Keysight MSO7034B, Teardown and Repair [Success]

[acbern]

Since this is probably a wide input range PSU (please check) the capacitor was selected for the lowest possible voltage (permissible input ripple voltage after rectifier). Since you are using it at 230V, it should work. I would still change it.

[HighVoltage]

Since this is probably a wide input range PSU (please check) the capacitor was selected for the lowest possible voltage (permissible input ripple voltage after rectifier). Since you are using it at 230V, it should work. I would still change it.

Hello acbern
yes, it is 100 to 240 V AC PSU
I pulled 2A out of the PSU and it seems tob e working perfectly

Installed the PSU and the scope works !
Look at this ... what a great Sunday it is !
What an easy fix that was.
Thank you all !

I will measure now the current the scope is pulling from the PSU and the ripple voltage.

[bingo600]

Great job 

Congrats with your new scope

/Bingo

[TheSteve]

Awesome! This is one of those deals and repairs we all dream of!

[HighVoltage]

Thanks,

I measured the current from the PSU to the scope.
Here are the results:
I am surprised that it does take 2.7A from the PSU.
The ripple voltage is about 10 mV AC (measured with 87V)

[nctnico]

While you still have it open: can you identify where the scope get it's signal to trigger on the AC mains (line trigger source)?

It's a great deal though! Now hack it into an MSO!

[mikeselectricstuff]

I'd definitely get the correct cap to make sure everything is working within spec. Hardly a major cost compared to the value of the scope.

[HighVoltage]

While you still have it open: can you identify where the scope get it's signal to trigger on the AC mains (line trigger source)?

Hmm, there is no other connection to the main board from AC, only the PSU +15V DC
May be the 50 /60 Hz is created on the main board, independent of the real AC ?

It's a great deal though! Now hack it into an MSO!

Yes, the hack will be next

I'd definitely get the correct cap to make sure everything is working within spec. Hardly a major cost compared to the value of the scope.

Yes, I already found an ordered the 330uF cap.
I will take some more detailed measurements, before and after changing the the cap.
Although I dont think it will make a big difference, you are correct to keep it in specs.

I just measured the AC ripple on the output of the +15V DC and it is around 2 mV AC on the 87V Fluke Multimeter

[tautech]

I measured the current from the PSU to the scope.
Here are the results:
I am surprised that it does take 2.7A from the PSU.
The ripple voltage is about 10 mV AC (measured with 87V)

Ha, who would have expected the HV DC cap as the point of failure, not common for a SMPS. 
So 2.7A (40W) is roughly what you'd expect and could have easily been supplied by a bench PSU for checks too.

Nice find HV and congrats on your cheap DSO. 

[nctnico]

To me it seems every repair technician has his (her) own list of common problems in switching power supplies but none of them apply to every SMPS out there.

[mikeselectricstuff]

This is how the MSO6000 does it - I imagine the 7000 will be similar
https://youtu.be/5lYbD9_eIko?t=366

[nctnico]

Thanks for pointing to that video! Judging from the photos it wouldn't surprise me if the power entry board in the 7000 series is the same just like the power supply.

[tautech]

This is how the MSO6000 does it - I imagine the 7000 will be similar
https://youtu.be/5lYbD9_eIko?t=366

Yep, line frequency reference from another PCB, but it's often done directly from the PSU itself however when a cheap generic SMPS is used for these Keysights you have to provide another Line source.

Does a single PCB with Line output configuration make for a more reliable setup or using 2 PCB's ?   

[HighVoltage]

Thanks Mike for this explanation.
I will have another look and make some detailed pictures, it looks it is the same setup in my 7034B

[CJay]

To me it seems every repair technician has his (her) own list of common problems in switching power supplies but none of them apply to every SMPS out there.

We do, but there's an awful lot of crossover and common faults, I think high value resistors on the primary, low value caps on the primary, leaky high value caps on the secondary and shorted secondary rectifiers would be the very first thing pretty much any tech who repairs these things will check for.

Excellent news from the OP BTW, congratulations on such an awesome score, envious doesn't really cover it.. 

[tautech]

To me it seems every repair technician has his (her) own list of common problems in switching power supplies but none of them apply to every SMPS out there.

We do, but there's an awful lot of crossover and common faults, I think high value resistors on the primary, low value caps on the primary, leaky high value caps on the secondary and shorted secondary rectifiers would be the very first thing pretty much any tech who repairs these things will check for.

