OK, not
quite as small as a single IC but still not large...
It looks as though I'm not going to be able to easily find a replacement 5MHz ocxo so I guess repair is the only option. Well, I gould get a new one for USD700 but that's a bit beyone what's reasonable :-/
The first problem is that there is a solder seam the whole way around the base as can be seen in these photos (click on picture for larger version)
Some exploratory attempts proved my initial suspicion that none of my irons were large enough - the biggest is about 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 or not 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. You can see them in one of the photos.
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 now 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, in terms of fixing it 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. I suspect a thorough clean followed by careful examination of all the joints and components is the way to go - certainly that seems to be the feeling on Time-Nuts
Whoops - things might have gotten a
little hot!
Three PCBs inside - the black stuff is remains of insulating foam.
This looks like the oven control PCB
And this is presumably the 7V reference.
TIP21 as heat source at the top of the photo - the crystal itself is enclosed in the box - not tearing that down as it comes up to design temp so if the fault were here it should still show itself.
Finally just another view of the internal construction.