Clark,
U1 pin 7 is the VSMTR signal and the voltage source comes from the
product detector U5 pin 2.
Either the product detector is damaged or U1 pin 7 has a shorted input.
To check which, remove U1 and bend pin 7 out a bit, then plug it back
into the socket with pin 7 hanging on the outside.
If U5 pin 2 (and pin 1) are then at 1.4 volts, U5 is likely OK.
U1 pins 21 through 28 drive the display directly, so that is why the
display is garbage. Specifically pin 23 drives segment C and pin 25
drives segment E - those are the two vertical bars on either side of the
lower portion of the display.
Yes, I would replace U1, and if the test above shows U5 is damaged,
replace it as well. Generally the SA612 ICs are quite hardy, but yes
damage can occur just the same.
73,
Don W3FPR
On 2/2/2015 6:04 PM, engineercm wrote:
While doing some rework, a hot lead fell out of my hand and dropped on the
board. A few segments on the display lit up briefly and then an eerie
silence. Except for the little map light, nothing works. I've checked the
DC voltages on the IC's and voltage regulators and the only voltages I can
find out of limit are on U1:
pin 7: 0 and should be 1.4
pin 23: .3 and should be 4.9
pin 25: .5 and should be 5.0
It would appear that the MCU is blown although I can't quite see how these
pins could keep the display from working. The hot lead dropped down in the
physical between the 3 blue buttons and the MCU. I've verified that the
xtal oscillator that drives the MCU is running.
Before I order a new MCU, are there any other components that I might have
blown due to carelessness that I can check?
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