lower than spec.

author: 9291Sam
author_rank: Contributor
timestamp: February 19, 2018, 08:50:45 pm
content: Should the power supply be good if the lights flashed on? I believed it must be a digital part of the circuit not working

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2018, 08:55:03 pm
content: Quote from: 9291Sam on February 19, 2018, 08:50:45 pmALWAYS check the PSU first. Always.The spec is in the service manual. Take somerealtime to read and understand at least the basic 'Principles of Operation".

author: 9291Sam
author_rank: Contributor
timestamp: February 19, 2018, 09:09:03 pm
content: Looking at it more the board which takes in the AC there is a 12v fan that is not spinning (the power connector is circled in the picture)http://prntscr.com/ih1lyj

author: tautech
author_rank: Super Contributor
timestamp: February 19, 2018, 09:13:42 pm
content: Quote from: 9291Sam on February 19, 2018, 09:09:03 pmFind the PSU output test points as listed in manuals then measure them; DC and AC (for ripple) and list each one here.Systematic approach.

author: 9291Sam
author_rank: Contributor
timestamp: February 19, 2018, 09:47:20 pm
content: Looked around in the manual and on the board and can not find any test points. or any lists of what voltages should be where

author: stevelup
author_rank: Regular Contributor
timestamp: February 19, 2018, 10:14:04 pm
content: Page 6-74 in the linked manual gives a step by step guide to troubleshooting the low voltage power supply.Follow that methodically and you should make some progress...

author: andy2000
author_rank: Frequent Contributor
timestamp: February 20, 2018, 02:53:09 am
content: Check the boards for signs of corrosion. The TDS460 I repaired recently was FULL of leaking SMD caps. It's a common problem with devices from the early to mid 90s. The power supply also had a bunch of leaking through hold caps which caused it to shut down.

author: 9291Sam
author_rank: Contributor
timestamp: February 20, 2018, 09:28:59 pm
content: I have looked at the board and I do not see any leaky caps. Also, everything is DIP so ...Thanks for the Idea

author: xwarp
author_rank: Frequent Contributor
timestamp: February 22, 2018, 01:54:27 am
content: No sense in pursuing it any further if all you are going to do is just look at it and call it good.Hint: The TDS 400 series scopes ALL suffer from the leaky caps. Some worse than others, but just a matter of time.My TDS460 had leaky caps, but the electrolyte had long since dried after it had done it's damage, which made it harder to see in some cases.

author: JoeO
author_rank: Frequent Contributor
timestamp: February 22, 2018, 02:47:14 am
content: Quote from: xwarp on February 22, 2018, 01:54:27 amI came to the same conclusion but waited for someone else to post about it. I don't understand where this person is coming from.He takes no advice and he is looking for a quick fix.You are wasting your time, answering this person.

author: maxxim
author_rank: Contributor
timestamp: January 11, 2020, 06:10:34 pm
content: Hi, what is the value of the tantalum capacitor on the underside power supply pcb (hybrid pin 13-19).Thanks