PY500

The PY500 is a booster diode for the horizontal timebase circuits of colour television receivers. Philips and Telefunken describe it as an indirectly heated valve for a series heater chain: its heater requires 300 mA and approximately 42 V. It uses a Magnoval base, with the cathode brought out to the top cap. Philips specifies an anode-to-cathode capacitance of 13 pF, whereas Telefunken gives 13.5 pF; these are the respective manufacturers' data and should not be merged into a single value.

To assess the PY500 in a circuit, current and voltage limits must be distinguished from typical measurements. Both datasheets give 440 mA as the maximum average anode current, 800 mA as the peak anode current and 11 W as the maximum anode dissipation under the design-centre rating system. The internal resistance of 45.5 Ω, by contrast, is measured at 440 mA and is not a limit. The permitted negative peak anode voltage is 5.6 kV; 7 kV is the absolute maximum, subject to pulses lasting no more than 22% of a cycle and in any case no longer than 18 µs. Philips also specifies a minimum resistance in the heater chain towards the mains: 100 Ω at 220 V RMS with respect to earth, or 50 Ω at 110 V. The instructions for pin 3 also require care: Philips recommends a 300 Ω resistor between pins 3 and 5, or a connection between pins 3 and 4 if the resistor is omitted; Telefunken instead describes pin 3 as connected to the protective grid and specifies a 330 Ω resistor to the heater pin closest to mains potential. When servicing a set, follow the circuit diagram and the datasheet for the installed version, and observe the pulse limits.