[time-nuts] Fluke monitor

GandalfG8 at aol.com GandalfG8 at aol.com
Mon May 17 09:12:59 UTC 2010

In a message dated 17/05/2010 06:13:05 GMT Daylight Time, Leigh at WA5ZNU.org  

You're  right about the current.  I just checked it with my DMM and got 
45ma.  My inline power meter must not be that it's not that accurate at 
the low  end.

The display works; the backlight works; the processor must be  doing 
something since it prints a variety of messages.  It's the  RS232 that 
has flakey.  The hardest thing at this point is to  unsolder the 16-pin 
header.  Bob suggests that a 12v regulator may  have failed.

That's encouraging re the current, I'm also inclined to the view that it's  
likely to be an RS232 issue but still wouldn't rule out a quality issue 
without  careful checking.
I'm not sure what Bob means when he suggests a 12v regulator may have died  
as there isn't one fitted to this unit.
The only regulator is the surface mount 5v unit which, from the measured  
current, I would say is working correctly but that's easy enough to  check 
Unsoldering such a header can be very difficult if you try to do it a  pin 
at a time, especially on a plated through PCB.
It is possible to clear one pin at a time with either a solder sucker or  
solder braid, or a combination of both.
I've also succesfully removed connectors and IC sockets from double  sided 
PCBs using a powered vacuum desoldering tool but it's generally less  likely 
to cause damage if you can melt all the solder at once to separate  the 
boards and then clean off the surplus after.
There's a variety of ways to do this, from hot air guns to specially shaped 
 iron tips, and as many opinions as to which might be the best  method:-).
I would prefer a shaped iron tip in  this particular instance but they 
don't seem to be so widely available  these days, at least not for the irons I 
use, so might also suggest flood  filling along the pins with molten solder 
as another option.
With this though you have to be very sure your iron has a big  enough tip 
and that there's enough heat reserve in the system to  ensure the whole lot 
doesn't solidify and leave you in a lot more mess than when  you started. PCB 
damage to pads and through plating is again a risk with  everything heated 
at once and practice on something non essential would be well  advised.
I still can't access my units right now and can't remember the exact  
physical setup but if the header pins are exposed and accessible between the two  
boards by far the best option is likely to be cutting them and sacrificing  
the header.
That might seem drastic but headers are much easier  to replace than 
circuit boards and all you need do then is remove each  pin separately which, with 
care, greatly reduces the risk of damage.
If there's a plastic moulding at one or other end of the header which  
still seems to be locking the pins together these are fairly easy to remove,  
softening with a nearby soldering iron should generally release any that seem  
well locked in place or a thin scalpel blade can be used to carefully cut  
through the plastic and separate the pins that way.
Emphasis though on "thin" blade, thicker blades such as those found in a  
Stanley knife can act as a wedge and force the pins apart which again  
carries risk of damage when close to the PCB.

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