8753E repair - help needed

[rfspezi]

I got a problem with my old 8753E.
Below approx. 16 MHz the red led on the A11 "phase lock" unit lights up indicating the PLL beeing unlocked.
I then measured the signal on the front SMA connector called "R CHANNEL OUT"and i get the signal rolloff shown below.
Although the curve is not sampled complete, one can clearly see the drop in level.
When i get the service guide right (page 2-31) the minimum level at the R CHANNEL must be < -35 dBm?

Does anybody have an idea where that attenuation comes from or where i should further investigate?
Or maybe somebody could measure his R CHANNEL OUT levels to have a reference?

[rfspezi]

I could fix the problem in the meantime.
Must have been a contact problem at the 6 GHz source unit.
After I disassembled and reassembled it for some component measuerements, it now works correctly and the R CHANNEL OUT supplies a constant -18 dBm signal. 

[rfspezi]

I guess there is still a problem in the super low band (10...300 kHz) of my 8753E?
After preset (no cal), i get the following S11 curve with an open 50 cm RG316 attached to port 1.
Although they write in the service guide that the signal has no ALC...

...The super low band frequencies (10 kHz to 300 kHz) are sent directly
from the Al2 reference board to the output of the A3 source assembly. This
band is not phased locked nor does it use the ALC. It is the basic amplified
output of the fractional-N synthesizer.

... I am not sure if the deviations are within normal range.
Maybe somebody with an 8753 can compare the curve to his own?

Thanks

[piotekp90]

Hi,

Have you solved problem with analyzer?
I have similar behaviour of my 8753E, at frequencies below 300kHz the UNLK LED is lights up, on this topic I attached video: https://community.keysight.com/thread/37013

[rfspezi]

Can you post a screenshot of your VNA with the exact same settings as i had in my post before?

[piotekp90]

Ok I will make a test and put the screen tomorrow. I can make more tests if you describe what tests you want.

[piotekp90]

I have similar curve as you. I have bought this analyzer as non working. Inside was a lot of dust. I disassembled all boards and cleaned it. After this I have replaced the CPU board and analyzer start working but the UNLK led light sometimes....

[mrf245]

RF unit in 8753E seems like same as other 8753C/D series.
Confirm which VCO unlock when appear, try to adjust VCO turning voltage from unlock boundary.

[rfspezi]

I have similar curve as you. I have bought this analyzer as non working. Inside was a lot of dust. I disassembled all boards and cleaned it. After this I have replaced the CPU board and analyzer start working but the UNLK led light sometimes....

Concerning the issue in my initial post, i could fix that - but this is not related to the 10...300 kHz range.
What i did to fix it can be found in this thread: https://www.eevblog.com/forum/testgear/8753-vna-ceramic-circuit-removal-inside-a3-source-assembly/msg1793270/#msg1793270

Since frequencies below 300 kHz are neither PLL nor ALC locked but directly supplied by the A2 synthesizer board, i think it is normal that the unlock LED lights up.
Because our devices both show almost identical curves, i would assume they are working as intended?

[TheSteve]

I can check mine as well next time I am in the lab. I am thinking it is normal though.

[TheSteve]

I can check mine as well next time I am in the lab. I am thinking it is normal though.

Edit - I can confirm mine matches what you have posted, so it is normal.

[piotekp90]

I can check mine as well next time I am in the lab. I am thinking it is normal though.

Edit - I can confirm mine matches what you have posted, so it is normal.

This is a good message:) So I think, we can assume that our VNA are working correctly. TheSteve Could you also check the UNLK LED in your analyzer?

[rfspezi]

If you got a sweep running, it is normal that the unlock led lights up each time the frequency jumps back to the start (and most likely also at the frequency band boarders).
If you set the VNA in CW mode, the unlock LED should stay off as long as you donˋt change the frequency.

[piotekp90]

In CW mode at frequencies below 300 kHz the UNLK is on continously but there is no error message and at frequency around 800MHz is similar.