Keysight MSOS104A Repair Attempt

[BeamDump]

Hi

What do you mean? It's generated in LTC6945, it's what it's suppose to do. Pin3 (status), which in this case has to be programmed through SPI as lock, even goes through resistor to the base of MMBT3904LT1 that goes to PLL2_Lock LED. 

4.6GHz is low for this VCO, it's suppose to be 5.

[Extrasolar]

Hi

I was able to trace the status Pad of the synthesizer to PLL1, and was unable to find the path for PLL2 !!

[BeamDump]

Hi

I also can't find any trace from the 2nd output of the buffer. Then the lock status is communicated through SPI?

Another question, when the PLL2 error pops up, are you able to perform any software function? Self test or anything at all.

[Extrasolar]

Hi

No it does not do anything in the sense that after the autocal routine it sprays out the same error and does not do anything. You can try an external Reference 10MHz and change the horizontal trigger.
I got it working for an instant by doing that. Another thing is that I could not detect the 10MHz reference at all on the board. Now all my focus is on the signal path from the oven controlled oscillator to the REF input of the Synthesizer.

Kind regards

[BeamDump]

Hi

Replaced the transistor in my VCO, it is a 2SC5606. Now the output is clean, stable and sits at 5.0GHZ at 4.3V. Plugged it back in to the board, PLL2 is locked and the LED is considerably brighter than it was before but the software error remains.
Since i don't see a test point for Y2 buffer out for PLL2 status and probing front and back was unsuccessful i can only assume it's sending the status via SPI. Then why bother routing pin 3 of the synthesizer to the buffer if the output wont be used...

Ordered spare LTC6945's and evaluation/communication boards to better understand the synthesizer. Buffers are difficult to find. Ordered some from the one place i could find them in China, see how that goes.

I'll try your method.

[Extrasolar]

Hi,

Thats very interesting !! Thank you for the information. Does any trace appear on the screen ?? 
If yes, then I believe you can factory restore the scope at this point, to clear the error message !!

[BeamDump]

Hi

Haven't tried yet. I've been waiting for parts to arrive and they finally have. Shipping times are horrible and companies stopped caring in the slightest.   
Got some spare LTC6945's and NC7WZ17 buffers. I'll post any updates later.

[max-bit]

Here, the control of the frequency synthesis system is probably damaged, and not the PLL (LT..) or VCO itself is damaged. Therefore, the system does not hold the frequency.
Check how the LT IC is controlled and whether there are control signals at the LT IC inputs.
That would explain why the oscilloscope ended up in the trash.
If the damage was simple, Keysight would fix it, but due to the FPGA (and / or ASIC) system, unfortunately, they do not replace a single IC, but replace the entire board.
And that probably costs as much as the entire oscilloscope.

[BeamDump]

Any FPGA/ASIC can be replaced. Maybe not in EU anymore, since most factories that would do such repair are either gone or won't provide the service. I replaced an A-ASIC in a Fluke 199c few years back. True, Fluke sold the IC for 120EUR but was possible.

All the frequency synthesis happens in the LTC6945 for the time base, that's it. It does have SPI where of course it can be programmed and read out.

They could easily sell the IC, knowing where the problem is but instead it's more profitable to them to charge 6K for initial inspection and possible board swap. Same reason why these scopes have no schematics or part lists.

Just got everything i need, now to get some time with it. December is the worst time.

[Extrasolar]

Hi All,

A second MSOS804 just failed at work, with the same problem !!
Will capture the SPI protocol as I have a few evaluation boards with the same synthesizer.
I could not find any problem on the signal chain from the oven to the synthesizer. Anyone knows the part number of the fully differential amplifier which is right after the Synthesizer (U33003) ??
I cannot find the Positive Supply for this Amplifier !!!!

[TheSteve]

Maybe temporarily add one to see what happens.

[KE5FX]

That might be an LMH5401 or something in the same family.  It may use both positive and negative supplies (e.g. -1.8 and +3.3).

[BeamDump]

Slightly better photo.

Sounds like a repeat of my story lately. Same thing with this one, no lock on PLL2?

[gamalot]

It looks like fanout buffer HMC720

https://www.analog.com/media/en/technical-documentation/data-sheets/hmc720.pdf

[BeamDump]

Yeah, all the pins match and it works only on a negative supply.

Extra, can you check the eye diagram coming out of yours?

[Extrasolar]

Hey,
Wow, mystery part no more , I have ordered a few HMC720s !!
I've received an ultra high bandwidth scope last week and will absolutely probe everything possible on that motherboard

So, on a systematic approach:
      1. Check Oven and signal path up to the synth.
      2. Check VCO
      3. Check Synth and Buffer
      4. if signal (+/-) is present at Buffers output (both outputs) then:
              Check  SPI comms.
      5. Check FPGA/ASIC 

Last week the same story repeated, one other MSOS804  broken with the same problem, and also a few weeks ago another MSOS404 same problem.
I wonder if Keysight uploads some artificial errors (functions) with updates on such scopes

[BeamDump]

Hi

Was thinking the same, that a flag went high somewhere during an intermittent event or maybe with an update. Can't speak about Keysight but one known offset printing quality control system was caught doing so.

So far swapped the n-synth and dual buffers, same thing. External ref also didn't show anything unfortunately.

What's the date on yours and what soft revision are they running?

[HighVoltage]

Last week the same story repeated, one other MSOS804  broken with the same problem, and also a few weeks ago another MSOS404 same problem.
I wonder if Keysight uploads some artificial errors (functions) with updates on such scopes

Too many problems with these S-Series scopes.
I always wanted one, maybe that is not such a good idea anymore.

Thanks for all the great info in this thread.

[Extrasolar]

Hi,
I dont recall the version and to get it I would have to reassemble the whole thing.
HMC720 chips on the way (the percs of being ADI employee ).

[BeamDump]

Hi,
I dont recall the version and to get it I would have to reassemble the whole thing.
HMC720 chips on the way (the percs of being ADI employee ).

Some perk there 
I ordered 2 my self. Price is really something on these. Then again, not your run of the mill part.

With that, would like to advice to stay away from the Chinese. Good thing i ordered two sets of NC7WZ17P6, one from their bay, other from sets of evaluation boards. Wondered why theirs didn't work. Just a case with pins sticking out and nothing inside. After i bought the 1st 5 pcs for 3EUR, price jumped to 42EUR.

[Extrasolar]

Hello,

I've replaced the HMC720 and now the PLL2 is intermittently locking. which means that the level of the signal is low despite being on the right frequency.
There is a dual buffer that is on the OCO output (50MHz) which seems to be bad, as the output drops a lot compared to the input.
I've discovered that the output from the VCO is bad (see photos).
I was probing with a high bandwidth oscilloscope about (6GHz), and I could verify the presence of the signal beyond the dual buffer at the output of the Synth.
So I am suspecting 2 things now, VCO improper output and maybe some sort of buffer for the PLL2 beyond the HMC720.
Will check it later this evening and then will also check the Synth SPI coms.

[seebeck]

The same old problems would surface, Keysight should pay attention to his product quality！

[Extrasolar]

Hi guys,

Hmm, I brought it back to life, well, for a short while !!
Most definitively, its a power issue. We have been looking in the wrong place so far.
Replace all the Power Rail Caps ( tantalum mostly ) around the timebase !! I have ordered a bunch and I'm impatiently waiting to perform the recap

Cheers !!!

[Extrasolar]

Second Unit in Operation after 1 Hour still no PLL2 Error !!

[HighVoltage]

Nice find.
Power Rail Caps, especially tantalum types are known to fail in test equipment. These should be visible in the ripple of the rails.