Keysight MSO7034B, Teardown and Repair [Success]

[HighVoltage]

I bought this Agilent / Keysight MSO7034B scope a while back in broken state really cheap.
Symptoms: Does not turn on at all.

So, I thought to make a teardown and repair attempt at the same time.
I never opened one of these scopes before and I have not seen a tear down before of a 7000B series scope.
To take the back cover off, remove the obvious screws, the handle and the feet.

So far I took the back cover off and found a lot of dirt on the fan and in the housing.
May be this was the killer for the PSU, although it looks pretty clean.

Here are my first pictures

Interestingly this PSU has lots of missing components.
Usually when I have repaired some Agilent gear before, the PSU was fully populated.
 

[joellinn]

Maybe someone salvaged it 
Did you try and measure voltages on the Header that goes into the scope? Are the fuses OK? What about the transformer?

[nctnico]

It seems some outputs are not used so that would explain the 'missing' components. I'd start by checking the PSU's output voltages and go from there. Don't assume something is broken; always measure to determine something needs replacing or not.

[HighVoltage]

Thanks, yes, will test the PSU next

I took the main board out first to clean it all of the dust.
Here are some pictures of the main board.

[HighVoltage]

And here are some pictures of the front end under the metal can.
Anything else I should take close up pictures of?

[SeanB]

Looks like a standard PSU board that produces multiple rails, and they only need 2 rails here ( likely 5V and 12V) so only populated those parts on the secondary side. My bets are the crinkled blue mains input cap upper left of the PSU by the mains input sent off the ceramic mains fuse as it failed short circuit. If not then suspect any high value resistors on the primary side, especially if the main transistor switch is not blown apart.

[HighVoltage]

The fuse is OK but the PSU is dead.
So, probably a resistor failure in the PSU?
I will continue the trouble shooting tomorrow...
The schematics are probably not available for the PSU ?


Just for completeness, here are some more pictures of the tear down.

[nctnico]

The fuse is OK but the PSU is dead.
So, probably a resistor failure in the PSU?

Don't guess but measure. First check all semiconductors. If those are OK apply power using a DC power supply to the low voltage part of the primary circuit (look up the datasheet for the required voltage) so you can check if the switching chip is doing something and the primary side MOSFET is switched correctly. Using a lab power supply makes this measurement safe to do. The next step is to apply power to the primary side capacitors using a different low voltage supply (30V to 40V and a low current limit setting). In this step you'll see some things happening on the secondary side as well.

Edit: this seems like some kind of self oscillating resonant converter with a current feedback from the secondary side. Applying DC at the input should give some insight in where the path ends.

[tautech]

Before you put a lot of time and effort into this is it not worth providing the voltages necessary from an external PSU to check the rest of the scope is functioning. 

[tautech]

Is that a dry joint on the inline fuse?

[CJay]

There is absolutely nothing special about that supply, it's an off the shelf Artesyn device and is replaceable for a few tens of dollars.

If you can't find the fault in a couple of hours, just buy a replacement or equivalent.

Totally unworth the effort to repair unless you are doing it for the challenge.

https://www.artesyn.com/power/assets/nfs110_ds_1213229025.pdf

15V 110W, single rail supply.

[tautech]

There is absolutely nothing special about that supply, it's an off the shelf Artesyn device and is replaceable for a few tens of dollars.

If you can't find the fault in a couple of hours, just buy a replacement or equivalent.

Totally unworth the effort to repair unless you are doing it for the challenge.

https://www.artesyn.com/power/assets/nfs110_ds_1213229025.pdf

15V 110W, single rail supply.

MTBF 125,000 hours min. 
Yeah right. 

[gamalot]

There is absolutely nothing special about that supply, it's an off the shelf Artesyn device and is replaceable for a few tens of dollars.

If you can't find the fault in a couple of hours, just buy a replacement or equivalent.

Totally unworth the effort to repair unless you are doing it for the challenge.

https://www.artesyn.com/power/assets/nfs110_ds_1213229025.pdf

15V 110W, single rail supply.

Totally agree! I‘ve bought a Meanwell 12V PSU to replace the TDK-Lambda one in my Keithley 2306.

[nctnico]

MTBF 125,000 hours min. 
Yeah right. 

Probably at 25deg Celcius; at higher temperatures the MTBF drops considerably and even then it is just a theoretical number.
It is quite interesting to find an off-the-shelve power supply in this kind of high end scope though.

[mikeselectricstuff]

By far the most common failure in SMPSs is an electrolytic cap on the startup supply - look for a low-voltage cap on the primary side and replace it, regardless of what it might measure. 
Remember the main primary cap will stay charged quite a while on a PSU in this state so take care!

