[time-nuts] Fluke Thunderbolt Monitor Revisited

GandalfG8 at aol.com GandalfG8 at aol.com
Sat Jun 5 15:04:25 UTC 2010

Further to earlier comments I have now traced the full schematic of the  
8051 replacement processor board in the iCruze display.
The schematic is so close to Didier's original that I don't think it's  
necessary to provide a copy but could do so if required.
Differences are described below and most of the detail is clearly  visible 
anyway in Brooke's excellent photo....
Other than D1 to D3 being fitted as zero ohm links, rather than diodes as  
they should be, the other differences are as follows....
1...The addition of the 78D05 regulator onboard. This is a conventional  
circuit with an SMD electrolytic capacitor in parallel with an SMD ceramic  
capacitor on the input and two of the same electrolytics and another  ceramic 
in parallel on the output.
The electrolytics are marked E105, which indicates 16 or 25 volt rating  
depending on which tables you read, and also seems to indicate they're only 
1uF  which seems low to me but the marking might again be manufacturer  
There are spaces for a through hole or wired LED with  SMD series resistor 
directly across the supply input but these are  unused.
2...Several more SMD ceramic capacitors have been added to the board  for 
additional supply decoupling.
3...The series input and base to ground resistors on the RS232 interface  
are both 180 ohms, rather than 10K and 3K3 respectively as in Didier's  
4...Didier's 10 pin JTAG interface connector has been replaced with a  5 
pin single row connector.
Both end pins are ground and the others match Didier's three connections to 
 the CPU.
On the rear of the PCB, the side normally visible, these pins are marked 1, 
 7, 4, indicating their relationship to the original circuit.
5...Although marked as R3 on the PCB the 3k3 resistor that Didier shows on  
the JTAG connector has been replaced with a ceramic capacitor.
I though this might be a one off error at first but Brooke's photo also  
shows a capacitor.
Just to be sure I did attempt to measure it and it indicates as open  
circuit on my DMM.
Although all the ceramic capacitors look to be identical none show any  
My LCR meter wasn't happy trying to measure them in circuit but  
"fortunately" the two adjacent to the contrast adjustment pot were only soldered  at 
one end so easy to remove.
These I measured as 100nF, or 0.1uF depending on which flavour you  
So there you have it, nothing significantly different from the original  
other than the lack of diodes and they really do need to be fitted if it's 
going  to be run with the 5 volt regulator doing it's job.
This one I've dismantled did run for at least a couple of months with the  
processor at 5 volts before being taken "off air" so even though the 
specified  "absolute" maximum is 4.2 volts there's obviously some tolerance on that 
but I  certainly wouldn't want to push my luck and run it now without the 

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