[time-nuts] 10811 warmup (Jason Rabel)

[Dr Bruce Griffiths]

Jason Rabel wrote:
> Lol, thanks Jack... :)
>
> Yeah, I pretty much figured it would overheat because it couldn't regulate
> the temperature properly. But also I would imagine you would burn yourself
> trying to handle it since the temp is somewhere around 185F give or take.
> They also say to let the oscillator cool for at least an hour before trying
> to remove it from the insulation (again to let it cool enough).
>
> I'll try to remember to take pictures of the disassembly.
>
> Jason
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Jack Hudler
> Sent: Sunday, January 21, 2007 1:04 AM
> To: richard at karlquist.com; 'Discussion of precise time and frequency
> measurement'
> Subject: Re: [time-nuts] 10811 warmup (Jason Rabel)
>
> Made'm red just for ya in the updated 10811 manual
>
> DO NOT OPERATE THE OVEN CIRCUITS WHEN THE OVEN MASS IS OUTSIDE OF THE
> OSCILLATOR
> INSULATED HOUSING. DOING SO WILL OVERHEAT THE OSCILLATOR CIRCUITS INSIDE THE
> OVEN MASS AND CAUSE PERMANENT DAMAGE. ALL OVEN TEST POINTS ARE AVAILABLE
> WITH
> THE OVEN MASS AND OVEN CONTROLLER CIRCUIT INSIDE THE HOUSING.
> WHEN OSCILLATOR COVER AND INSULATOR ARE REMOVED THERMAL FUSE WILL NOT
> PROTECT
> CIRCUIT FROM OVERHEATING.
>  
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf
> Of Rick Karlquist
> Sent: Sunday, January 21, 2007 12:03 AM
> To: Discussion of precise time and frequency measurement
> Cc: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] 10811 warmup (Jason Rabel)
>
> Jason Rabel wrote:
>   
>> I must of read at least 15 times 'do not power up the oven circuit' in the
>> documentation... lol.
>>     
>
> I am trying to think of why they say this.  I don't see why
> you can't power the oven when it is disassembled.  I can see
> that it might not reach 80 degrees or might oscillate and
> it would be too hot to handle.
>
> Rick Karlquist N6RK
>
>
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>
>   
Jason

As long as the thermistor is still within the oven mass and the 
temperature regulation circuit is functioning OK, the oven should 
actually be OK.
However the temperature control loop may oscillate as the thermal time 
constant for which it has been compensated has been drastically 
decreased by removing the insulation from the oven mass.
If the oscillation amplitude is sufficient it is conceivable that some 
damage due to excessive temperature may occur.
However, the dissipation of the oven heater transistors is limited by 
the control circuit so the temperatures reached may not be high enough 
to damage components.
The turn on current limit circuit limits the heater transistor emitter 
current to about 0.52A with a 24V heater supply, limiting the maximum 
dissipation in the heater transistors to about 12.5W with a 24V heater 
supply, which is probably insufficient for the inner oven mass to get 
very hot once the insulation is removed.
The principal problem is personal burn injury .

Bruce