[time-nuts] Symmetricom 1111C OCXO question

[Azelio Boriani]

Other idea than leaving the 6K8 resistor?


On Thu, Apr 3, 2014 at 5:08 PM, Javier Herrero <jherrero at hvsistemas.es>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
>
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