[time-nuts] DIY FE-5680A lobotomy (disable temp compensation)

[wb6bnq]

Hi Scott,

I reviewed the material at the N5TNL site and it leaves me wondering.  
There are at least three different and separate temperature controlling 
areas inside the FEI-5680.  One of which has cannot be messed with due 
to the fact that it is inherent in its design.  That would be the 
posistor (i.e., a PTC resistor) that is attached to the 60 MHz crystal 
that acts as a general heater.

The other two locations are on the physics package itself.  One for the 
lamp area and the other is for the cavity chamber.  SO.........

The lamp area, I surmise is rather simple as it does not require tight 
temperature control like the cavity chamber area would need.  The lamp 
area just needs a temperature to change and maintain the Rb into a gas form.

The cavity chamber area is way more sensitive to temperature as it 
affects the pressure, frequency and stability of the Rb in the cavity.  
I suspect that the control mechanism for this area to be more complicated.

The block diagram, while pretty general in nature, does show that the 
system (i.e., internal computer) has an A/D  monitoring 4 inputs. Three 
of the A/D channels are observing "system data" and the fourth is 
temperature.  What it does not tell us is what temperature or system 
data it is monitoring.

SO....... Here is the rub, the only temperature that is truly critical 
is the cavity temperature.  BUT, the system computer does not really 
control that temperature, except possibly monitor it, as indicated on 
the block diagram.

Thus the real question is, is that the temperature being referred to in 
the monitoring process ?  In other words have you traced out the 
connections to see what is driving the pin you think is the temperature 
input ?

    I would think that the temperature reading would have a steadily
    climbing curve from the application of power to some steady state
    (relative) value.  However, the curve that you selected as
    temperature seems to rise and become steady (relative) some period
    of time after application of power and seems associated with the
    unit going into a lock condition.  OR, so you indicate on some of
    your graphs.

The next big question is have you monitored the frequency and its 
stability, externally, to observe what effects are taking place when you 
disable this input to the A/D ?

I realize Bert is trying to take FEI to a tighter level, but I wonder if 
it would be better to add thermal mass to buffer external ambient 
changes rather than screw with the internal control mechanisms.  
Particularly seeing as how we have no knowledge of the what the internal 
firmware is doing.  By thermal mass I mean on all sides of the unit.  
The only way to really achieve that would be make a stirred oil bath 
container with the FEI suspended in the center of said bath.

That sounds complicated and messy but may be easier than it appears.  An 
appropriate container would be:

http://www.worldkitchen.com/en/snapware-food-storage/1098437.html

It is made out of polypropylene and can handle at least 130 degrees "C" 
and it holds just under two and half gallons of oil.  Light mineral oil 
runs around $15 a gallon, so two gallons would be the right amount 
leaving a little room at the top.  Wicking is a problem with wires and 
cables but using connectors attached to the lid solves that problem by 
breaking the wicking surface.  The final question is how much, if any, 
external oil cooling would be necessary.  That would have to be 
experimentally determined.  The mineral oil, by the way, has a higher 
flash point then the container and is electrically NON-conductive.

Food for thought,

Bill....WB6BNQ


Scott Newell wrote:

> Bert asked me to send an update on the FE-5680 tempco mod progress.
>
> It appears that the FE-5680A temperature signal (or maybe it's really 
> a current sense signal?) can be disabled by removing a single 10k 0805 
> surface mount resistor.
>
> Using Elio Corbolante's terrific high-res scans, I've noted the 
> resistor location: http://www.n5tnl.com/time/fe-5680a/lobotomy.png
>
> Why would you want to disable temperature compensation? As we've seen, 
> the unit's firmware will adjust the DDS frequency as the temperature 
> signal changes. If you're using the '5680 inside a control loop, it's 
> likely to conflict. By removing the resistor, that channel of the 12 
> bit ADC will be tied to ground through an existing 2.21k resistor. The 
> unit will see a constant 0 counts from the ADC and assume it's really 
> cold.
>
> I modified one unit and monitored it for a few hours over a range of 
> temps, running it nice and hot with no heatsink, then blasting it with 
> a fan and placing it on an ice-cold heatsink. I observed no change in 
> the DDS tuning words.
>
> It's a really easy mod--remove four screws, set aside the insulator 
> sheet, and apply your hot leucotome/soldering iron.
>
>
> I've also found a simple mod to replace the temperature signal with 
> the output of the unused trimpot. This allows you to simulate any 
> temperature you want. If there's any interest, I'll set up a test and 
> monitor the DDS tuning words as the unit's firmware tries to adjust to 
> the fake temp signal.
>
>