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Can't get U1 voltage down
Posted:
Sat Oct 30, 2004 10:04 amby kid
When measuring the voltage between pins 3&4 and 4&5 of U1 the lowest I can get it is 24.9 volts. I checked all of the things mentioned on the web site like backwards caps and diodes or shorts around U1,2,3. Any advice on what to do?
Also what is the happiest voltage for SC25 opamp?
Posted:
Sat Oct 30, 2004 11:11 amby tpryan
Either something is very wrong or you're cranking the adjustment pots in the wrong direction. That sounds like the highest voltage you might expect to see, not the lowest. Are you measuring +/-27V at the power connector?
Whatever you do, don't stick the SC25 in there with +/-25V on it. It won't blow up immediately, but it won't be happy long-term. I run them at +/-18, but they'll work fine at +/-24V. There's no real need to run them with such a large rail voltage, however, since the 1:2 output transformer doubles the output voltage.
Posted:
Sat Oct 30, 2004 11:43 amby kid
I checke the voltagwe at the power connector and I am getting +27/-27 volts. I built a second A12 and it is measuring pefectly. R27/R29 don't seem to be doing anything. I turned them in both directions just to see a change and nothing happens.
Posted:
Sat Oct 30, 2004 11:51 amby tpryan
Are you sure you haven't inadvertently swapped the regulator ICs themselves? Since you've got a working one, you're in a better position to troubleshoot the bad one than I am at the moment. Measure the voltages at each regulator pin and tell us what you find.
Posted:
Sat Oct 30, 2004 12:42 pmby kid
That was it. I swapped the regulators around and everything is measuring fine. You are right Tim about desoldering a component with plated through holes not being easy. One other thing now is what should the volateg be adjusted to with the SC25?
One other problem is the red diode is not glowing.
Posted:
Sat Oct 30, 2004 2:07 pmby Category 5
kid \$m[1]:That was it. I swapped the regulators around and everything is measuring fine. You are right Tim about desoldering a component with plated through holes not being easy. One other thing now is what should the volateg be adjusted to with the SC25?
One other problem is the red diode is not glowing.
The red diode is probably in backwards. Since it is a diode it can only work one way.
You should run the op-amps at +/-20v. That will certainly give you more headroom than you'll need for digital recording. As Tim said, they're fine with up to 24v, but why run them that hot when you'll never need the headroom?
Shane