According to the Echrorec schematics the bias oscillator circuitry generates a 50KHz sinusoidal waveform and a current of 0.6mA flows through the record head. A simple test can be performed to confirm this and that the bias oscillator is functioning within established parameters. The first part of the test involves using an oscilloscope to directly measure the peak-to-peak amplitude and frequency of the bias voltage across the record head—measurements taken from my own model T7E were 300VP-P, which is 105VRMS (Note this is a calculated RMS value and measuring the bias voltage with a digital voltmeter may yield an incorrect, low voltage) and 58KHz. The inductance and resistance of the record head are also required, however I don’t own an inductance meter so just used the value 0.9H from the Photovox datasheet. The bias current can now be calculated using the generalised form of Ohm’s Law below:
IRMS = VRMS / √(R2 + XL2)
where, √(R2 + XL2) represents the impedance, Z of the record head
IRMS = 210 / √(6002 + (2π × 58,000 × 0.9)2) = 0.64mA
Perhaps making actual measurements of the record head resistance and inductance on my T7E will yield the exact 0.6mA bias current indicated on the Binson schematic, however 0.64mA is close enough for government work.
