suddenly unable power up Tektronix 2235 (analog) oscilloscope

[mmFooD]

I have a tektronix 2235 that I have been using for a couple of months now. I forgot to turn it off for a couple of hours this morning with the probe connected to measure ripple/noise from a switch mode regulator. When finally I turned it off I didn't notice anything suspicious but after a while when I tried to start it up again it didn't work.

Normally when pressing the power button the CRT makes that same, kind of static noise that old TV's does, for a second or two. Now my scope makes this noice but repeatedly for only a tenth of a second or so and the power led is also flashing in the same manner. It goes on like this without showing anything on the display until I turn it off.

So I left it for a while before trying to open it up to see if I could find some obvious error inside but for some reason the last screw was a very small torx which I didn't have at the time so I will have to fetch one tomorrow. But I was wondering if anyone have some idea as to what could be broken? Or at least what to look for.

I'd be very thankful for any help on this issue!

[tautech]

Welcome to the forum.

Sounds like the PSU is in tick mode and some guidance can probably be found in this recent thred:
https://www.eevblog.com/forum/repair/tektronix-2232-scope-in-need-of-a-doctor/

[David Hess]

The aluminum electrolytic capacitors in the power supply have probably worn out and are in need of replacement.

[oldway]

Check all the diodes on secondary of ferrite transformer T948....probably one of them is faulty (short circuited).

[TerraHertz]

[oldway]

What do you mean with this off topic answer ?
Are you naming who answered as "bastards" ?
I did not like your answer at all....
I reported it to moderator as off topic and insulting and I will delete my present post if your post is deleted.

[EEVblog]

Are you naming who answered as "bastards" ?

No, he's not, it's an aussie joke.

[JLNY]

I was recently having a similar issue with a 2230. Checking the PSU rails revealed that the power was pulsing on and off. Re-capping the unit immediately and completely fixed the problem for me. I also recapped a Tek 2445 recently and it booted up just fine again after that. Almost all of the Tek 22XX and 24XX scopes are suffering from capacitor failure at this point, but once you fix them they work fine and are very nice units.

[oldway]

Are you naming who answered as "bastards" ?

No, he's not, it's an aussie joke.

He should take more care because it's an international forum, not everybody knows aussie joke and not everybody is fluent english speaking.

Translated in french, that's a very bad insult.... Bastard = connard....

[BradC]

Are you naming who answered as "bastards" ?

No, he's not, it's an aussie joke.

He should take more care because it's an international forum, not everybody knows aussie joke and not everybody is fluent english speaking.

Translated in french, that's a very bad insult.... Bastard = connard....

Twaddle. You should take more care to understand what is being said before taking offence. If we all spent out lives tiptoeing around global sensitivities we'd all become Californian. Thankfully we live in a world where I'm free to speak and you are free to take offence. You don't have to of course.

Interestingly "Start ya bastard" is now a Nulon product. The Aussie equivalent of the old CRC aerostart. Very high ether content (so watch for destructive knock on startup). Part number when I last checked was Nulon SYB-001.

[oldway]

Twaddle. You should take more care to understand what is being said before taking offence. If we all spent out lives tiptoeing around global sensitivities we'd all become Californian. Thankfully we live in a world where I'm free to speak and you are free to take offence. You don't have to of course.

Interestingly "Start ya bastard" is now a Nulon product. The Aussie equivalent of the old CRC aerostart. Very high ether content (so watch for destructive knock on startup). Part number when I last checked was Nulon SYB-001.

I would like to know what care I could take other than to use Google translate ?
Perhaps you want I pay a native english speaking guy to explain me why Google Translate is wrong ?
We are on a technical forum, I did not expected for an "aussie joke" because it is not the right place for jokes...We are trying to help TS to repair his Tek 2235....

When I am trying to help somebody, I don't ask for thanks, but I do not accept to take offense...

[Falkra]

Translated in french, that's a very bad insult.... Bastard = connard....

Why would you need to translate this to french ?
Anyway, the b***d would be the oscilloscope in the joke, so I don't think it hurts the oscilloscope.

[oldway]

Translated in french, that's a very bad insult.... Bastard = connard....

Why would you need to translate this to french ?
Anyway, the b***d would be the oscilloscope in the joke, so I don't think it hurts the oscilloscope.

Because I have only a very limited knowledge of English language and "bastard" is not a word that I learned on school.
It was posted just after my post, so it seems to be an answer to what I wrote.

[Falkra]

Don't worry, that was not against you, no pb.

[oldway]

Don't worry, that was not against you, no pb.

Yes, I know, Dave explained it was just a joke....Time to go back to the repair of Tek 2235....

[grumpydoc]

Translated in french, that's a very bad insult.... Bastard = connard....

What have ducks got to do with it?

