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DB877 teardown

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muvideo:
Hello, I'm playing with this metal-box-with-dials:


It's heavy metal construction, the dials are working ok, but
can be seen some sign of oil on the surface, it's not good,
probably bomebody tried to lubricate it with spray oil.

Electrically it is a decade resistance box, so the dials
set the resistance between the terminals. The unit
is an 8 decade, so it has a good resolution of 0.1ohm
considering the maximum resistance value of 12Mohm.

Each switch stage contains 5 "2" resistors and 1 "1" resistor,
in the even positions the switch progressively inserts in series
the "2" resistors, in odd position the "1" resistor is added to the serie.
for example the kohms is made up of 1kohm resistor plus 5 2kohm.

On first inspection my unit shows a "little" problem: it's electrically open,
not a good sign...

Cracking open the unit wasnt simple, it was held
closed by only two screws but these were very hard to
unscew. Here is the internal view of the decade box,
full of precision resistors:


There are signs of corrosion on the standoffs, and
signs of overheating on some of the resistor.
Here one of the 20ohm resistors was swapped with a
wirewound 25ohm potentiometer, under it there
is a burned 200 ohm. Seem that the precedent
owner used the unis like a load  >:(


This is one of the switch selectors, it has a ceramic body
with a polymer center "cursor",


Here is another view of the higher values selector, the white
cylinders are 100/200k resistors and 1/2Meg ones.


This is a particular of the 20kohm resistor, it was interrupted by
a dent in the wire, for testing I unwound a pair of turns and
soldered them back, losing about 400 ohm in the process:


"1Meg +-.005" on the 1Meg resistor:


One of the 2 Meg resistors, without protecting tape:


This is the 200ohm resistor mica sheet, once cleaned:


The different colour of the wire is the zone where the resistor
was grinded to trim it to exact value:


I cleaned and washed all the selectors, and mica resistors.
Tha mechanical construction is very good, and the selectors just
"fall" in the detents with a pleasant click.
I measured all the resistors with the multimeter, the values are
around what shuld be, but the multimeter was calibrated back in 2004,
so dont know who is wrong, but it's impressive that almost
all the resistors showed on the meter display values few counts
far from nominal value.
Of the working ones, there are only 2 or 3 that seem a little low.
The definitley broken ones are 20 200 20kohms, initially, but
now I need also 4 of the 5 2Meg ones.

In fact I suspected a little leackage from the high value decade:
the total resistance value was lower than the sum of the single
resistors, so I decided to test the decade with a current higher
than the multimeter one,
to see if there was some voltage coefficient on the resistors.
I connected the fluke 343 and set it up on 200V per resistor,
the current was dead on 100uA, but few minutes later it dropped
to 50-60uA, unstable. The result was that I killed the more
delicate, more costly and difficult to find resistor in the box :(

The resistor are rated at higher values (300V each max, 90mW, 300uA)
I can only think that there was some insulation problem with these
old units, and the 50V per section started some corrosion
or hotspot that interrupted the wire.

Now I'm searching for replacement resistors.
For lower values Frank Stellmach on volt-nuts adviced the serie 8E16 and 8G16
on http://www.rhopointcomponents.com
The price is very good, and much less than vishay metal foil ones.
Only problem now is to find the 2Meg ones.

Fabio.

lowimpedance:
I have one of these at work, also I have the 'instruction sheet' that came with it. Contains circuit, specs, calibration, maintenance and parts list all of 2 pages!.
I can scan it if you would like to have it ?.(if you don't already that is). let me know.

John

muvideo:

--- Quote from: lowimpedance on June 20, 2012, 07:05:20 am ---I have one of these at work, also I have the 'instruction sheet' that came with it. Contains circuit, specs, calibration, maintenance and parts list all of 2 pages!.
I can scan it if you would like to have it ?.(if you don't already that is). let me know.

John

--- End quote ---

Thank you, I have a pair of versions of the two page sheet, it floats on the internet,
I imagine is the same :
http://128.238.9.201/~kurt/manuals/manuals/Other/ESI%20DB877%20Instruction.pdf

Fabio.

amyk:
Couldn't you rewind the resistors yourself using the existing resistance wire (adding more if necessary)? The only problem is how to calibrate them, although you only need access to a precision DMM once.

muvideo:

--- Quote from: amyk on June 20, 2012, 08:35:09 am ---Couldn't you rewind the resistors yourself using the existing resistance wire (adding more if necessary)? The only problem is how to calibrate them, although you only need access to a precision DMM once.

--- End quote ---

The first thing I tried was to unwind the resistors to try to find the point if interruption.
Soon I realized that was impossible.
That wire is around 30um diameter, probably a pair of km long and brittle :)

Fabio.

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