HP 8753D CAUTION: NO IF FOUND, need help

[wd5jfr]

Just got this 8753D with options 002, 006, 010, 1D5.   The R CHANNEL  jumper was sharply bent at one end but not shorted or open.  The VNA powers up immediately to a working display with  "NO IF FOUND:  CHECK R INPUT LEVEL".  Later I found that after I press PRESET it displays: INITIALIZING INSTRUMENT, PLEASE WAIT  and then after a few seconds SELF TEST PASSED. Then I replaced the bent jumper with a 4" coax with no change visible.  After a bit of reading I decided to try a Minimum R Channel Level Test from the service guide.  The first try I got REFERENCE VALUE -70 dB and then TEST PORT POWER -55 dBm.  I soon came to the conclusion that I had no idea of what I was doing so here I am looking for some guidance.  I posted some pics on groups.io  HP
Thanks
Hank

[mankan]

Hank, the service manual is your friend, however you must know it is written to identify the faulty module. As a hobbyist you cannot buy new instrument modules based on old service manuals. For the 8753D I recommend reading the 8753E manual instead but doublecheck details in the 8753D version-

To me it sounds like you should go down the source troubleshooting path stated in the manual. But keep in mind that the YIG oscillator and source parts might be perfectly fine and you end up like me that one of the sampler gains had to be increased a bit more than the service manual said.

[wd5jfr]

Thanks mankan, I'll open it up and see what I find.  The unit came with APC-SMA adapters both male  so I've got order some cables and adapter as one of my old cables is bad.  I've  an 8753E service manual on-line but no 8753D to be found, so if anyone can guide me or has D service for sale, please let me know.
Hank

[mankan]

Here you go: http://www.ko4bb.com/getsimple/index.php?id=download&file=HP_Agilent/HP_8753_Network_Analyzer/HP_8753D_Vector_Network_Analyzer__Service_Manual-08753-90405.pdf
I also recommend downloading the CLIP: http://www.ko4bb.com/getsimple/index.php?id=download&file=HP_Agilent/Agilent_8753D_Network_Analyzer__Service_Manual-8753DCLIPmanual.pdf

[wd5jfr]

Opened it up, seal still on the top screw, looks nice and clean inside with A11 board red led on for phase unlock.  Down loaded the manuals, thanks for URL.  I've been there in the past but Google didn't find it when I searched.  Now can do some trouble shooting.  I wonder if there's a common part failure for this light?

[Michael-VK2BEA]

Yes, the common failure part is the YIG oscillator (HP 5086-7934). Unfortunately there's no easy fix and replacements (donors from other units) are very expensive.
(there is a wire-bonded transistor that would need to be replaced if you had the transistor and wire-bonding equipment)
I'd see if you get any output from that oscillator before jumping to conclusions though!

Michael