Hello John,
A bit of background to the 80KHz low pass filter (LPF):
I wanted to preserve the original appearance/function of the HP334A & make it easy to remove the filter to restore it back to its "pristine" state. That meant no additional holes on the front panel etc. So, I looked where I might be able to be put the 80KHz filter in/out switch. I found a "dummy plug" adjacent to the IEC power plug on the rear panel. Whilst this wasn't the most ergonomic position, it would work for me with the way I use the HP334A (reaching around the back to turn the filter in/out).
The filter works in conjunction with the existing 400Hz high pass filter (HPF) i.e. in the same signal path. The latter only operates in the "Set Level" & "Distortion" & NOT in "Voltmeter" mode which means the new filter does likewise.
The existing 400Hz HPF is a passive LC type (3 pole) between the rejection amplifier (A3) & the meter amplifier (A2). The meter amplifier has a 600 ohm input impedance attenuator (S3R1 - S3R11) which terminates the 400Hz filter i.e. the 600 ohm input impedance forms part of the filter design. The new 80KHz filter could either be an active filter (using ICs or transistors etc) or be a passive filter. In either case, it must be able to operate into 600 ohm as well.
In the end, I chose a passive filter (2 pole) because of its simplicity & I had the parts on hand. With the signal levels (300uV to 0.3V RMS) & the inductors used, I didn't want to put it next to the IEC power plug with the filter in/out switch (noise/hum injection). So, the in/out switch operates a 24V relay which switches the 80KHz in/out. The relay draws about 8.5mA (0.2W) which represents a 5% increase on the HP334A power consumption (4W) which should be OK...
The relay simplifies the whole layout. It allows the 80KHz filter board to be colocated close to the existing 400Hz filter & the adjacent A3 board whilst allowing flexibility as to where the 80KHz filter in/out switch is located.
I'm not sure how to attach images "in-line" but there should be 7 images attached.
"01_80KHz 2pole LPF for 600 ohm schematic.jpg" is the schematic of the 2 pole filter designed to be terminated into 600 ohm. Apologies for it being hand drawn, remember it was one off. The 24V relay is a RY-24W.
"02_80KHz LPF integration schematic.jpg" is an extract of page 7-7/7-8 of the manual showing how the new 80KHz filter is placed in series (upstream) of the 400Hz switch/filter. It looks messy but it basically describes the transposition of the blue wire between A3 board & the 400Hz switch to the 80KHz filter board. It will make more sense when you look at the next image.
"03_80KHz Filter Wiring (marked up).jpg" shows the 80KHz filter located in the same internal cavity as the 400Hz filter. The blue wire has been moved from A3 pin 2 onto the 80KHz filter & a new purple wire from the 80KHz filter going back to A3 pin 2. This is how the 80KHz filter has been inserted in series (upstream) with the original 400Hz filter.
"04_80KHz Filter +25V wiring (red).jpg" shows how the new 80KHz filter derives its +25V for the RY-24W relay.
"05_80KHz Filter In_Out Switch (Internal).jpg" shows the inside view of the filter 80KHz in/out switch. It's a "monster" switch only because it needed to fill the existing hole in the rear panel.
"06_80KHz Filter In_Out Switch (External).jpg" shows what the switch looks like from the outside.
"07_Theoretical 80KHz 3 pole LPF (didn't have parts).jpg" are the components for a 80KHz 3 pole LPF (600ohm) if you have them (in lieu of the 2 pole).
I hope this helps...
Alan