On Tue, 21 Feb 2012 06:45:08 -0500, JW wrote:

A heated "discussion" between JL and JT prompted me to post this. I often
get these HP 8116A generators in for repair that have processor reset
problems. CR3 and CR4 in the reset circuit become leaky which pull the
LM324 input negative and seems to drive the LM324 outputs into unexpected
states.

Is it that leaky?

CR4, driving + input of U27A would have to be ~2Meg or less to upset
U27A.

LM324, like LM339, on positive input only, will reverse phase at the
output, if + input exceeds Vbe below negative rail. So CR4 is
explainable.

CR3 goes to a negative input, so the output could possibly go
positive, but that would a normal behavior for the LM324.

So what are the "unexpected states"?

1N4150 is rated at 50V reverse. Has "leakage" bee verified?

On a power up the voltage at CR3 and CR4 swings from about +15 to
-15V or so. They are driven positive first by a LM348 through a diode,
then pulled down through a 100K resistor to -15V. See the two jpgs I
posted for the schematic. My questions are 1. Is it expected that the
slightly negative voltage at the input would do this? and 2. Is there a
reason why CR3 and CR4 fail so often? The diodes are 1N4150. Obviously
there isn't much current flowing through the diodes... Maybe there was a
bad bunch of parts?

Thanks.

...Jim Thompson
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