"Chuck Harris" wrote in message
...
Norm Dresner wrote:


My point in checking for oscillations was that sometimes an oscillating
Op
Amp can give some very strange readings on a simply DMM.

Norm


It is probably just a blown opamp, but I have also seen this behavior
on some manufacturers 741's when any input is forced to be higher than
1.2V from the + rail.

As a simple check, short the inv and non inv inputs of the opamp together,
and then:

a) check their voltage relative to the +supply. and
b) check the output voltage.

-Chuck Harris

Chuck

I really don't think it'll do much good to short the inputs together. The
non-inverting input was 6.2V (the voltage from a zener) and the
non-inverting input was about 8.0-8.2V while the output was stuck at ~28V.
(It's being operated single supply with a ~30V filtered, unregulated output
from the diode bridge and capacitor). If that much unbalance doesn't swing
the output off the positive rail, then I don't feel there's much hope for it
at all.

This 741 is the error amplifier with an MJE340 pass transistor which is
supposed to output a regulated +20V based on the zener input. This is an
ancient Tektronix FG501 plug-in function generator which I bought used
knowing that the pilot light came on and that it was otherwise untested.

Norm