HP8566B input signal level drops by 20dBm - Page 1
Forum section: electronics / repair

author: vincei
author_rank: Contributor
timestamp: January 25, 2017, 03:07:28 pm
content: Hi Guys,I just noticed something weird on my HP8566B. On the 0-2.5GHz range as I move the center frequency up or down thru 2.25Ghz there is a click and I believe that is as expected. The thing I noticed is on the high side of 2.25Ghz my reading are exactly 20db lower than on the low side of 2.25Gz. I've tested this every which way from Tuesday and the problem? is there.I've put 0 to -30dm out of my signal generator into the SA and "selected track signal" with a span of 10mhz and walked the frequency up in 10's of MHz. Once I up thru 2.25Ghz the signal drops to -20 to -50dBm after the click depending on what the signal input level was. I've done the instrument calibration but this persists.I'm assuming this is a failed/failing input attenuator at above 2.25GHz? I haven't looked deeply yet but thought I'd throw this out there before getting in real dirty.If it helps I can upload a youTube video that shows this phenomenon.btw. This is my first ever EEVBlog post so a big greeting to you all.-Vince

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 25, 2017, 04:17:04 pm
content: My 8566A had damage to the input where the rigid coax behind the input connector had a broken solder joint. You could see the input jump by wiggling the connector. I can't remember if it affected the whole bandwidth equally.

author: MadTux
author_rank: Frequent Contributor
timestamp: January 25, 2017, 09:24:54 pm
content: That click you hear is a relay which switches from baseband to harmonic mixer above something around 2.3GHz.The harmonic mixer has a yig tuned filter in front of it which is most probably misaligned. See if it gets better by pressing the 'PRESEL PEAK' button on the SA box.If not, you could try to manually tune the YTF by using a lab power supply. If this also doesn't fix it, either the relay(s) are broken or the YTF/mixer assembly is broken.

author: vincei
author_rank: Contributor
timestamp: January 26, 2017, 12:07:03 am
content: Hi MadTux. Perfect. That fixed it (presel peak). Signal level is the same on both sides of the frequency switch overPhew, didn't really want to get a screw driver out.Thanks so much! FWIW. I love my 8566B. Bought on eBay and almost destroyed by the clowns that shipped it. She's humming away doing good things after some TLC!Once again, Thank you!-Vince

author: vincei
author_rank: Contributor
timestamp: January 26, 2017, 12:09:03 am
content: Quote from: ManateeMafia on January 25, 2017, 04:17:04 pmPhew, thankfully it turn out not to be broken coax. Operator error? Maybe?Hope you got it fixed!Cheers,Vince

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 26, 2017, 05:14:36 pm
content: It was repaired with my big tipped soldering iron. So far it has worked well. I also invested in a torque wrench for those connectors. It was worth the investment and I recommend one if you ever get to that point.A couple other issues I have/had was a bad film capacitor causing the dreaded YTO unlocked message. I still have an issue with the trim pot to adjust the reference level. It has a dead spot so I need to replace it when I upgrade the display to LCD.The section of pcb's sitting in front of the fan get caked in dust so I ended cleaning those boards to keep them as cool as possible.

author: G0HZU
author_rank: Super Contributor
timestamp: January 26, 2017, 05:50:53 pm
content: Quote from: vincei on January 26, 2017, 12:07:03 amIt might be worth trying  [PRESEL PEAK] to set it to the factory default tuning for the preselector.Otherwise, you may find you have made it better in one place but worse in others. My 8566 works at its flattest across the 2-22GHz range with the factory default settings for the preselector tuning

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 26, 2017, 06:03:21 pm
content: Good news. These beasts are a work of art and are best appreciated from the outside. I hope to not go back inside mine for a while.

author: vincei
author_rank: Contributor
timestamp: January 27, 2017, 08:12:36 pm
content: It might be worth trying  [PRESEL PEAK] to set it to the factory default tuning for the preselector.Otherwise, you may find you have made it better in one place but worse in others. My 8566 works at its flattest across the 2-22GHz range with the factory default settings for the preselector tuningOh, cool. I'll give that a try. So many knobs so little time. Or maybe I need to spend more time with the manual. :-)Thanks!

author: vincei
author_rank: Contributor
timestamp: January 27, 2017, 08:17:25 pm
content: Quote from: G0HZU on January 26, 2017, 05:50:53 pmOh, cool. I'll give that a try. So many knobs so little time. Or maybe I need to spend more time with the manual.Thanks!