Some days ago a friend of mine brought to me a fluke 8520A.
He bought this instrument about 4 years ago, it was functional,
and from then it sat unused in a moisty “lab” for all this time.
Last week we powered up it, and it started regularly but soon
we realized that it had some problems: Vdc measured correctly
but Vac measured a fixed value and Ohm gave an High Voltage error.
I have not much experience in repairing instruments, and
this beauty has the same age than me, but it was not hard
to understand it's problems.
Fortunately the fluke is very well documented, so I downloaded
the service manual. The second time I started the instrument
it simply stopped working, with freezed display. After a reboot
it's display wasn't even lighting up.
Here are the 4 faults I found:
the first was to understand why the display wasn't functioning, the
CPU was reading the memory addresses but wasn't doing what it
should have done. The cpu is an MK3880 from mostek, reading about
it I learned that it's a Z80 clone, so I swapped with an old Z80 and the
instrument came back to life. This took me a pair of days familiarizing
with the instrument circuits
The second problem was the keyboard, some buttons didn't respond.
They were electrically ok, but one of the outputs of a MM74C906 was
open. I didn't want to wait for a replacement so I built a small board that
replaced the buffer output.
Third problem was a 7908 neg. regulator, temporary solution is a 7905
with a led and a pair of diodes to bring it to approx -8V.
Now I was ready to start the self test, but I was wrong: the instrument
was upset because it had too low resistance on it's inputs, it read
between 2 and 3 nanoSiemens and it wanted it to be less than 1nS, i.e. >1GOhm.
I tried to heat up the High Ohms input components but the conductance
worsened. At last I cleaned with some TCE the bottom side of one of the input
relays and the conductance rapidly fell down to 0,3-0,7nS.
The point I cleaned was visually clean and dry
Now the input reads always >0,6 nS and raises with temperature, is it normal?
I imagine it's a cleaning problem?
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