The HP/Agilent/Keysight 85685A Preselector is a great addition to any 8566B or 8568B spectrum analyzer. The preselector converts the analyzer into a test receiver, by increasing its dynamic range by 30 dB, down to very low noise levels.

Recently, I got a 85685A for repair, only knowing that it doesn’t work as it should. With some checks, it was very quickly evident that there must be a issue with the RF attenuator, or its driver.

This defect is clearly seen when looking at a test signal at various attenuation levels of the 85685A. The signal should stay at the same level, irrespective of the attenuator setting, but as soon as you go from 10 dB to 20 dB, the signal vanishes almost completely. This is not good.

This is the RF attenuator, a Wavetek OEM part. Unfortunately, there is no service manual for the 85685A available, so we need to figure it out by ourselves.

First, determined the switch matrix for the attenuator controls, by probing the control inputs at various attenuation settings. Pretty clear, there are 10 dB – 5 dB – 20 dB – 20 dB segments inside, which are activated by pulling the respective control input low. Easy enough.

After some disassembly of the case (removing the rear panel), you can get access to the four screws holding the attenuator to the case.
Notably, the case of the 85685A uses Torx screws, unlike most other HP equipment using this style of enclosure.

Underneath the label, there is now hidden screw to get to the internals of the attenuator, all is glued closed and sealed with silver epoxy. To break it open without destruction, I milled a small slot from the side of the unit. Probably could have milled a bit shallower, and a bit less, but OK.

With the slot, the lid is easily removed using a screw driver. Make sure not to bend the lid too much.

Looking inside, it is pretty obvious that someone must have fired a lot of power into the unit, when set a 20 dB input attenuation. Checked all other segments with a 8752A network analyzer, and all good, except for one of the 20 dB segments, as expected.

How does a 20 dB attenuator work? There are several topologies, Wavetek used a so call pi-arrangement of resistors. Only two of the resistors are blown, the output resistor is OK (this is also clear from the fact that most power is dissipated in the left two resistors).

The switching of the attenuator segments is done with miniature RF relais, similar to these. At over EUR 40 a piece – glad these are all good.

The relais are DPDT switches, soldered flush to the board (which is a PTFE composite board), for best RF performance.

For repair, we need to replace the resistors with good new parts – but there are hard to come by, with not even a Wavetek datasheet available for the attenuator, let alone, these parts.

Several attempts were made to get the best (lowest) SWR, and the best flatness, at very close to 20 dB attenuation.

First, used a combination of 1206 SMD resistors to get close to the values needed.

This is the flatness of the “good” 20 dB segment:

This is the flatness of the “1206 repaired” 20 dB segment:

Another style of repair, with the same parts, now soldered directly between the legs of the relais:

… no improvement, still quite some reduction of attenuation above 2 GHz.

Now, tried with a series arrangement of 0805 resistors for the 250 Ohm resistor (giving lower stray capacitance).

… quite some improvement!

Red is the good attenuator section, blue is the repaired section, at 0.2 dB/div scale!

I would call it good enough!

A quick SWR test (non-calibrated) for “through” and “actuated” setting of the repaired segment (and terminated in a 15 dB precision 18 GHz rated attenuator at the output) showed low SWR (keep in mind, the 85685A will only work up to 2 GHz anyway).

All sealed up with silver epoxy – a bit old stuff around here, but still working. And, used some Cu tape (with conductive glue, 3M type 3313), to make sure all is sealed well and forever.

Now, with the attenuator fixed and working, one more thing to consider – the power handling capacity. The 85685A is rated for up to 30 dBm (1 Watt) average power. Not sure if the SMD resistors used can handle it – they are a bit smaller than the original Wavetek parts. So I decided to swap the control lines for the two 20 dB segments. This way, the “good”=Wavetek segment No. 3 will always take most of the power, and the repaired section (SMD resistors) will only be needed for the highest attenuations, and never see any more than 10 dBm of power, even at the maximum allowable input. Still, I will keep a search going on a spare 0955-0235 programmable attenuator, for a reasonable price (some of these being offered for USD 100, which is a bit more than I want to spend for a 25 year old part of unknown nature and condition).

Finally, all assembled back together, and performed a flatness/attenuator test, by supplying a signal at -40 dBm from a 8642B generator. Measured amplitude at 1 kHz resolution BW is pretty flat over all attenuator settings and frequencies.

Let me know in case you have any 85685A units for repair….