Electronics > Repair

HP 8720C VNA - S2, S3 20GHZ Microwave Switch control voltages?

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Hello. I recently picked up a HP 8720C Network Analyzer on Ebay. It was listed as "dim screen" and "phase lock failures". I fixed the screen via some pot adjustments and I have tracked down the phase lock failure to two RF switches that are not letting through the YIG Oscillator output signal. I looked up the switches but I cannot find the exact model datasheet online. However, I found one on kratosmed.com that looks the same except that it is marked 2677 rather 1699 that my switches have printed on them. I suspect it is a compatible unit but i cannot be sure. I'm not sure anyone would know if I contacted them directly and asked. Perhaps its worth a try.


The above kratos RF Switch states that a port is turned on by setting its LGC control pin to 7V. It is turned off by setting the same pin to -1V.

I measured on my 8720 system and it is feeding -4.53V to the LGC pin for J1 and +13.03 to the LGC pin controlling J2.

Also, these control voltages don't change when I set the 8720 into either low or high band when in CW mode. They also stay the same when sweeping the full 0-20 GHz.

I hoping that someone here has some deep knowledge of these units and can confirm whether the control voltages are bad (and possible have burned my RF switches).

I have attached some relevant information.


More attached files.

Attached application notes explaining internal workings of these RF Switches.

One more piece of information: The control voltages are coming from the interface control board as shows in the attached picture. I will remove the board and see where the vias lead to.

Update: The RF switches are controlled via the two 1858-0008 DIP-14 ICs on the Test Set Interface Board. I have Googled and I cannot find any reference to these ICs. It seems Motorora made them custom for HP? Image attached.

Edit: Found a reference to this part here:

It is a Motorola SPQ526X. FETs. I have yet to find a datasheet...

Edit: I found that an equivalent part number is MHQ6001. The ICs contain two NPN and two PNP transistors.



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