>If there's nothing across the shunt, there should be no difference voltage
>across the input pins. Try putting a short across them and looking at the
>output voltage: it should be zero. If not, change the op-amp.
Did the next best thing: got voltage and resistance readings from
AlphaPower. All readings agree within reasonable tolerance, except output
from the op amp. For example, I have 312 mV on the inverting input, 309.5 mV
on the non-inverting input and 189.5 mV on the output. AlphaPower measures
295 mV on the inverting input, 294 mV on the non-inverting input and .3 mV
on the output. The difference in voltages at the inputs can be accounted for
by the difference in 36V supply voltages between my amplifier and the
amplifier at the factory. Everything else was nominal, including voltages on
the rest of the pins, pin resistance to ground and output resistance to the
D/A converter. The case is now pretty strong for the op amp being the
culprit.
>The high voltage op-amps (there's a similar type available from National)
>always worry me slightly about reliability at maximum supply volts. I
>sometimes wonder how good the process control is to make sure that they
>don't get a long term reliability hazard that would be very hard to find in
>a factory test without life testing at full voltage and high temperatrure.
>That isn't done unless it has to be, because of cost.
Well, although I don't know much about it, I have to agree with you. In
searching for a replacement op amp, I found that the Motorloa MC1536 is no
longer being made. There is a lower voltage version (MC1436) but it won't
work in the circuit. National Semiconductor used to make an equivalent, but
it's obsolete now, too. I figure either these chips didn't sell very well or
they had a high failure rate. I only found two other manufacturers with high
voltage op amps, Harris and Burr-Brown. The Harris unit is pretty close in
specs. The Burr-Brown looks like a much better device as far as supply and
temperature limits, but I'm not sure it's electrical characteristics would
be correct.
The 87A HV board has since been slightly modified, and one of the changes
was a new lower voltage op amp in the plate current measuring circuit. The
36V supply gets stepped down to a lower value for the chip now. AlphaPower
says that the design notes don't indicate that the chip was changed due to
any problem with the higher voltage op amp, or even due to lack of
availability of the MC1536 chip. They've never seen the strange plate
current reading I have nor any failure of that particular op amp.
The factory was out of spares, but not to worry. Scott Erhorn at AlphaPower
found a supplier for some MC1536's and will have one drop shipped to me. I
really appreciated him going the extra mile and locating the part for me.
>If in doubt, change the op amp. Resolder the joints on the pcb, too. I'm
>always surprised at how often that cures problems - I shouldn't be after 35
>years in the industry, (20 of them in an IC manufacturing firm) but there
>you are.
I gave that a half-hearted go. I did notice some flickering in the lowest
plate current LED when I was taking measurements on the non-inverting input,
so I thought maybe the solder joint was bad. This didn't make a whole lot of
sense, because I was measuring voltage at the top of the pin, not at the
solder pad, and it was nominal. Could be a loose pin connection inside the
chip, though, which could explain the odd readings. Anyway, I gave the two
input pins a quick touchup with the soldering iron from the top of the
board. I didn't want to do a full-blown resolder job until I have a
replacement MC1536 chip here -- just in case the resolder exacerbates
whatever problem is going on inside the chip now. For example, if the chip
output went to the max, the amp plate current protection circuit would trip
at turnon and I wouldn't be able to use the amp. Also, I'd have had to
remove and replace the board, which I need to do when the new chip gets
here, anyway. I'll probably just replace the chip, then do up a test circuit
for the old one. If it works outside the amp, then I'd say the odds are that
it was cold solder joints on the chip or desoldering produced enough heat to
fix whatever was wrong inside.
73, Dick, WC1M
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