Keysight MSOS104A Repair Attempt

[Extrasolar]

I found a MSOS104A 1GHz Oscilloscope from Keysight. It was marked as offline and not working so I thought I could try to repair it and eventually picked it up and set it on my bench.
The problem seems to be related to the timebase clock and PLL2 as we can see on the image.
"Timebase error detected in PLL2", perhaps a missing clock, maybe some buffer decided to take a vacation, or the internal 10MHz Oven Controlled Oscillator is bad...
By feeding an external 10MHz Reference and the specified magnitude and changing the trigger type on the horizontal scale, for a brief moment I was able to get a Square Wave on the screen as Ch1 was connected to the Cal terminal on the front panel. So, maybe there is a bad buffer or routing mechanism for the clock (I'm just assuming at this point) that is intermittent...
There is absolutely nothing online on the public domain except for a "General Service Guide" which is really reduced to only "Board Swap" kind of repairs.
The Acquisition Board looks really intimidating but hopefully the problem might be isolated only to the Timebase section.

[Extrasolar]

So, There are 4 Clock related sections, a clock Ref (10MHz) output, Oven Controlled Oscillator , Main VCO and PLL gen and a negative 11.5V Oscillator made of discrete components.
I've tested the OCO and by powering it up locally and I was able to see an output at 10MHz which confirms this section to be working (also I checked a few buffers and all working).
For the moment will continue to the VCO section.

[Extrasolar]

I tried to do the same with the VCO, 5V rail (was pulling 5mA), Applied a Voltage on the Tuner (PIN2) and the SMU went in Current compliance (it was pulling 100mA at 2.1V) 
To me this is a good indicator that the VCO is bad. I decided to remove it and try to power up in a different environment.

[KE5FX]

 I can't see what the PLL chip part number is, but it's possible that the VCO tuning line simply went straight back to it with the loop filter components in parallel.  PFDs with charge pump outputs often don't use series R, and in that case, you might have forward-biased some protection diodes somewhere. 

[Extrasolar]

I desoldered the lid or the shield of the VCO, but managed to knock out a few components, no problem tho because I got everything back and soldered.
My suspicion that the VCO was toasted got confirmed, the part was pulling nothing, and I was not able to get an output from it.
Here is also the Diagram of the Crystek CVCO55 602934 1702-02 (seems a custom part number as I was not able to find this part online).
I have ordered a few Crystek VCO with output ranging from 500MHz to 2GHz, with the same form factor, lets hope for the best at this point 

[Extrasolar]

The VCO is driven by a LTC6945 Ultralow Noise and Spurious 0.35GHz to 6GHz Integer-N Synthesizer, Neat !! (I have a few of those parts !!)
I was able to RE the schematics of the synthesizer block.
Unfortunately I must wait for the VCOs to continue this repair, but in the meantime I will continue to probe around the Timebase Section, just to rule out any other problem it might have ...
Any help on finding the datasheet of the VCO will be greatly appreciated !!

[gamalot]

The component with 'UA' top marking on it looks like a RF BJT 2SC5606, not a NMOS.

https://vakits.com/sites/default/files/2SC5606.pdf

[Extrasolar]

True about the Transistor, thank you Gamalot !!
I tried today to fix the VCO and kind of succeeded, well sort of !!
The VCO outputs around 840MHz, when powered up correctly and by correctly I mean had to short the capacitor at the base with tweezers !!
It works, but has a lot of phase noise, unstable and intermittent operation.

[picburner]

What you have indicated as C5 most likely is an inductor or resistor instead.
The base/gate of 'UA' could not be polarized otherwise, that's why shorting it with the tweezers the circuit starts to work.

[wraper]

What you have indicated as C5 most likely is an inductor or resistor instead.
The base/gate of 'UA' could not be polarized otherwise, that's why shorting it with the tweezers the circuit starts to work.

IMHO he probably switched some components while resoldering them, thus ended up with capacitor on the base (if it's 2SC5606) as by its looks it's certainly a capacitor.

[Extrasolar]

Hi wraper,

There were 3 parts that were knocked off when I removed the lid.
I opened up a similar VCO and saw the Resistor (820 Ohms) forming a divider with a 1.5k resistor. The inductor is a larger package than the capacitor and there was only one pad that could actually accommodates it.
I think there might have been a resistor as well in parallel(on top) of the capacitor at the base.
Nonetheless, will try to rearrange the inductor to see what happens ...

[gamalot]

Some SMD multilayer inductors look similar to MLCCs.

[Extrasolar]

The VCO is now working. I soldered the inductor to the BJTs base. Its operational from 4.635GHz with 0.5V to 4.703GHz with 4.7V at the tuner.

[emoco]

Hi,
I just got the same warning on my scope. Did the VCO solve the problem?

