[volt-nuts] Update on 720A

[Chuck Harris]

Hi David,

It isn't your soldering quality or ability that will degrade
the instrument, it is rather the quality of the parts you
add, or replace, and what you do to fudge things into sort of
working, that will damage/degrade the instrument.

Devices like the 720A are firmly in the category of magical
devices.   Resistors, switches, pots, and even the plating,
and the solder alloy are critical to the ultimate performance
the 720A can achieve.

Fluke worked every angle they could think of to make the 720A
a precise and very stable device.  If you change a resistor
type in a critical place, the whole recipe can fall apart.

The resistors they used aren't the sort that will simply drift
off value.  They are wirewound using a very special alloy, and
are essentially good forever.  If they have changed, either
someone has changed them, or something drastic has happened,
like they have been overloaded, or an internal contact has
failed.

The pots used as trimmers should have only been trimmed a couple
of times over the life of the instrument... hardly enough to wear
them out... though corrosion is always possible.

My concern is you seemed to be going at this repair like you
were working on an oscilloscope, rather than a super precision
instrument.

-Chuck Harris

David C. Partridge wrote:
> I can't replace the bad resistors in the A decade - they are in the oil bath and
> the manual says that in this case you sent the unit back to Fluke for a re-build.
> These days, I suspect they won't even do re-builds at all (or only at *silly*
> money), but make you buy a new one, and may well not supply *any* parts unless
> they do the repair (as happened to a friend of mine with another instrument).
> 
> I'm really not convinced that "the very act of replacing the resistors is likely
> to cause additional damage" - I've been around delicate and high-Z electronics for
> a while now and can get in and out in unsoldering pretty handily.  I'm also very
> aware of the critical importance of not messing up the conformal coating which is
> there for a reason (and re-sealing it where necessary).
> 
> I accept that if the primary resistors keep on drifting then I am "in a hole" and
> should probably save my pennies for another one.
> 
> The problem is that the only source (that I know of) of used 720As in UK is either
> eBay or very occasionally a cal. lab. that is closing.   "Known good" isn't
> normally a term that applies to either.
> 
> Dave
> 
> 
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