Peculiar case of R&S SMP03

[CrazyTiger]

Hi All,

I was passed down a scrapped R&S SMP03 with failure written down as YFOM failure during internal Cal, somewhere around Mid November last year. When I first got the unit. The instrument was able to pass the internal adjustment, level and frequency were spot on all the way to 27 GHz.

End of December 2023. I moved all my instrument to a temporary rack, as i was doing the flooring at my room. During this time, to save space on the rack, all the instrument was stored on  its side instead of its normal resting position. Mid January I started to move back things and started to power up instruments. This time the SMP03 came up with YFOM failure, FM coil warning.

WARNING 134 FM input of YFO module overdriven;
ERROR 180 Calibration failed; YFOM: pretune;

So I started opening up the instrument and my suspect was the FM coil of the YIG oscillator itself.
The instrument levels were still good, but the frequency was off by around 60MHz. A dead FM coil would lead this, as the main coil handles the coarse setting and FM coil will do the fine adjustment. According to the service manual the FM coil should measure somewhere around 1.5 ohm, mine was measuring 0.5 Ohm(not sure if the manual is correct on this). So my theory was a shorted FM coil could do it. A shorted YFOM coil will draw higher current, so the warning made sense for sure. Removing the control voltage of the FM coil, took out the warning. Further proving the FM coil is indeed over driven.

But still there was the case of why the instrument worked when i received it. So i inquired how the instrument was stored in the first place. Prior to error, the instrument was in storage for long sitting on its side. So when they tested, it has failures. Then it was awaiting to be scrapped on its normal position before it came to me. Which made wonder if I rest it flat, would that fix it. So the entire last week it was siting on its default position. Today morning I powered it up. It passed all internal calibration and frequency is spot on.  So seems like God of Gravity did some magic.

I could live with this, but  a failure like this eventually will be permanent at some point. I tried to very gently knock the yig, to see if that changes the state, seems like it does nothing. I am yet to measure the fm coil resistance again. Another post by member in the forum measured about the same as me, but seems like he is facing same problem like mine. Any ideas?

https://www.eevblog.com/forum/testgear/rs-smp02-err233-ypll-unlocked/

[m k]

Tilt it only a bit and see what happens.
Not very effective if you must wait weeks before next set.

Maybe it's a secondary thing.
There are sort of heavy parts and pcbs are sort of wide.

My experience is from lower frequency things but a connection problem can be *very* delicate.