Congratulations on a job well done and a saved prize piece of technology.
John WA4WDL
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From: "Frank Stellmach" <[email protected]>
Sent: Saturday, January 07, 2012 9:17 AM
To: <[email protected]>
Subject: [time-nuts] HP 10811-60111 - successful repair of NTC damage
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
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