George,
Do you have the PL1000 Service Manual? If you are doing the PL2000 Series I or II, there are three OPAMPs to deal with. Z1 is a RC4739 Dual OPAMP and only amplifies the Phono input to the rest of the circuit. Z2 is also a RC4739 and only amplifies the Rear Channel outputs of the PL2000. The key OPAMP is Z3, which is a RC4136 Quad OPAMP, as you know.
All you need is an oscilloscope to see the high frequency oscillations. Just put it on the Main outputs with nothing on the inputs at first. If you see oscillations as you change the time base, with the voltage set very low (millivolts scale), then you can check the output of each OPAMP to find the offender. If no oscillations appear with no input signal, try a 2kHz sinewave signal at 200mV (not the Phono input, that one only needs 2mV). Adjust the scope to magnify the sine wave to look for "ringing" on top of the sinewave signal (it will look like fuzz).
The fix is to install one grounded 100nF capacitor on each rail near the DIP socket (+pin 14 and -pin 7 for the RC4739 and +pin 11 and -pin 7 for the RC4136) as Glen suggested, but the PL2000 OPAMP rails are short, so there is not much chance to pick up oscillations from rail loops.