[time-nuts] Fluke monitor

GandalfG8 at aol.com GandalfG8 at aol.com
Fri Jun 4 09:57:57 UTC 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 
the case.

As soon as I plugged it in, it worked.   It's been on about 15 minutes 
now with no problems.
Hi Leigh
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 
the diodes.
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 
previously described.
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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