Modifying an HP-8711C/HP8712C network analyzer from 75 to 50 ohm.

[prutser]

I was able to buy an HP-8711C for not too much and could change it into a 8712C which has VNA capabilities. Unfortunately it is a 75ohm unit and I would like to use it for 50 ohm.

I wonder if there are other people who looked into this or did change it.  As far as I could see it should not be too difficult:

- The RF in port is internally 50 ohm, the only diffrence is the 'N-SMA Connector' at the front. It has a built in series resistor of 25 ohms.
  So with replacing this, the input port is covered. (maybe this with a minimum loss pad for the other port is a reasonable compromise)

- The transmission/reflection port is more difficult. As far as I could see the directional couplers need to be replaced for 50 ohm versions, but they are quite standard and available.
- There are a few (~ 6) components which need to be changed (different value, I compared the partslist of the 50 ohm and the 75 ohm receiver PCB.)
- The most difficult part: SMA connector on the PCB and the rigid cable to the front have to be changed for a 50 ohm version.  (original probably hard to get or expensive)
- Software wise, it 'thinks' it is a 75 ohm unit. On the receiver board are  2 ID pins, one is tied to Vcc and the other to ground. that has to be swapped for 50 ohm.
  probably the software then recognizes it as a 50 ohm receiver board.

Anyone also looked into this, or tried it ?

   

[eDan]

Hi,
I came across your question regarding the conversion of a HP8711C from 75 ohms to 50 ohms.
Did you solve your issue?
I am trying to do the same and thought we might be able to correspond.
I have a lot of documentation but not for the specific unit I have. The lowest unit which connects to the source output in my
unit is a 8712-60055 as printed on a sticker on the PCB. I note that if the
hard line to the S1 terminal is to be 75ohm, then the connector on the PCB also needs to be replaced.
Which leads to the question are there other components which should be replaced?
Tough to know without the schematics.
dan

[rainbow]

I have an HP 8713C 75 Ohm and I would like to make it 50 Ohm.
Any progress?
Thank you

[lugaw]

This guy replaced a few parts and he now have a 50ohm input.

[rainbow]

This guy replaced a few parts and he now have a 50ohm input.

Thanks lugaw. Seems feasible for the 8711C but the 8713C should have an HP directional coupler that you need to change and it costs a lot. Maybe it has an internal 25 Ohm resistor and nothing more but I can't find the service manual with schematics (only text).