Electronics > Metrology

Keysight MSOS104A Repair Attempt

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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.

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.

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.

 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. 

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  :D


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