[HighVoltage]

It is quite interesting to find an off-the-shelve power supply in this kind of high end scope though.

Yes, this is a big surprise to me as well.
Although I think Keysight has stopped production, it is still available for sale in Germany for Euro 17.396 + VAT = Euro 20.701 (and that is without options)
I would have expected a high end power supply.

Thanks for all your suggestions, I think I will find the problem tomorrow.

[tautech]

As Dave would say, you'll be on a winner, winner chicken dinner if it's an easy fix.
Hope it is, good luck.

[wraper]

Before you put a lot of time and effort into this is it not worth providing the voltages necessary from an external PSU to check the rest of the scope is functioning. 

Very bad advise. Never do that unless you have a very good reason and triple check that you are doing this right. Little mistake and all the expensive hardware will be dead beyond repair.

[nctnico]

Before you put a lot of time and effort into this is it not worth providing the voltages necessary from an external PSU to check the rest of the scope is functioning. 

Very bad advise. Never do that unless you have a very good reason and triple check that you are doing this right. Little mistake and all the expensive hardware will be dead beyond repair.

I have to agree. I'm also reluctant to do this because it is easy to swap + with - and there usually isn't any reverse polarity protection on the boards. Secondly there can be requirements for rise-time, power good signals, etc.

[tautech]

Before you put a lot of time and effort into this is it not worth providing the voltages necessary from an external PSU to check the rest of the scope is functioning. 

Very bad advise. Never do that unless you have a very good reason and triple check that you are doing this right. Little mistake and all the expensive hardware will be dead beyond repair.

I have to agree. I'm also reluctant to do this because it is easy to swap + with - and there usually isn't any reverse polarity protection on the boards. Secondly there can be requirements for rise-time, power good signals, etc.

Oh really guys are we all novices or something.
Oh wait isn't that what current adjustable bench PSU's are for?

[tautech]

Manuals for those following along at home.
http://www.docs-library.com/pdf/1/6/agilent-7012.html#
Hit the "Go to link" and another tab with the pdf will open and download.

PSU checks are described on P77

Basic as these PSU's are, anybody that has fixed scopes will know there'll be a Line frequency feed to the Mobo, possibly a mains gnd too and if it's a single rail supply there'll be multiple conductors to supply the current, hence the additional pins on the connector.

[CJay]

MTBF 125,000 hours min. 
Yeah right. 

Probably at 25deg Celcius; at higher temperatures the MTBF drops considerably and even then it is just a theoretical number.
It is quite interesting to find an off-the-shelve power supply in this kind of high end scope though.

~14 years MTBF continuous use, it'll be extrapolated from a series of 'torture' tests, high temperature, load etc. (Artesyn may well release data  that shows how they derate MTBF for load and temperature and that will give a cllue as to how they test) but I do have PSUs that are considerably older and have been in use for at least ten years (on equipment like gas analysers) so for a quality part like an artesyn, I'd expect it to last a good number of years

It is a bit of a surprise to find an off the shelf part but the secret sauce on a lot of these 'scopes is the acquisition so it makes sense to have good quality off the shelf bits chosen for specification.

[SeanB]

Check R1, change the power transistor with the same type, change the low value resistor in the emitter, change all the little jellybean transistors and check all resistors are not open and it will work, providing the secondary side diodes or capacitors are not shorted. Also check the crowbar thyristor CR12 is not shorted and stopping the power supply.

[nctnico]

Before you put a lot of time and effort into this is it not worth providing the voltages necessary from an external PSU to check the rest of the scope is functioning. 

Very bad advise. Never do that unless you have a very good reason and triple check that you are doing this right. Little mistake and all the expensive hardware will be dead beyond repair.

I have to agree. I'm also reluctant to do this because it is easy to swap + with - and there usually isn't any reverse polarity protection on the boards. Secondly there can be requirements for rise-time, power good signals, etc.

Oh really guys are we all novices or something.

No, wisdom gained by experience 

Oh wait isn't that what current adjustable bench PSU's are for?

At 110W you'll need a chunky bench PSU and also need to worry about power supply risetime and switch-on overshoots if the PSU is crappy enough.

BTW: it seems line triggering isn't done through the PSU but there may be a small current transformer on the mains entry board.

[HighVoltage]

I think I found the culprit

The large high voltage DC capacitor was bad.
This was a Nippon Chemi-Con 330uF 400V
But I only have a Rubicon 220uF 400V at hand.

Installed the Rubicon and the power supply works.
This power supply only has a single 15 V DC output.

I will hook up a DC load and look at the ripple on the output.

What do you all think.
Should I get a new 330uF cap or should this 220uF cap do the job?