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Network analyzer repair tips

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sreed

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phase lock cal failed

Hi,

Is anyone out there fimiliar with the repair of HP 8752A network analyzers??
I have one unit on my hands which needs to be serviced and I'm looking for some tips.

Thanks
 

pit

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phase lock cal failed 8720es

Hello!

I have operation and programming manual, some error messages are explained there. I can upload the pdf if required, but I have no service manual.


CU
 

E-design

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service refurbish network analyzers

There's no customer service manual with circuits available. You can go onto agilents site and download the manual with fault finding and cal procedures. This will only help you to identify the faulty module or card. The only diagram that the manual provides is a block diagram. What is the problem with your unit?
 

sreed

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hp8752a service manual

Hi,
The problem with my unit is when it's turned on with
no inputs the display is distorted. But on another unit under the same conditions, a flat line was displayed.
Thanks
 

sreed

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8753a repair tips

Thanks. I will try it.
 

pit

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network analyzer repair

sreed said:
The problem with my unit is when it's turned on with
no inputs the display is distorted. But on another unit under the same conditions, a flat line was displayed.
A more or less flat line at a very low level is ok. If it cannot receive anything it should measure nothing.

What happens if you connect a 50 Ohm load to the input? And what happens if you connect the RF output to the input? Set output to a moderate output level (0 dBm) and make a sweep, you should get a more or less flat response.

What happens if you are calibrating the device? Do you get a calibration error or any other error message?


CU
 

sreed

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network analyzer phase lock lost

Hi Pit,
I have not tried connecting any load or inputs.I'm trying to get a load from the customer for testing purposes. When unit is powered up the message below is displayed:

"Caution: Phase Lock Cal Failed"

Does this tell you anything.......Thanks
 

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phase locked lost network analyzer 8720es

There is a procedure in the manual that you may follow to find out what is causing this condition. Can be quite a few things, from simple cal to something more serious
 

sreed

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failed to phase lock

Is there a website on which this manual is psoted? I can't find it at www.agilent.com.
 

pit

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hp 8752a manual

sreed said:
When unit is powered up the message below is displayed:
"Caution: Phase Lock Cal Failed"
You didn't mention this yet, this changes a few things.
Is it possible that someone applied too much input power and destroyed the mixer? Or did this error message appear during regular operation?

Some of the HP network analyzers have a built-in time bomb: they have a CMOS RAM which is supplied by a Li battery. That CMOS RAM contains important information like serial number of the device, configuration parameters etc. This CMOS RAM also stores information about additional features of the device etc., for example if TDR is enabled, BASIC enabled, which frequency range the device may use etc. If that information is lost due to low battery the device is dead.

I don't think that is the case with your device, if the CMOS RAM failes there is a different error message normally.


CU
 

sreed

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network analyzer service manual

This unit was used on a production line and as such nobody realised when the error message popped up.If too much power was applied do you think the fault might be on the phase lock board?? If so which component would most likely be affected in your experience.I have pulled out the board and need a place to start checking...Thanks
 

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network analyzer repair services

sreed said:
If too much power was applied do you think the fault might be on the phase lock board?
No, if too much input power was applied something in the RF section is broken, probably something in the first mixer stage. As the receiver is then blind (it doesn't see any output any longer) the PLL calibration failes as a cause of the primary fault.

You can check the following:

Set RF output to 0 dBm and look if there is any RF coming out of the transmitter. It should be a sweep according to the settings of the device. If this works apply the 0 dBm output to the input with a simple N cable and check through the mixer stages and the IF down to the final demodulator where the information is lost. As the 8752A is a VNA it should have two receivers for I and Q vectors, so you should check both in every direction (for all four S parameters).

You can place a switchable damper (about 6 or 10 dB) between RF out and RF in and switch it while you are measuring the internal signals, so you can see the deviations.

Be aware of the fact that failures in the RF section can be VERY expensive in HP equipment, so under certain cirumstances a used device in good condition might be cheaper than repairing the existing one.


CU
 

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