[time-nuts] Symmetricom 1111C OCXO question

Javier Herrero jherrero at hvsistemas.es
Thu Apr 3 11:08:49 EDT 2014


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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