[oldway]

ducks = canards, nothing to do with connards....In Belgium or in France, you name somebody "connard", you risk serious troubles 

[grumpydoc]

ducks = canards, nothing to do with connards....In Belgium or in France, you name somebody "connard", you risk serious troubles 

I know, it was an attempt at humour

[mmFooD]

Turns out that torx size isn't that common, so it will take a little longer to access the PSU board.

I found the schematic

I guess it is a good thing to replace all of the caps since it probably hasn't been done. But since I don't have an ESR-meter availible. What caps should I focus on? The 2200 uF one  (C904) I guess and then the 1000 one (C940) and the ones around the power supply.

Checking the diodes is a good tip as well!

A related question, should I use low-ESR caps or will any name brand ones due?

[David Hess]

Where did you find a schematic?  That is better than the one I have.  I have been using the 2236 schematic for the 2235.

The high ripple current capacitors are C906 and C940.  These oscilloscopes are old enough however that the other aluminum electrolytic capacitors are worn out also so they should be changed as well.  Typical low impedance capacitors intended for switching power supply applications are suitable.

The X and Y class capacitors on the input, C900, C903, C902, and C904 should also be changed for safety.

[oldway]

You are going to make a fundamental mistake  ... I have 40 years of experience in electronic project and repair and I can guarantee it for you.

Your oscilloscope has crashed suddenly and completely, it is not a "wear" of a capacitor that caused this.

If there is something wrong with a capactor, it is short circuited, not to low capacitance, nor too high ESR....

And a short circuit, you can spot easily with a multimeter.

Start by searching for the defective component.

To repair, touch the circuits as little as possible unless absolutely necessary.

By replacing a bunch of components that are not defective, you may create failures that did not exist like:
- damaging / interrupt tracks or metallizations.
- making connections and mounting errors
- cause bad contacts by stressing solders and connectors
- install capacitors that have been off for a long time and will require a very large inrush current, and this can damage the circuits
- Make polarity errors of the capacitors.

In short, you are gone to create full of unnecessary problems ...

Sometimes, you have really to recap some gears, not because capacitors are old, but because:
- they are bad quality capacitors.
- they are known as having a lot of failures (for exemple, some SMD electrolytics capacitors, some types of tantale capacitors, ...)
- they are working very hot and their livetime is reduced for this reason.

But only recap if it is absolutely necessary....Measure the capacitors you have replaced....if they are ok, it will proove you have made a great mistake. 
 

[2N3055]

Where did you find a schematic?  That is better than the one I have.  I have been using the 2236 schematic for the 2235.

There you are, from the Tek itself..

http://www.tek.com/manual/2235-service-manual

[tautech]

You are going to make a fundamental mistake  ... I have 40 years of experience in electronic project and repair and I can guarantee it for you.

Your oscilloscope has crashed suddenly and completely, it is not a "wear" of a capacitor that caused this.

If there is something wrong with a capactor, it is short circuited, not to low capacitance, nor too high ESR....

And a short circuit, you can spot easily with a multimeter.

Start by searching for the defective component.

To repair, touch the circuits as little as possible unless absolutely necessary.

By replacing a bunch of components that are not defective, you may create failures that did not exist like:
- damaging / interrupt tracks or metallizations.
- making connections and mounting errors
- cause bad contacts by stressing solders and connectors
- install capacitors that have been off for a long time and will require a very large inrush current, and this can damage the circuits
- Make polarity errors of the capacitors.

In short, you are gone to create full of unnecessary problems ...

Sometimes, you have really to recap some gears, not because capacitors are old, but because:
- they are bad quality capacitors.
- they are known as having a lot of failures (for exemple, some SMD electrolytics capacitors, some types of tantale capacitors, ...)
- they are working very hot and their livetime is reduced for this reason.

But only recap if it is absolutely necessary....Measure the capacitors you have replaced....if they are ok, it will proove you have made a great mistake. 

+1 

Some strong clues to the failure could be in the thread I linked earlier if the OP takes the time to study it.
Like this would raise a big red flag for me:

[oldway]

A rule to follow:
When you find a capacitor that has leaked, replace all other capacitors of the same brand that you find in the device even if they do not leak.

But if I can give you a good advice to prolong the life of your 2235, it is to modify it and install a fan like the 2236.

(For installation, see manual service of 2236)

Power supply without fan is working too hot

[David Hess]

You are going to make a fundamental mistake  ... I have 40 years of experience in electronic project and repair and I can guarantee it for you.

Your oscilloscope has crashed suddenly and completely, it is not a "wear" of a capacitor that caused this.

If there is something wrong with a capacitor, it is short circuited, not to low capacitance, nor too high ESR....

Without considering that the aluminum electrolytic capacitors are old enough in this model to be worn out, I disagree because:

Normally when pressing the power button the CRT makes that same, kind of static noise that old TV's does, for a second or two. Now my scope makes this noise but repeatedly for only a tenth of a second or so and the power led is also flashing in the same manner.