[time-nuts] 10811 Repair (was warmup / bad batch)

[Dr Bruce Griffiths]

Dr Bruce Griffiths wrote:
> Jason Rabel wrote:
>   
>> What voltage goes to the crystal? I may have had my scope scale too high and
>> just didn't notice, but at first glance I don't think it was getting power.
>>
>> Jason
>>  
>>
>> -----Original Message-----
>>
>> Yes, the black wires are the thermistor.
>>
>> The crystal has three wires.  Two are actually connected
>> to the crystal electrodes.  The third is the case.  The
>> case is fastened to the casting with a screw.  The only
>> way the casting gets an electrical ground is by way of the
>> crystal ground screw and ground wire to the board.
>> The wires are welded to the crystal, so if you want to
>> disconnect one of them, you have to unsolder it from the
>> PC board.
>>
>> Rick Karlquist N6RK
>>
>>
>> _______________________________________________
>> time-nuts mailing list
>> time-nuts at febo.com
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>
>>   
>>     
> Jason
>
> There should be no dc (with respect to ground) on any of  the crystal leads.
> The ac voltage developed across the crystal is determined by the 
> parallel combination of the effective crystal inductance at the 
> operating frequency and the crystal ESR plus the impedance of the 
> trimmer network. Since the crystal current is 1mA rms (see HP journal 
> March 1981 p24) and the crystal ESR is around 50 ohms the AC voltage 
> across R4 in the schematic will only be about 50 mv rms.
>
> Bruce
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>
>   
Jason

Since the oscillator circuit is checked by substituting a 10uH inductor 
for the crystal then the crystal impedance must be around
50 + j628 ohms at the circuit resonance.
The trimmer network has an impedance of -j568 ohms and the common base 
transistor has an input impedance of about 8 ohms plus an inductance of 
a few nanohenries.
with a 1mA crystal current the voltage across R4 is consequently about 
80mV rms.

Bruce