Viavi CellAdvisor JD785A repair - Page 1
Forum section: electronics / repair

author: nielsm
author_rank: Newbie
timestamp: January 17, 2022, 04:58:23 pm
content: I have a Viavi CellAdvisor JD785A that is failing a RF board test and suspect that the front end has been damaged as measurements are around 80dB below where they should be.I'm going to try and repair the RF board myself and removed the top of the case that holds the keyboard and screen. The RF board appears to be behind the main module and VNA boards. Has anyone disassembled one of these units before? What's the best way to to remove these boards and access the RF board?I've attached a photo showing the main board.

author: nielsm
author_rank: Newbie
timestamp: January 18, 2022, 02:41:47 pm
content: I've now removed the main board and the CAA (cable antenna analyser) board which sits underneath it. I marked the MCX connectors with a felt tip pen so that I don't get them mixed up when I put the unit back together again.Next step is to remove the RF board so that I can get access to the PCB.

author: Qw3rtzuiop
author_rank: Frequent Contributor
timestamp: January 18, 2022, 04:28:01 pm
content: 80db is a lot and i think that probably makes your repair easier.You can also take a look if its a software issue like this one:https://www.eevblog.com/forum/repair/jdsu-jd745a-base-station-analyzer-repair-log/msg2993802/

author: nielsm
author_rank: Newbie
timestamp: January 19, 2022, 11:22:36 am
content: Thanks Qw3rtzuiop, I took a look at that topic before I started disassembling my unit and it gave me some good indicators on what I could expect.I measured the return loss of the input port of the CAA unit using a RigExpert AA600 antenna analyser and its better than 20dB from 1MHz to 600MHz. The return loss on the input port of the RF board is extremely poor at around 0dB.A couple of the components around the RF input connector appear burnt.D15 doesn't appear to be in place, maybe it was never there?D14 and R179 look like they've been destroyed?Not sure what U81 is. It only has 2 marked on the case.Does anyone have any idea what D14 needs to be replaced with? I guess R179 needs to be 50R as it appears to be directly across the input.Thanks in advance.

author: nielsm
author_rank: Newbie
timestamp: January 20, 2022, 05:09:05 pm
content: Made a little more progress today. Removed R179 and cleaned up the board. It looks like D14 was not in place either.I cannot figure out what U81, U62 and U78 are. Does anyone know what type of device they are? The total resistance across U62 and U78 is 80 ohms and 50 ohms to ground on each terminal. U81 appears to be shorted to ground and has probably failed but not sure what I could replace it with.Hoping that someone will be able to help.

author: nielsm
author_rank: Newbie
timestamp: January 27, 2022, 05:48:56 pm
content: I think that U81, U62 and U78 are fixed chip attenuators.Does anyone recognise these and possibly the manufacturer? I may have to replace U81 which I think should be 2dB.Thanks.

author: Qw3rtzuiop
author_rank: Frequent Contributor
timestamp: January 27, 2022, 07:53:11 pm
content: Could be one oft those:https://www.smithsinterconnect.com/products/rf-mw-mmw-components/resistive-components/fixed-attenuators-small-signal/Especially the TS03 series looks similar

author: benj38
author_rank: Frequent Contributor
timestamp: January 28, 2022, 07:42:56 pm
content: Indeed, these look like fixed attenuators.If you remove an intact one you can easily measure the resistance between each of the terminals and figure out the exact attenuation (these are usually Pi pads, and many times you can actually see the three resistive elements at the bottom of the chip) in dB and see if it matches the number on the chip. If it matches you can be very confident that you have figured it out. All that remains then is to estimate the wattage which can be done by looking at the physical dimensions and comparing to the current offerings in manufacturers' catalogs.

author: nielsm
author_rank: Newbie
timestamp: March 08, 2022, 02:16:02 pm
content: R179 (50R) has been replaced and I had previously removed R134 (510R) for testing so I replaced that as well.After several weeks of waiting for components I received replacements for U81, 2dB chip attenuator and U50, GaAs SPDT switch. I took them to a local electronics repair shop but unfortunately they could not remove either U81 or U50 with their hot air gun rework station, probably due to the large ground plane. I'm not sure what to do now as using a Dremel tool or similar to remove U81 and U50 may damage the board beyond repair.I think I may reassemble the unit and continue to use the VNA part as that was working OK.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 08, 2022, 02:33:59 pm
content: You need to preheat the board. That will make removal of the components much easier. Heat the whole board to around 150C then hit the components you want to remove with the hot air, they'll usually come off no problem.

author: kissmu
author_rank: Contributor
timestamp: May 24, 2022, 11:35:57 am
content: I have a similar demaged unit.I will got it in the next days.I think my unit has same issue than the Nielsm unit. (based on ebay description)But i couldn't identify the correct part number of U50 switch ic.Can anybody help me?Thanks.

author: Amphion
author_rank: Contributor
timestamp: February 05, 2023, 06:42:29 pm
content: I’m looking at getting one of these with a failed CAA board. I’m curious if anybody has success here?