[time-nuts] 5MHz ocxo- Opening Solder sealed cans

paul swed paulswedb at gmail.com
Fri Jul 27 01:51:11 UTC 2012


OK
I get to add 10 cents here.
Yes indeed various foam stuff goes to heck after many years and can indeed
become this strange goo-ie stuff. Or it gets all flakey. Well the good news
is stuff seems to work and that can be frustrating. So I would suggest the
very deep dive and look at all of the solder joints. Variable caps
absolutely go to heck. Had to change one on an RB that fixed the exciter.
Good luck
Paul
WB8TSL

On Thu, Jul 26, 2012 at 5:58 PM, Paul Flinders <paul at flinders.org> wrote:

> On 26/07/12 20:49, Tom Miller wrote:
>
>> Pick up a few sticks of ChipQuik and mix it in with a good iron. Then,
>> you may do just what you say. It should melt below about 95°C.
>> A good hot air heat gun would most likely do the trick. Maybe use some
>> solder wick first to lower the amount of tin/lead solder. Then add the
>> chipquik.
>>
>> You might also just solder a tab on the bottom then you could clamp the
>> top in a leather jawed vise. Hit it with a hot air gun while pulling on the
>> base tab.
>>
>>
> Well, a few exploratory attempts confirmed my initial suspicion that I
> don't have a soldering iron big enough for the job - the largest I have is
> 60W or so, and the hot air gun didn't touch it even set to 480deg C
>
> So, I reached for the next larger source of heat which is a small
> blowtorch I have, set a small flame and ran it round the whole seam then
> pulled the top off with a gloved hand. Things did get a bit warm but only
> for a short while.
>
> The first thing I encountered was some insulating foam - this was rather
> sticky. Not sure whether it had gone like that over time or due to my
> heating the whole thing a bit too much. On three sides the foam was
> separated from the case by what looks like some thin SRBP. This had
> blistered a bit so obviously things were a bit hot just there but the foam
> was much the same whether "protected" by the SRBP so I'm wondering whether
> this has just disintegrated over time.
>
> Pulling/washing the foam off reveals three PCBs and a further enclosed
> metal case with the crystal. A TIP21 bolted to this acts as the heating
> element. Originally there had been some cable ties anchoring the crystal
> enclosure but these were brittle - presumably from the heat of the oven.
>
> Hooking the unit up to 12V and an oscilloscope shows that it has survived
> the encounter with the blow torch and produces a nice 5MHz sine wave at
> 2.5V p-p (into 10Meg ohm). Irritatingly it doesn't really want to misbehave
> - presumably because whatever didn't like the heat is now cooler with the
> insulating foam removed. The inner metal case gets too hot to touch but the
> thermostat seems to be working because the current drawn drops from about
> 380mA to 240 or so. It should probably be lower but, again, without the
> usual amount of insulation it's likely to draw more keeping the oven up to
> temperature.
>
> So, as it doesn't want to misbehave, I'm not totally clear which way to
> go. The output does drop to about 1.8V p-p when the oven is fully warm
> which might be related to the original fault. I suppose the fact that the
> fault has "gone away" eliminates the oven assembly or crystal itself as the
> source of the problem.
>
> Photos for comments or curiosity. The black stuff all over the PCBs is the
> remains of the foam.
>
> http://www.wild-pc.co.uk/**images/P1020364.jpg<http://www.wild-pc.co.uk/images/P1020364.jpg>
> http://www.wild-pc.co.uk/**images/P1020367.jpg<http://www.wild-pc.co.uk/images/P1020367.jpg>
> http://www.wild-pc.co.uk/**images/P1020368.jpg<http://www.wild-pc.co.uk/images/P1020368.jpg>
> http://www.wild-pc.co.uk/**images/P1020369.jpg<http://www.wild-pc.co.uk/images/P1020369.jpg>
> http://www.wild-pc.co.uk/**images/P1020370.jpg<http://www.wild-pc.co.uk/images/P1020370.jpg>
>
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