Author Topic: Textronix TDS 420 Repair  (Read 1754 times)

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Offline 9291Sam

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Textronix TDS 420 Repair
« on: February 19, 2018, 08:14:51 pm »
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/ih0rle

I have no idea where to start with this any suggestions?
 :-BROKE
 

Offline tautech

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Re: Textronix TDS 420 Repair
« Reply #1 on: February 19, 2018, 08:30:23 pm »
Welcome to the forum.

Let's start with the manuals:
http://w140.com/tekwiki/wiki/TDS420

Have 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
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Offline 9291Sam

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Re: Textronix TDS 420 Repair
« Reply #2 on: February 19, 2018, 08:37:40 pm »
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)
« Last Edit: February 19, 2018, 08:40:27 pm by 9291Sam »
 

Offline tautech

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Re: Textronix TDS 420 Repair
« Reply #3 on: February 19, 2018, 08:47:30 pm »
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.
Typically you need first ensure the PSU meets spec. Do this before anything else. Don't worry about the HV rails initially.

Quote
No ESR meter is the big one
Not so much, you can do a lot with just a capacitance meter and just flag/replace anything that's ~10% lower than spec.
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Offline 9291Sam

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Re: Textronix TDS 420 Repair
« Reply #4 on: February 19, 2018, 08:50:45 pm »
Should the power supply be good if the lights flashed on? I believed it must be a digital part of the circuit not working
 

Offline tautech

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Re: Textronix TDS 420 Repair
« Reply #5 on: February 19, 2018, 08:55:03 pm »
Should the power supply be good if the lights flashed on? I believed it must be a digital part of the circuit not working
ALWAYS check the PSU first. Always.
The spec is in the service manual. Take some real time to read and understand at least the basic 'Principles of Operation".
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Offline 9291Sam

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Re: Textronix TDS 420 Repair
« Reply #6 on: February 19, 2018, 09:09:03 pm »
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
 

Offline tautech

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Re: Textronix TDS 420 Repair
« Reply #7 on: February 19, 2018, 09:13:42 pm »
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
Find the PSU output test points as listed in manuals then measure them; DC and AC (for ripple) and list each one here.

Systematic approach.  ;)
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Offline 9291Sam

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Re: Textronix TDS 420 Repair
« Reply #8 on: February 19, 2018, 09:47:20 pm »
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
 

Offline stevelup

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Re: Textronix TDS 420 Repair
« Reply #9 on: February 19, 2018, 10:14:04 pm »
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...
 

Offline andy2000

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Re: Textronix TDS 420 Repair
« Reply #10 on: February 20, 2018, 02:53:09 am »
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. 
 

Offline 9291Sam

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Re: Textronix TDS 420 Repair
« Reply #11 on: February 20, 2018, 09:28:59 pm »
I have looked at the board and I do not see any leaky caps. Also, everything is DIP so ...
Thanks for the Idea
 

Offline xwarp

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Re: Textronix TDS 420 Repair
« Reply #12 on: February 22, 2018, 01:54:27 am »
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.
 

Offline JoeO

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Re: Textronix TDS 420 Repair
« Reply #13 on: February 22, 2018, 02:47:14 am »
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.
I 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.
The day Al Gore was born there were 7,000 polar bears on Earth.
Today, only 26,000 remain.
 

Offline maxxim

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Re: Textronix TDS 420 Repair
« Reply #14 on: January 11, 2020, 06:10:34 pm »
Hi, what is the value of the tantalum capacitor on the underside power supply pcb (hybrid pin 13-19).

Thanks
 


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