Hi Normally that sort of circuit has a “boot strap” pull-up resistor that weakly biases the diode to get things running at start up.
Bob On Apr 3, 2014, at 11:08 AM, Javier Herrero <[email protected]> wrote: > Hello all, > > I've a 1111C OCXO (inside a Symmetricom 5045A that is inside an Oscilloquartz > OSA 5585) that refuses to start. It seems that one voltage regulator is not > operating, with only 2V at its output while it should have (I think...) > around 12V. I've partially analyzed the circuit, and it is nothing from other > world: a 6.9V reference diode, driving an operational amplifier, driving a > NPN transistor. But the 6.9V reference diode is biased from the regulator > output, so... if the regulator does not provide output, the zener is not > biased, and then... the regulator does not provide output. So it must be some > kind of start-up circuit, that makes the transistor to conduct a bit, or to > bias the diode, when power is applied. > > I've been looking around, but not found it yet. I have make a quick test by > placing a 6k8 resistor between the collector and the base of the transistor > to make it conduct, and then the regulator starts, provides adequate output > (12.5V), and also the 5MHz output appears. > > Any idea is welcome :) > > Best regards, > > Javier > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
