On Thu, 1 Mar 2012 14:30:32 -0500, "John Smith"
put finger to keyboard and composed:

The mains voltage selector is set correctly for the area I'm working in. So
+75V and -75V rails seem to be correct.

That means that quite a few components are operating close to spec or
slightly in excess of spec.

I removed the replacement TA7317P and fitted socket strip.

With the power on, here are the voltage readings I now get. Note that no
chip is fitted, just the diodes mentioned above.

1. -15V
2. 8.8mV
3. 0.17V
4. 0V
5. -75V
6. 0V
7. 0V
8. 2.67V
9. 2.7V

The voltage on C314 is varying between -42 and -49V, presumably due to
ripple. I'll scope it tomorrow. The voltage at pin 1 of the socket is also
varying a bit too.

I'd change the capacitor, but I don't believe that level of ripple
would be enough to influence the circuit.

AISI, pin #1 senses the current in the 0.33 ohm load sharing/sensing
resistors R33, R34, R35, and R36. It would require about 2A to turn on
Q324. Q325 would then turn on after a delay determined by C311 and
R357, sending pin #1 of the TA7317P high and causing the relay to
open.

My next move will be to test the remaining TA7317P I bought, with a 22K
resistor to pin 9 and pin 4 grounded with no other pins connected and see
what I get on pin 9.

I suspect that your replacement parts may have a string of 3 diodes on
pin #9. If the voltage at pin #5 is -0.6V, then this would tend to
confirm it.

I notice that the datasheet identifies each diode as a Q rather than a
D, so I suspect that the original part uses diode-connected
transistors rather than actual diodes. Perhaps your replacement part
uses the latter?

- Franc Zabkar
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