Sense Of R174 And R175; Hum Problems; Increasing The Gate Voltage To +12V - Spectral Audio ProTone Service Manual

4.3 Sense of R174 and R175

These NTC resistors improves the temperature stability of the two VCOs a lot. It is important that, if
you add these NTCs, also change the resistors R81 and R86 to 5k1. Use hot glue to glue them
onto U16. Sold them parallel to R81 and R86:
After this change, a readjustment must be done.

4.4 Hum Problems

In some circumstances, the internal connection from GND to the metal case creates a hum noise
because of closed earth loops.
In this case it may help to cut this internal GND connection.
You will find it (on later Proton's series only) on the digital pcb, a pretty wide track to one screw. Cut
it with a sharp cutter.

4.5 Increasing the Gate voltage to +12V

To increase the Gate voltage from +5V to +12V, two additional resistors and one NPN transistor are
necessary.
Apart from this, port 1.3 (pin 4) of the microprocessor must be connected to GND that the gate
signal output from the microprocessor is inverted.
The circuit change appears as follows:
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