[time-nuts] Symmetricom 1111C OCXO question

[Bob Camp]

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 <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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