Electronics > Repair

Viavi CellAdvisor JD785A repair

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nielsm:
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.

nielsm:
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.

Qw3rtzuiop:
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/

nielsm:
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.

nielsm:
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.

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