Norm Dresner wrote:
"mike" wrote in message ...

Norm Dresner wrote:

Just to be clear, I recommended you use the lm317 to power the new 741.
The sine distortion is gonna be critically dependent on the stability
of this reference voltage. I never used a 317 anywhere I cared about
voltage stability and have no direct experience. May be just fine.
Also, if you use it to replace the supply, you also need to worry about
voltage differential when you short the 20V supply. Don't remember what
the max differential for a lm317 might be.

While you're at it, put some resistance in
pin 3 of U410 so C406 won't blast the input on power down.

mike


My point in checking for oscillations was that sometimes an oscillating

Op

Amp can give some very strange readings on a simply DMM.

Norm



Aha! I understand -- use the IC regulator to drop the voltage to the Op Amp
to a level where I could use a commercial grade safely. Good point.
Regarding max voltage inputs, I think I can safely use an LM7824 to drop
whatever the input is down to 24 volts to drive a jellybean 741 and I've got
one or two in some drawer somewhere around here.

Here's your quandry. You need high enough volts to supply 20V + 1 diode
drop + 2 resistor drops + the headroom the 741 needs to supply that
current. 24V might not be enough??? I haven't looked up the specs on a
741 in 25 years.
I always designed to a minimum trough voltage of 22V. It may be higher
depending on the max load on C400. You should measure the trough voltage
at maximum output and low line. That'll give you how much headroom you
have for the preregulator. It's probably gonna be a negative number.
That's why you'll have to decide how low you can let the line go and how
fine you can cut the 741 headroom.





As for C406, my circuit diagram shows a diode CR410 whose anode is connected
to the + side of the capacitor and the cathode to the V+ input to the Op Amp
to keep its input voltage no more than a diode drop above the supply
voltage. I couldn't find the diode on the circuit board anywhere or in the
parts location diagram but it's clearly on the schematic. If I can't find
it, I'll put a 1N4148 in as a "jumper"

The diode is not on my early schematic. The FG501 was one of the first
TM500 units designed. Over time the more glaring design mistakes got fixed.




Thanks
Norm




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