CHECK THE VOLTAGE ACROSS THE ZENER.
The original MXR Phase 90 derived the bias voltage for everything; the modulation JFETs, the op-amps and the final buffer transistor, from the zener diode. I believe in the originals this used to be 3 Volts. Likely this voltage was chosen because it was just greater than the maximum VgsOff for the modulation FETs and the main purpose of the zener was to provide a stable bias for the FETs. It does mean that the D.C. operating point for the rest of the circuit ran rather below the ideal of half of the supply, or 4.5V. As a consequence the transistor at the output of the pedal was biased to only a couple of volts above ground and the output signal tended to clip against the ground rail. I originally suggested the extra 68K to adjust the bias of the output transistor nearer to half the supply allowing for more signal swing before clipping. That was before I realized why it produces variable results, the variation is likely due to the zener diode not doing what it is supposed to.
Later versions of the Phase 90, including most kits increased the value of the zener to 5.1V which re-biases everything and helps to avoid clipping - and providing the JFET bias is still set so the JFETs are running somewhere in the middle of their range as variable resistors - the circuit still phases as it should.
HOWEVER this all assumes that the zener is in fact providing the required stabilised voltage. All zeners require passing enough current to operate in their stable voltage range. Zeners designed for higher power operation require much more current than the low power zeners.
SO IT IS IMPORTANT that you check the voltage you are getting across the zener in the phase royal. The original Phase 90 used a zener rated at 250mW. I believe that at least some Phase Royal kits shipped with 1 Watt zeners. A 5.1V 250mW zener is rated to produce a stable 5.1V when passing 5mA. At 9V supply, for 5mA through the zener the feed resistor would need to be 820 ohm, although you may get away with a larger value. In the original Phase 90 with a 3V zener the feed resistor was 10K !! Which was probably pushing things too far. The original Phase 90s only ran on batteries and they drained batteries rather quickly, so MXR were trying to minimise current drain. If a 1Watt zener is used, fed from a 10K, it is very unlikely that a stable 5.1V will appear across the diode.
If you find your zener voltage is low and you have a 1Watt zener in circuit, replace it with a 250mW zener and drop the value of the feed resistor. An alternative to using a zener is to use a 5V three terminal 100mA voltage regulator, a 78L05. As a three terminal device this of course is not an exact drop in replacement for the zener, but it isn't too difficult to rearrange the connections. 78L05's may be easier to find than axial lead low power zeners.
|