On 17/01/2014 01:34, Shaun wrote:


"Michael Black" wrote in message
news:alpine.LNX.2.02.1401161520120.31862@darkstar. example.org...

On Thu, 16 Jan 2014, N_Cook wrote:

Otherwise trusty meter, whenever I try on a Weston Cell it is always
the same voltage registered. But now on the ohms scale will sometimes
read 10 or 12 meg with no probes in place. Good battery.
Something to do with the screen shielding? Without any metal covering,
ie the top cover removed and no replacement for the now missing sticky
backed foil over the shielded tinplate box, the meter will not pass
POST, all segments lit on display permanent and no beep on changing
rotary sw setting, normal ?.
So start with cover in place , turn to ohms, and remove cover , it
goes to the floating 10s of megs. Twizzle sticking does not show
suspect switch , preset or anything, and gradually drifts to
over-scale, 30M.
Something going leaky with age?

My DMM acted like that at one point, and I took it apart, thinking the
switch was dirty. But no.

It turned out the fuse was bad. It still had continuity, but not zero
ohms. A new fuse fixed the problem.

Michael


There is also a resistor that fuses open (fusistor) when too much
current flows. It's usually light brown or blue in color near the input
jacks. It's either 1 Kohm or 1 Megohm. It has been over 18 years since
I worked on these so all the details arn't clear. If it's blown, it
will give funny reading too. For cleaning the PCB - use electronics
grade Isopropyl alcohol and a new clean tooth brush. Scrub the board
top and bottom near the input jacks, then rinse with more alcohol and
let it dry properly.

Shaun


I've had that failure before on another Fluke and the symptom then is
that readings are about 1 percent of what they should be, no wavering.

Another rain-soaked night in the UK, but trying out this 77, shows
33meg for open probes and Weston Cell reading 1.017V (on DCV scale) as
it always has been over 20 years and using the same cell