Electronics > Repair

JDSU JD745A / GC747a Base Station Analyzer - Repair log

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Qw3rtzuiop:
Hello,

i have a broken JD745A aka GC747A here which i will try to repair.

It is a 4GHz mobile spectrum analyzer with decoding and vna functions.
Unfortunately i didn't find much resources on the internet.  If anyone has some sort of service manual or block diagramm i would be happy.

The major defect is visible on the two images. The displayed level is way to low except for the range from 20 MHz to 105 MHz.
And if the preamp is on that range changes up to 680 MHz.

I started to draw some diagrams to get a better understanding of this device.

Despite this problem the connectors are pretty unreliable. The contacts are dirty or even corroded.
Maybe some IPA will do it. Otherwise they need to be replaced.
A movement of those cable will result in a reboot of the unit which needs some time.

Qw3rtzuiop:
I was able to remove some loose connections.
Now its time do take a deeper look at the rf-frontend.

Today i've just done some research and collected datasheets for the main components.
Next thing to do is drawing a block diagram.

Qw3rtzuiop:
After some more experiments it turns out to be a digital problem |O
Even with a disconnected ADC input the problem remains. Im suspecting broken calibration tables.
Unfortunately i didn't find a way into the memory yet.
Telnet and FTP are secured by unknown passwords. I don't know if have a chance by listening to the JTAG ports of the FPGA and CPU.
My JTAG experience is non existing but I'll try it later on.

testpoint1:
such meter from the JDSU actually is from Gencomm of Korea, so the RF circuit is quite fair, the performance may not so good (speed and dynamic range etc), also I found some flash eprom corrupt problem, for your case, just check which chip include the data you need, copy from a normal meter is OK.

Qw3rtzuiop:
Major breakthrough update!

In the meantime i desoldered the nor flash to take copies of them. I didnt succeed to analyze them but thankfully @dxl was able to do it.

He also pointed out some possibilities to get root access. The device runs the file recovery/recovery.000 with root privilege from an attached usb stick at startup in order to start a recovery of a bricked device.

That way i modified the etc/securetty to allow the root access over telnet.
I attached the root telnet "script" and the contents of the /gencomm/cal folder. If someone has a working device i would appreciate if she/he can share their contents of the /cal folder for comparison.

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