[HighVoltage]

Nice repair!

Any idea what the failure mode was for the VCO?

[Extrasolar]

Hello,

Sadly the VCO did not resolve the problem, but on the bright side, I now have it characterized completely 
I've swapped the LTC6945 Synthesizer with a new one and still did not work.
I am going to investigate further at the output of the Synthesizer, because somehow I have a feeling that some buffer might have failed   
It has to be for sure a HF switch or buffer at some point, because by feeding an external 10MHz signal, I was able to briefly see the Cal Out quadrature appear on the screen.

Regards

[emoco]

Thank you for the update.

[emoco]

If it is of any help. After having my DSOS104A disassembled, connectors removed, and assembled again the error ""Timebase error detected in PLL2" did not appear. But the message comes back occasionally when the scope is started.

[Extrasolar]

Intermittent problems, hmmm this is a bit problematic, it is a strong indicator that probably a power rail on the PLL2 is in fact intermittent !!
I will check a few signals on the Timebase as I can see the PLL2_Lock LED indicator does not turn on. Again I suspect that some signal is not present.
There is a Fully Differential amplifier that is polarized only by a negative 3.3 rail (very strange  ) that "Buffers/Amplifies" the Synthesizers output (4.64GHz to 6GHz). I have prepared a HF probe to check for the input - output and will post updates soon.

[BeamDump]

Lots of interesting discoveries here. Thank you for putting in the effort.

In the processes of repairing one my self. Same error.

From power ON only the PLL1_lock LED is on but once the software starts PLL2 comes on too.
Measured RF output from the VCO, during OS boot it sits at 4,950GHz, once the soft starts loading and PLL2 LED comes on, frequency jumps to 4,978.900 and oscillates between that and 4,979.800. Not sure how there can be a lock in this case.
5.12V on VCC, 4.6V on Tuner

Any idea where it's possible to get a similar VCO? It's Keysight authorized part and Crystek won't sell without their permission. Have some contact in there, see how that goes.

[Extrasolar]

Hi there !!

I've made some progress on this and reverse engineered the entire signal chain from the OXO to the output of the Synthesizer.
I will be probing around now as I strongly suspect that either the fully Differential Amplifier next to the Synthesizer or the OXO output buffer (ref input of the synthesizer)is bad.
In the case of the first problem, I am putting up together a simple passive RF probe and will be using a Spectrum Analyzer to "see" the signal.
The OXO buffer is easy enough to be probed with a normal scope. One thing that is absolutely needed is a cooling fan on the ADC, otherwise it will get toasted.
Will post the results this week !!

Regards

[BeamDump]

I would say don't run the acquisition board without the fans hitting it's full surface, apart from the ADC every other heat sink and memory get's roasty toasty fairly quickly.

In my case it's definitely the VCO, output is sporadic. Measured in the acquisition board and out. Too bad i don't have a signal or spectrum analyzer that can go over 4 GHz but my 20GHz CSA8000 does the trick to "see" the signal. Interestingly that with such a mess the board still lights up the PLL2_Lock LED.



Ordered a few Crystek VCO's in this range and some LTC6945's for experimentation. Otherwise conversation with Keysight's Finish department has reached a dead end. Part can't be sold and can't be authorized for selling by the Crystek. Instead send the scope in for "repair". Further clarification on "repair" - they don't do any component level repair and acquisition board gets swapped. 

[vtp]

One of the Infiniium S Series scopes in office threw that error some months ago.

It was repaired under factory warranty and apparently is a known problem to KS.

Excerpts from service note:

The Problem - Under certain circumstances a PLL2 error may occur on the S-series products.

ADMINISTRATIVE INFORMATION
------------------------------------
Action [X] On specified failure

Location category: [X] Service center

Availability: Product's support life
No charge available until: December 31, 2022

Just FYI.

[BeamDump]

Probably. A colleague with an S-series had a PLL2 error today. Guess it is plagued with this issue. Wonder if it's Keysight or Crystek to blame. If they are well familiar with this, then why indeed sell a ~50EUR part when they can charge 6K for repairs.

If I'm to believe what I've been told, this is suppose to be 5GHz VCO. 

Desoldered the transistor from my VCO:

[Extrasolar]

Hi,

I can fully characterize the VCO, frequency bandwidth and Voltage control range.
The only problem is that my VCO might be faulty as well 
Last time I checked it was spot on 4.6... GHz and remained stable with no noticeable phase noise, 0.5V to 4.7V Control. (removed from Acquisition Board).
I have attached the Synthesizer RE Schematics below, the circuit if fairly easy to follow... My question is, where is the PLL2 synthesized/generated ??
There is another Synthesizer on the board, slightly different part but overall same functionality (which I haven't checked yet... )