"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
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