Fairchild 660 question.

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Well-known member
Nov 8, 2005
While some of you have been eating turkey and apple pie, I've sussed out what's happening with the 660 negative supply.

The transformer voltage is critical  which is probably why Rein stuck a 1.5k resistor (R51) in the primary of the TX to fine tune it.

On the EMI report copy of the schematic, the voltage for the grid bias in the circle is almost illegible (it was in a fold on the photocopier) but the minus sign and a 4V is just visible, in fact it can only be -24V which is the same as the figure in the circle at the power supply itself.

The voltages in the circles are when the CV is -67V which is near total cut-off.  This means that next to no current is flowing in the negative supply and so the two voltages are the same.  When  quiescent, the bias is -19.5 from  -22V so with the 220 ohm dropper resistor, the current is just over 11mA.

The bias voltage must not change too much during compression or else the control amp will reduce it's output  of CV and be counter productive.  In the negative supply, the parallel resistor R50, also 1.5k, acts like a current sink to hold the voltage  fairly constant between load and off load.  A zener might be better but could put noise in the signal amp.

So the on and off load voltages must be very carefully set up with an appropriate transformer for the 660 negative supply.

I'll report on this again when I have a few transformers to play with next week.

By the way, I've just noticed that EMI have put the K under the 330 ohm bias resistors we were sweating over last week Doh!