[time-nuts] HP 10811-60111 - successful repair of NTC damage

[Frank Stellmach]

Dear time-nuts,

the 10811, sitting in my HP5370B, showed an intermittent heater 
circuitry failure.
It turned out, that the NTC temperature sensor was damaged (1982 vintage).

At room temperature it was conducting (100kOhm), but became open during 
heat up phase, eventually conducting properly at around 82°C, but 
failing from time to time, i.e. the star symbol on the 5370B turned on 
frequently, and the reference frequency fluctuated strongly.

Normally this is a total damage of this OCXO, as the NTC is epoxied 
completely into the oven mass, and this whole assembly would have been 
to be replaced, according to the manual.
I doubt, that it's available any more, but definitely not for a 
reasonable price.

I found a compatible NTC replacement, regarding diameter (2,41mm max) 
and electrical parameters, i.e. Rn(25°C) = 100kOhm, B = 4540K, and an 
even better accuracy of 1%, i.e. 8.6kOhm at  82.8°C +/- < 1°C.
It's from epcos, S57867S104F140. Visit epcos.com for data sheet and R(T) 
table.

This NTC has 50mm short and blank wires; I couldn't get the PTFE 
insulated S861.. type.
So the wires had to be insulated with a heat shrink tube, and the NTCs 
wires had to be assembled straight, without bending them 180 degrees 
through the 2nd hole in the oven mass. That's also much better regarding 
reliability.

After complete disassembly and carefully milling out the old NTC and the 
epoxy with a 2.5mm drill,  I applied thermal grease at the new NTCs 
head, and fixed the wires with a small drop of epoxy at the other side 
of the oven mass.

The reassembly was a little bit delicate, due to the short wires, and 
because the insulation of the wires was thicker than before. But 
finally, the heater circuitry stabilized normally within 10 minutes. 
Obviously, the thermal gain was not perceptibly affected by this 
slightly different assembly.

The OCXO now works fine since two weeks, and its drift is in the order 
of < 1e-9 again.

Frank




-------------- next part --------------
A non-text attachment was scrubbed...
Name: oven_mass_back.jpg
Type: image/jpeg
Size: 39395 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20120107/65ab7eb8/attachment-0002.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: oven_mass_front.jpg
Type: image/jpeg
Size: 38503 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20120107/65ab7eb8/attachment-0003.jpg>