Sansui 1000A CRS02 filter replacement - restoration

The Sansui 1000A tube amplifier (each channel) contains three RC filters to manipulate low and high tones, labelled as: CRS02,
CRS-10A, and CRS01 (see schematics below). They have significant influence on sound quality, since the signal goes through them. The CRS01 and CRS-10A may be bypassed by LOW FILTER and HIGH FILTER switches, so they can be left unchanged. The CRS02 can't be bypassed and cooperates with bass and treble potentiometers.

Fig.1. Sansui 1000A RC filters in amplifier section

The original CRS02 (Fig.3) is an integrated ceramic circuit. It contains 3 resistors (2x100k, 15k) and 3 capacitors (150pF, 3nF and 30nF) as shown on the schematics (Fig.1). Since the CRS02 circuit is over 50 years old and probably contains elements of lowered parameters now, I have decided to replace it with a new, self-made one containing descrete elements of good quality. Most important are the capacitors. I've used polypropylene caps from WIMA (150pF, 3.3nF and 33nF, all 100V). To build the circuit I've used an universal PCB as shown on Fig.4.

Fig.2. Empty space between potentiometers after removing original CRS02s.

Fig.3. The original CRS02 ceramic filters removed.
Pin numbering as on the schematics (yellow spots).

Fig.4. New CRS02 filter made of modern descrete elements on an universal PCB.

Wires in new circuits should be as short as possible to reduce hum, but the main source of hum (and noise) in Sansui 1000A is from cables, switches and potentiometers placed before the grid of the first V18 (12AX7) triode. Unfortunately, we don't have big influence on reducing it without modyfing the original construction.

To protect the circuits, they have been wrapped with an insulating tape, but also heat-shrink tubes can be used.

Now numbering the cables as on the original circuit (Fig.3) and mounting it inside the amplituner...

and the second one:

That's it!

Hearable results after replacement

Basses are deeper and many subtle details are heard. Noises seem to be lower.

WARNING! Beware of HIGH VOLTAGE inside the amplituner! All you do, you do on your own risk!