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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Jul Mon 17, 2017 1:22 pm 
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Joined: Nov Tue 15, 2016 10:42 pm
Posts: 2
Thank you so much.

I have to correct myself. My oscilloscope is a TDS420A.
SMD capacitors don't show leakage anyway, as I think I'll use this scope I'm planning to replace all electrolytics.
I have to write a list.
During disassembly I saw that few of the caps were replaced. Maybe this means that the unit had problems with leakage caps?
I read somewhere that some Tek socpes use electrolytics in the signal path and that their ESR value is required for working well. Changing them with tantalum ones of same values would like to put a different ESR value in that point and the unit would not work correctly.
Is this true for the 420A?


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Mon 02, 2019 7:51 pm 
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Joined: Sep Mon 02, 2019 7:26 pm
Posts: 6
It seems long time, when anybody is writing to this sector.

My TDS420 is from 1990.
It has worked well until this year.
It is a 4 channel digital oscilloscope.

It started to go repair mode after I started it.
Then after few start, it was working well.
I have to press clear error button.
It's display shows Mathematical prosessor error like a ten years.
Now it cannot go to normal mode and nothing happens when I press clear error button,
I try to change all small capacitrors(10uF) in main borad.
There was about 40-50 them.
No help.
Then I measure power supply. It was working almost well.
Only -7V power supply doesn't allow the load 400 ohm. It went to -4V. It was a small power supply,
So I was not worried.
Other power supplies can carry the 400 ohm load.

Can anybody give me a hint what to do next?


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Tue 03, 2019 6:01 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8198
Location: Sunbury, Ohio 43074
need a little more description on what you mean by the 400 ohm load. Normally I'd say a power supply that is supposed to produce -7V, that doesn't, might be part of the problem. Does this supply power other parts of the instrument?

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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Tue 03, 2019 6:40 pm 
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Joined: Sep Mon 02, 2019 7:26 pm
Posts: 6
Barry H Bennett

I have no schematic about DS420, nor about the power supply.

I have found yeaterdays, that it should carry 8A!
http://remotesmart.wdfiles.com/local--f ... ersion.png
I found this power supply ad.

It shoud not feed 5V , because it is -voltage supply.
I can supply either -15V power, it is too much.
I have no idea what this supply feeds.

Today I open the power supply again and measure this -7V supply again.
It is actyally -6.4V power supply.
Is the voltage fixed?

Now it shows no voltage loss with 400Ohm load.

It was only testload, I had no idea what kind a loads I needed.
Now I have, because I know how much they should carry..

I can measure it futher.

Regards
Pekka Ritamaki


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Tue 03, 2019 7:25 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8198
Location: Sunbury, Ohio 43074
Pekka, without a service manual, repairing a scope of this type might be impossible. You can find the manual here. It includes a troubleshooting flow chart: https://www.tek.com/oscilloscope/tds420-manual/tds410-tds420-tds460-service-manual

You might try one of the Tektronix forums, maybe someone over there has some direct experience with this one.

Here's one forum ... there others. Just do a google search. http://w140.com/tekwiki/wiki/

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https://www.bbtvtestequipment.com


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Tue 03, 2019 8:42 pm 
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Joined: Sep Mon 02, 2019 7:26 pm
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Barry

Than you for your help!

The sevice manual was not like I hope to find.
But is much better than before.

I go to Tek forum, I hope that I get help there, but thank you.

When I was Munich repairing Tek scopes two years, there was all schematics availabe.
I repaireired two to three units per day in Rhode & Schawarz .

One Tek unit was difficult, when I put it on, immeadeality ( after heat delay) +350V and -350V power tubes came red and burn.
When I measure both power supplies to ground, they do not show short circuit.
What was the thouble?

Regards
Pekka Ritamaki


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Tue 03, 2019 8:48 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8198
Location: Sunbury, Ohio 43074
One Tek unit was difficult, when I put it on, immeadeality ( after heat delay) +350V and -350V power tubes came red and burn.
When I measure both power supplies to ground, they do not show short circuit.
What was the thouble?

I am going to guess that one or more power supply filter capacitors were to blame for the red plates. Most likely other cause would be a heavy load across the +/- supply, possibly High Voltage? Just wild guesses.... have to know the scope and take a look at the schematic.

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https://www.bbtvtestequipment.com


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Wed 04, 2019 9:55 am 
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Joined: Sep Mon 02, 2019 7:26 pm
Posts: 6
Barry

You are almoust right.

There was horisontial side offset potentiometer.
It has +350V and -350V in both ends.
The wires were at down side of the potentiometer.
There was full of the carbon waste inside of the potentiometer and they made a shortcircuit between + and - 350V.
The cleaning the potentiomerter inside solve the problem, but I burned 2*2 big tubes.

It was about 1970 when there was lot of tube oscillocopes.

I have not joined to TEK forum.
I have only read Tek forum.

Regads Pekka Ritamaki, Finland


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Wed 04, 2019 1:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 21943
Location: Somers, CT
Very helpful Tek advice can be found on this forum:

https://groups.io/g/TekScopes/topics

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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Thu 05, 2019 2:23 pm 
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Joined: Sep Mon 02, 2019 7:26 pm
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Peter

Thank you, I will see it.

I open the power supply once again and made some measurements.
I take the big prosesor board out and measures batteries.
One was 2 V and other 3V.
I understand they shoud not provent the button to work.
I did some SMD capacitor repair ( I think they had bad joints).

When I put it to working condition, I shows all starting things, but now it do not work at power supply button at front panel.
I have to turn it off one backside.
I have not used this button before.

It is difficult to made so complex device repiar wihout schemati diagram.

Pekka Ritamaki


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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Thu 05, 2019 3:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 21943
Location: Somers, CT
Good luck! I just went through a Tek 497P and a Tek 2467 that both needed full recaps. Bad SMD caps in both units, and a few axial lead tantalum caps that actually leaked and damaged multilayer boards! Tek had a problem when they first adapted SMD electrolytic caps into their designs... either the caps had manufacturing issues or they were damaged during installation or board cleaning after soldering. Even the line bypass caps were cracked and ready to fail. Unfortunately, the Tektronix documentation is not as good as HP's, and I've noticed many errors in their manuals.

Pete

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 Post subject: Re: Some hints on Fixing Tektronix TDS-420 Oscilloscopes
PostPosted: Sep Thu 05, 2019 9:35 pm 
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Joined: Sep Mon 02, 2019 7:26 pm
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Peter

Great you have done two repairs!

Are you professional TEK repairman?
I was about 50 years then.
==
I open my unit again, and found that I had left +5V single wire which went starting the power supply board, open.
Then I open the input board and found about seven SMD capacitors. which I repalced ( 10uF).
I measure them and found that in two there was a little capacitance.
Others did'nt show any capacitance.

I was thinking that it will help. but no.
I found one smd capacitor in CPU board and replaced it, but no help.

My Tek TDS-420 shows FP/fail and FP fail.
It has shown these failures about ten years.
I have only pushed Clear button and it will go to normal, but now it stops in these errors.

Pekka Ritamaki


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