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Author Topic: FLUKE 8000A series of 1978 - Beware of internal NICD batteries health status  (Read 37 times)

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Online Kiriakos GR

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This topic is here to alert you if you own any FLUKE 8000A series of 1978 benchtop DMM, this is a revision including battery option, among with double pairs of NICD battery packages.

I did spotted the problem at my own FLUKE 8050A back at 7-2010, R26 resistance this found as externally burned and destroyed.
R26 resistance this serves a double purpose,  at power OFF due this resistance both battery packs they get charged, then at power ON this resistance it power up the DMM by sending now only 1/3 of charging energy at both battery packs.

But what happens if entire 4X NICD batteries health status this is terrible?  One healthy NI-CAD cell when this is fully charged, this translates that charging current this fall down to a minimum of 30~50mA but not less than that.
With four single healthy cells in series the lower charging current this is now approximately 120~200mA.

But when a single NICAD gets old and damaged, charging current never fall down to minimum, and this causing overload - overheat at R26 resistance.
 
At my FLUKE 8050A, I did notice a weird symptom which this alerted me and helped me, so I to discover the true source of the problem.     
At power ON and after 20 minutes or so, the DMM display was getting OFF.

The reason for that, was that (overload - overheat) at R26 resistance (caused due the charging circuit) this now increased beyond wattage handling of R26.
 
While back at 2010 I did replace R26 with higher quality Film-metal resistor, this is unable to get burned, the overheat now increases just this resistor temperature and measuring value increases to the point that the rest of the circuit cannot be served due it (LCD goes OFF).

By replacing all damaged NI-CAD cells, then the charging circuit it does find back it balance, and R26 resistance does not get stressed any more.  ;)

 
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