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BillDWVA · 27th Nov 2017, 5:38 pm

Hi Levente,

Have you got access to an oscilloscope and an external audio amplifier that you can temporarily connect up to the output(s) of the recording amplifier?

I would suggest checking with an oscilloscope that the bias oscillator is working properly when first switched to 'Record' and also that the output of the record amp is 'interfacing' correctly. You say it is not a standard Ferrograph circuit but there should be what is called a 'bias trap' in series with the record amp output (on the circuit it will be a tuned circuit consisting of a capacitor in parallel with a small choke, sometimes with a resistor and and capacitor in series) to stop the bias signal from saturating the audio amplifier. Be careful, the bias oscillator can generate quite high voltages in the wiring to the record head!

The bias oscillator will be running at a frequency of about 100kHz so is not audible but something is causing a 'modulation' that is your audible 'whining'.

27th Nov 2017, 5:38 pm	#4
BillDWVA Pentode Join Date: Nov 2010 Location: Tenby, Pembrokeshire, Wales, UK. Posts: 139	Re: Ferrograph Mono 1/2 track Hi Levente, Have you got access to an oscilloscope and an external audio amplifier that you can temporarily connect up to the output(s) of the recording amplifier? I would suggest checking with an oscilloscope that the bias oscillator is working properly when first switched to 'Record' and also that the output of the record amp is 'interfacing' correctly. You say it is not a standard Ferrograph circuit but there should be what is called a 'bias trap' in series with the record amp output (on the circuit it will be a tuned circuit consisting of a capacitor in parallel with a small choke, sometimes with a resistor and and capacitor in series) to stop the bias signal from saturating the audio amplifier. Be careful, the bias oscillator can generate quite high voltages in the wiring to the record head! The bias oscillator will be running at a frequency of about 100kHz so is not audible but something is causing a 'modulation' that is your audible 'whining'.