The PCBs contained 4 gain blocks each of which is a discrete current feedback amplifier. As far as I know this is the first time this CFA topology was used in a commercial audio product anywhere.
This photo is of the most recent version of the CPR2.

Older PCBs can be upgraded to the latest standard. This involves fitting high spec. film capacitors to the RIAA stage and trimming the RIAA resistor values for greater precision. The stages are optimised for minimum noise. The jFET muting is protected by using shunt TVS diodes. Coupling capacitors are special low ESR types.
If you have an older PCB the best way to upgrade it is to send it in for an exchange with on already upgraded one as this is a lot quicker. Please contact me for pricing.
610-35 Version (Early)
This is a 1981 PCB, note the plethora of electrolytic capacitors! These can be updated quite effectively but do need a quiet PSU. See below.

CPR2 (later) version
See her for information on muting faults How to Fix Mute Faults on CPR1 &CPR2 – Brian Powell Audio
Power Supply Suggestion for this PCB .The early PCBs had zener diodes as crude shunt regulators and are of very poor performance. It would be better to supply the PCBs by means two series regulators such as LM317 and LM337 from a raw DC of +/- 25V. The output voltage should be set for +/-9V up to +/- 11V to stay below the zener voltage. Alternatively remove the zeners D5, D6 from to PCB to use up to +/- 15V for the PSU.