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Fixing old Fluke DMM, why the carbon resistor?

I zapped an old Fluke 23 yesterday by inadvertantly applying about
several KV which burnt a 1 Megohm 5% carbon composition resistor which
is in series with an overvoltage MOV type thing with a split in it.
This resistor is connected to the + input.

I don't have the service manual or schematic, does anyone know if this
needs to be replaced with a carbon, other resistors are metal film?
 
D

David L. Jones

Jan 1, 1970
0
I zapped an old Fluke 23 yesterday by inadvertantly applying about
several KV which burnt a 1 Megohm 5% carbon composition resistor which
is in series with an overvoltage MOV type thing with a split in it.
This resistor is connected to the + input.

I don't have the service manual or schematic, does anyone know if this
needs to be replaced with a carbon, other resistors are metal film?

The service manual is here (two parts):
http://www.eserviceinfo.com/index.php?what=search2&searchstring=77
23 is the same as the 73.

Parts list shows it as a "cer" type, that would be ceramic.

Dave.
 
I wonder if that is the same substrate as used in fusible resistors often
used in similar sacrificial applications?

It appears that the fusible resistors are explicitly stated, and this
one, R2 is not one of them.
 
P

Phil Allison

Jan 1, 1970
0
Parts list shows it as a "cer" type, that would be ceramic.

** Nonsense.

"cer" indicates a " cermet" aka " metal glaze " resistor.


Got it, thanks Dave.


** But where ??


....... Phil
 
P

Phil Allison

Jan 1, 1970
0
"David L. Jones"
"Phil Allison"

**Restoring the crucial snipped line:

" "cer" indicates a " cermet" aka " metal glaze " resistor. "
Not at all.


** Fraid it does - you OVER- SNIPPING ASREHOLE!!



** Bollocks.



...... Phil
 
F

Franc Zabkar

Jan 1, 1970
0
Not at all. Could well be ceramic composition, like this one for
example:
http://www.ohmite.com/catalog/pdf/ox_oy_series.pdf
Designed for high energy surges and the like.

The actual part number is in the BOM if Geoff is interested to know
what the exact one is.

Dave.

R2 has a part number of RG1/2-105M-5%. and is described as ...

Res, Cer, 1M, 1W, 5%

FWIW, the following datasheet is for RG series "PRECISION NON MILITARY
METAL GLAZE™ RESISTORS":
http://www.irctt.com/pdf_files/RG.pdf

The RG 1/2 types are rated at 1W, 5%, ±200 ppm/°C.

The construction is described as "Metal Glaze thick film element fired
at 1000°C to solid ceramic core".

The Fluke manual lists the following resistors from the same
manufacturer:

R1 Res, MF, 1K, fusible, 5%, p/n FA8466
R20 Res, WW, fusible, .36, 2W p/n SPF3605

FWIW, IRC also have FA8025 series (FA8225, FA8325, FA8425), 2%, metal
film, flameproof resistors on a ceramic rod:
http://www.irctt.com/pdf_files/FA8025.pdf

.... and SP20F series 1W, fusible, wirewound resistors:
http://www.irctt.com/pdf_files/SP20.pdf

- Franc Zabkar
 
P

Phil Allison

Jan 1, 1970
0
"Franc Zabkar"
"David L. Jones"
"Phil Allison"
R2 has a part number of RG1/2-105M-5%. and is described as ...

Res, Cer, 1M, 1W, 5%

FWIW, the following datasheet is for RG series "PRECISION NON MILITARY
METAL GLAZE (tm) RESISTORS":
http://www.irctt.com/pdf_files/RG.pdf

The RG 1/2 types are rated at 1W, 5%, ±200 ppm/°C.

The construction is described as "Metal Glaze thick film element fired
at 1000°C to solid ceramic core".


** QED.

Good research - Franc !





..... Phil
 
E

Eeyore

Jan 1, 1970
0
Den said:
I wonder if that is the same substrate as used in fusible resistors often
used in similar sacrificial applications?

My thinking too. Carbon resistors can cause really interesting fires. Get a
flameproof / fusible one.

Graham
 
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