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

[Azelio Boriani]

Good work. I love to restore fine old piece of equipment with attention not
just to get them working but really restoring the original design.

On Sat, Jan 7, 2012 at 3:17 PM, Frank Stellmach
<frank.stellmach at freenet.de>wrote:

> 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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