HP made a series of Dynamic Signal Analyzers that were advertised as DC - 102KHz. While they inputs are DC safe to a limited extent the actual frequency response is 195.31mHz to 102.4KHz. The series included:

• HP 35660A - Introduced 1989 - Retired 1992
• HP 35665A - Introduced 1991 - Retired 2001
• HP 35670A - Introduced 1993 - Portable (More luggable)

The HP 35665A & HP 35660A are heavy devices. From a user standpoint the HP 35665A was nicer not just because of the improved specifications, features and performance but because it added a knob to the front panel which was made with the same tooling used for the earlier model. It also added MS-DOS formatting vs the HP 35660A which only supported HP-LIF disk format.

Because of the wide dynamic range of the instrument it's input has coupling that can be changed from grounded to floating single ended (do not exceed +-4V from ground).

Upgrades

HP/Agilent/Keysight obviously left the business of selling this whole class of device. They still offer service for the HP 35670A but support for the HP 35660 and HP 35665A are understandably over so buying upgrade options for install via floppy disk from HP is also over.

The options (and I suspect serial #) are stored in a socketed I2C EEPROM. Not all features are possible to enable without all the SRAM present in the unit but GLK Instruments offers upgrades via I2C replacement. Another user studied dumps of the EEPROM and documented how to do this yourself.

To add the memory required for all options and work with larger amounts of data the users Sören_Marodören & mightyohm on eevblog forum showed you can replace the memory chips yourself.

Maintence

Warnings: I have seen the insides of 3 of these so far. The following should be checked:

• The power supply has Class X and Class Y mains filter capacitors from RIFA in their clear encapsulation that cracks over time.
• The memory PCB has a lithium battery for the saved data, settings, programs, calibration data, time etc. I have never seen one of these leak but given the age of the devices that battery should probably be swapped out too.

Opening the power supply is tricky as the HP 35665A was expected to be in a high vibration environment so the chassis for the power supply is held closed with both screws and a hard to reach a plastic clamp that connects one of the PCBs to the opposing side of the chassis. To open it a set of long medical forceps is recommended.

If repairs are required on the main board of the power supply be careful as it is a (>2) multilayer PCB with many parts soldered through hole without thermal relief. Preheating is advised.