[time-nuts] Fluke monitor

Leigh L. Klotz, Jr WA5ZNU Leigh at WA5ZNU.org
Wed May 12 16:41:28 UTC 2010

On 05/06/2010 12:29 PM, Arthur Dent wrote:
> "But now here is the finding..... The monitor board has the 7805 SMD version and
> I was feeding from +12Volt and after a while the SMD voltage regulator overheated
> and started to drop the voltage.... and it was the reason to receive/display garbled
> msg.....the display is built into the same box as the gpsdo..... the temp of the gpsdo
> was around 45 Celsius, so inside the box the temp also was elevated so no wonder
> why the SMD voltage reg overheated...."
> Ernie,
> Glad to hear you were able to find the problem. Quite often you can use a can of spray
> coolant to identify thermal problems like this one. The regulator is rated 0.5A but the
> amount of power it was asked to dissipate caused the thermal shutdown. It was working
> just the way it was designed.
> One suggestion I would make is to epoxy a small heatsink onto the chip to make sure
> this doesn't happen again. The 75 ohm resistor you're using will probably take care of
> the problem permanently but even a small heatsink cut and formed from a soda can
> wouldn't hurt.
>                               -Arthur

My fluke.l monitor stopped working today after one overnight session.  
LH is connected at the same time and is reporting good data from the tbolt.

I currently think that it's perhaps a clock speed issue, not a voltage 
issue, because I'm getting 5V out of the regulator right now and it's 
not working.

Here's how I got here:

The monitor is a recent model and has the regulator. I left it on 
overnight, in its original case and out in the open, hooked to a 13.5V 
supply. I had it on a ammeter the whole time, and it never draws more 
than 0.02A.  The backlight is fine and the display works.
It says "No Message" and occasionally gives garbled displays of other data.
Once it said "No Message / PowerSupply Fail" but not again.

Turning it off for an hour didn't help.

I tried it at a lower voltage (8V) and it works some, alternating 
between "No Message" and some data or otehr every few minutes, but the 
data doesn't appear to be accurate (claiming 2D/3D mode when it's not, 
including question marks in numeric fields, etc).

With 12.2V-13.5V in, I get 4.99-5.01V from the square via on the left of 
the board to the 3 round vias next to it, yet I still get "No message" 
or garbled data.


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