[volt-nuts] Wanted wirewound precision resistors and ESI DB877 help

[Fabio Eboli]

Frank Stellmach <frank.stellmach at freenet.de> ha scritto:
> The low ohm ww resistors can be orderes from:  
> http://www.rhopointcomponents.com/   - Resistors - Precision Through  
> hole. Series 8E16 and 8G16 are 0.1 and 0.01%, TC3, low long term  
> drift.

Thank you for the tip, the prices seem reasonable.

> 2MOhm is difficult and very expensive to realize in wirewound  
> technology, as wire has to be very long.

Yes, this is the problem, I should have specified that the resistors
arent required to be necessarily wirewound, but the important is long
term stability, I have still original 1Meg and 2Meg and I can trim
the new resistors to the ones still alive, assuming these are still
ok. It seem so because the direct resistance measurement seem ok,
and measuring the current once connected to the fluke 343 source
returns perfect value, but now I fear to repeat the process.

>
> 100µA give 20mW only, so they obviously have some internal problem,  
> perhaps from humidity and/or age.
> Or the High Voltage (200V) has killed them, local short circuit and  
> kind of "hotspot", due to deteriorated isolation.

I dont have experience with such kind of high precision high value
resistors, per ESI/TEGAM specs the 2Meg have maximum ratings
of 90mW 300uA 300V per resistor, so I was way below the maximum.
I opened one broken resistor ( they are open, with only a tape
on the outside) and the wire is oily, there was also somethin like
wax in some points on the outside. I think you are right that the
wire insulation is probably compromised.

> But your unit has obviously some other severe problems due to age or  
> environmental abuse.
> I'd check all other resistors before investing any Cent.

Can you advice a tecnique to check the other resistors?
I disassembled the selectors and measured every single resistor,
there are a pair of resistors that seem a little drifted toward lower
than spec value, these are the same that seem a little overheated.

Am I correct in thinking that, if there are corrosion problems,
probably the problem will be in the 100/200k and 1/2Meg?
These are long thin wire around a small ceramic former.
The other resistors are wound around a mica sheet and the wire
is all on the outside surface, so can be easily cleaned and checked
visually with microscope.


There was also an hint that something was wrong with the high value decade
before the disaster.
I measured the single 2meg resistors and after I measured the series,
the resistance of total series was slightly lower
that the sum of the single units, so for example 10Meg mesured 7-8kohm
low while the sum of single 2Meg resistor was low by 3-400ohms each.
I tought this was due to leackage on the selector switch (it's ceramic).

Fabio.

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