And the HV DC primary side cap is low on the list ^^, right? 

[mikeselectricstuff]

To me it seems every repair technician has his (her) own list of common problems in switching power supplies but none of them apply to every SMPS out there.

We do, but there's an awful lot of crossover and common faults, I think high value resistors on the primary, low value caps on the primary, leaky high value caps on the secondary and shorted secondary rectifiers would be the very first thing pretty much any tech who repairs these things will check for.

And the HV DC primary side cap is low on the list ^^, right? 

Don't think I've ever seen it, but maybe because manufacturers realise it's a critical part and take more selecting it, as opposed to things like standby supply caps and resistors  where a jellybean part is used despite it frequently be being exposed to significant heat or voltage stress respectively.   

[CJay]

To me it seems every repair technician has his (her) own list of common problems in switching power supplies but none of them apply to every SMPS out there.

We do, but there's an awful lot of crossover and common faults, I think high value resistors on the primary, low value caps on the primary, leaky high value caps on the secondary and shorted secondary rectifiers would be the very first thing pretty much any tech who repairs these things will check for.

And the HV DC primary side cap is low on the list ^^, right? 

It really isn't a common fault in my experience, in the ten years or so I repaired PSUs as part of my job I can't remember replacing more than a handful of them that had failed in anything other than spectacular fashion

In my telly repair career, it was a different story, the multi value, single can capacitors were a bugger for failing in some sets (G11, duff rivets).

For PSU repair work I found the component tester on the Hameg 'scopes makes the 'gross' fault finding really quick (we could turn around ~40 PSUs a day per tech if we got a large batch of the same types) so the primary reservoir would be picked up really quickly if it were faulty while testing the bridge/diodes and, depending on topology, the switching transistor(s) because it would look 'wrong'.

[wraper]

Ha, who would have expected the HV DC cap as the point of failure, not common for a SMPS. 

Not something PSUs are plagued with but is not something rare either.

[HighVoltage]

For PSU repair work I found the component tester on the Hameg 'scopes makes the 'gross' fault finding really quick

This is still a feature on the small Rohde & Schwarz scopes.
I never thought about using it "in circuit", interesting idea.

[CJay]

For PSU repair work I found the component tester on the Hameg 'scopes makes the 'gross' fault finding really quick

This is still a feature on the small Rohde & Schwarz scopes.
I never thought about using it "in circuit", interesting idea.

I find it useful for things like power supplies, amplifiers, analogue power things, but I know other people aren't keen.

I guess it's just personal taste and it really depends how often you repair things like that.

It's useful for gross faults in circuit (shorted or open junctions, open circuit resistors, leaky, short or open  caps), I can 'sweep' a board in a few seconds for those with a component tester and desolder parts that look suspicious to get in deeper with a meter or component analyser and confirm or disprove suspicious traces.

They're not so great on 'subtle' things like high value resistors that have gone higher or nF caps but combined with a multimeter and some experience, they're really effective.

[HighVoltage]

While you still have it open: can you identify where the scope get it's signal to trigger on the AC mains (line trigger source)?

Ok, here are some pictures of the AC board and the connection to the mains AC trigger.
It looks like an optocoupler in between the AC and the wire to the main board.

Even this little PCB is made by Agilent and looks great and much better than the PSC PCB.

 

[HighVoltage]

Now hack it into an MSO!

And here are two pictures of the screen, before and after the hack.

Thanks to some forum members for providing the 6.16 FW version. (No longer on the Keysight website available)
The Hack is very simple, as long as the Firmware is 06.16.0001.

[HighVoltage]

Once the Service Menu is turned ON via telnet access, this new Menu is available under "License" in the "Utility" section.

[HighVoltage]

I'd definitely get the correct cap to make sure everything is working within spec. Hardly a major cost compared to the value of the scope.

OK, to conclude this thread, I just installed the new 400V 330uF capacitor and all works great.

Interestingly, the new cap is much larger

Original Nippon Chemi-Con
- 400V 330uF
- 25 mm diameter
- 40 mm tall
- 29.8 Grams

New VishayBC
- 400V 330uF
- 35 mm diameter
- 50.7 mm tall
- 64 Grams
This new cap has an ESR of 0.07 Ohm
But only measures 298 uF
May be this one is better, because it has a larger volume.
But who knows, I have never used a Vishay electrolytic capacitor before.

Thank you all for help and suggestions, it is really appreciated.