HP8568B Issues with levels and bandwidth settings - Page 1
Forum section: electronics / testgear

author: Sambo521
author_rank: Newbie
timestamp: August 21, 2019, 07:09:42 pm
content: Hey Guys and Gals,Just acquired a HP8568B Spectrum Analyzer and it was in tough shape. Missing the interconnect cables and the A4A7 Card (Ordered one from Fleabay).Bought some cables off fleabay (Ouch) so I could power up the thing.What I have found is that it will receive a signal but the display shows about -60Dbm with an input of -10Dbm.Also, changing the Resolution Bandwidth affects the displayed signal level fairly severe.I also noticed that the signal completely goes away when you change from LOG to LIN.Sounds like multiple issues to me and I'm still learning how this thing works.Oh and I also noticed that the step gain display is kinda weird, the first two steps are very shallow in respect to the rest of the staircase. I did check the control lines and found them switching properly.I will be checking the RF section today using my Rigol to verify stage gain and go from there I guessLooks like I have my work cut out for me.Any thoughts or suggestions would be appreciated

author: tkamiya
author_rank: Super Contributor
timestamp: August 22, 2019, 01:29:31 am
content: I'm just responding to say, if you need parts, let me know! I have a parted out 8568B and I'll be glad to send whatever you want to you for the cost of shipping.Level could be the attenuation. Bandwidth could be wacked alignment. Looks like the previous owner(s) were not so attentive, so anything is possible.

author: 0culus
author_rank: Super Contributor
timestamp: August 22, 2019, 05:59:57 am
content: Dead attenuators happen. If you poke around the HP-Agilent-Keysight mailing list on groups.io you'll find some posts about rebuilding them. If it hasn't been blasted by some idiot transmitting into the front end (sounds like, however, that's not outside the realm of possibility given the shape it's in), there are tiny rubber o-rings that serve to cushion things from the solenoids that disintegrate and cause things to not work right anymore. The 68B attenuator is fairly easy to remove, just need to pull the front panel. Carefully open it and examine the resistive pads with good magnification for obvious signs of cracking, burning, and charring. Those would be sure signs someone put way too much power in and the attenuator is junk. Possibly the mixer too...

author: Sambo521
author_rank: Newbie
timestamp: August 22, 2019, 01:34:03 pm
content: I think I have a bad A4A8 module. According to the gain per stage chart everything's good from the RF section all the way to the A4A8 module. Losing quite a bit of gain there.Also, looks like the attenuators are working properly as well.Also, the A4A7 module is missing so I would need the A4A7 and A4A8 module to start with.

author: Sambo521
author_rank: Newbie
timestamp: August 23, 2019, 02:13:54 am
content: I could use aA4A8 module. Think it’s faulty.

author: Sambo521
author_rank: Newbie
timestamp: August 24, 2019, 12:11:35 am
content: Okay, the attenuators Check fine , there is no loss between the input and the input stage.What do I check next?I could use a manual on the RF section , shooting blind here.Thanks for all the help so far.

author: tkamiya
author_rank: Super Contributor
timestamp: August 25, 2019, 05:42:49 pm
content: The interconnecting cables are about $200 on eBay. Outrageously expensive.