Batboard

I like to listen with an open PL but that seems to open the low pass filter so I hear the PL buzz.
Is there a way to turn on the low pass filer (block low frequency) without activating the PL decoder.

Once you activate the 'monitor' function, you actually turned OFF the PL decoder in the radio, so you are now in CSQ (carrier squelch) which doesn't cause the PL decoder to function.

This buzz you mention is probably caused by some other malady in the receiver chain.

If you are accessing LAB software and digging into the real core of the radio and making changes to areas in the receiver that you should not be making, or changing values without the aid of proper test equipment, then you are going to have a radio that will no longer be able to operate properly, and stay within the FCC bandwidth for your model.

Areas you should never 'play' with are modulation acceptance, transmitter PA bias, receive filter adjustments or anything else you can't properly test for as these can horribly mess up the radio's operation, IMD performance, spurious emissions and adjacent channel rejection to name a few.

Also, you should never change the VCO crossover frequencies UNLESS you are replacing the transceiver board with a newer one, and your board is a newer revision that requires changing the VCO crossover points in both the receiver and transmitter. This is explained in the service manual also.
This will cause the radio to lose tuning range, frequency errors, and channel bandwidth, not to mention a loss in TX power over the factory designed bandspread, and can increse co-channel interference from the transmitter's spurious emissions.

Of course, the 800/900 band radios do not use VCO crossover adjustments, only the VHF and UHF models use this setting.

There are numerous SRN's covering this in the service manual for the Jedi series radios.