Teardown of Bliley OCXO module

[dnessett]

Relative to the transistor: from the provided datasheets the 2SJ117 seems that is produced in a TO-220 package that is not this case. The marking of the MJD117 is J117and it is from the same manufacturer...
You may try to do some measurements without removing it from the circuit board and, if the rest of the circuit permits it, we may be enlighten, just for the curiosity.

Is the device labeled J117 a mosfet (2SJ117) or a BJT (MDJ117)? It is hard to tell. Figure 10 shows the device up close. Notice the middle lead is either broken off or missing. The schematic of a MDJ117 in the DPAK case 369c shows the middle pin missing, as in the picture. However, the code above the MDJ117 designation (451) comprises 3 numbers (YWW), but according to the spec, it should be 4 numbers (AYWW), where A signifies the assembly location, Y the year and WW the work week. In addition, the middle lead looks like it is sheared off, which would suggest it is not the MJD117.



Since the device is in circuit, measuring the voltages across the leads is problematic in regards to identifying the device as either a 2SJ117 or a MDJ117. For one thing the MDJ117 is a PNP darlington pair, which would not display the traditional BJT characteristics. But, for the record here are values (using a DMM in diode mode):


 1 -> 3     .438v
 3 -> 1     .438v
 1 -> 2     .589v
 2 -> 1    1.269v
 2 -> 3    1.234v
 3 -> 2     .546v

NB: I originally mislabeled the pins on the image wrt to the table. They are now correct.

[dnessett]

On the other hand, shearing off the gate of a mosfet would be rather bizzare.

[pigrew]

On the other hand, shearing off the gate of a mosfet would be rather bizzare.

Pin 2 should be the drain, and it is shorted to the tab, so no connections are actually being lost since the tab can still be used. (They may have torn it off to provide extra space on the PCB for more components)

(My GPSDO is showing very slight thermal oscillations, and I'm wondering if there are design issues with the OCXO. Once I create a nice case for the GPSDO, I'll make better thermal measurements.)

[metrologist]

This is great. I have an MTI OCXO that has a failed heater, and I thought of doing the same sort of teardown. First thought on the coiled wires was that they were the heater. I was hoping a failed oven would be a plausible fix.

[dnessett]

I have been trying without success to find an indication that ON ever made a 2SJ117. I could not find the 2SJ117 on either Mouser or Digikey when I searched for it, so it seems like an obsolete part. Since the device on the board is clearly an ON part, I think this tips the scales in favor of the MJD117; although to be entirely certain, I would have to desolder it from the PC board and test it properly.