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

[Ron Ward]

Hi:
I have been looking at the poor quality solder joints on the oscillator.
If you have a steady hand and a small tipped soldering with silver
baring (about 2% silver) solder, I would re-solder the connections. Many
of them do not look properly "wetted" and cold. Some of the heat from
removing the top may have re-soldered a problem connection.
Ron

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Paul Flinders
Sent: Thursday, July 26, 2012 2:59 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] 5MHz ocxo- Opening Solder sealed cans

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/P1020367.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/P1020370.jpg

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