[time-nuts] Fluke monitor
GandalfG8 at aol.com
GandalfG8 at aol.com
Fri Jun 4 05:57:57 EDT 2010
In a message dated 04/06/2010 05:47:32 GMT Daylight Time, Leigh at WA5ZNU.org
fixed my fluke.l monitor.
This evening I got mail today from Bob Mokia:
The problem is too much volts on CPU (8051F330D).
Must have diodes 1n4148 etc at D1 and D2. Drop cpu volts to 3.6volts.
Maximum volts from data sheet is 4.2 volts.
D1, D2, and D3 are zero-ohm SMT resistors in series from the 5V
regulator output. They are visible from the CPU board edge. Without
removing the CPU board from the display, I was able to remove D1 and
tack a pair of 1N4148's in series from the D1 plus pad to D2, both right
at the board edge. I left D2 and D3 in place. I made the leads as
short as possible, but still had to bend the parts up a bit to fit it in
As soon as I plugged it in, it worked. It's been on about 15 minutes
now with no problems.
Congratulations on getting your monitor fixed.
That's great news and very interesting, many thanks for sharing it.
Mine are still packed away but I'm getting closer to being up and running
again so will check them over when I can and adjust as necessary.
Marking the component positions D1, D2, D3 presumably implies that the
designer of the PCB allowed for this from the start so raises the question,"
why wasn't it implemented as such?"
Measurements I made following the initial confusion over supply
requirements did confirm that the display module will still function at 3.6 volts but
I would have expected the contrast to require adjustment if the supply to
that had also been dropped so perhaps the regulator output splits before
That might also explain also why the option to fit the diodes was given in
the first place, instead of just using a lower output regulator.
Perhaps the design of the original iCruze processor board was rather
blindly copied, with variation where necessary to accomodate the different
package and/or pin out of the 8051, but otherwise left the same and without due
consideration given to the voltage requirements of the replacement
Also of concern is the fact that your unit, and it appears some others
too, did work as expected for quite a while before showing the symptoms you
Those symptoms then being consistent, at least without dropping the supply
voltage, suggests the possibility of some form of irreversible change, so
I wonder if something in the processor itself, perhaps a protection device,
might have been permanently damaged?
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