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Author TB3031 ISSUE 2/OCT 2004: Problem with PSU
janvanvolt
 Hi there, just finished my TB3031 (still working on the midi2cv ...) and while trying to get 5.333V tuning, i am stuck with wrong values. Somebody might have a clue. It does NOT seem to be a short somewhere, but the power readings are really odd (and point to something for the expert, which i am not) Power Readings: input ac = 16v AC output +12v and G1 = 8.25V constant output -12v and G1 = -8.94V constant tp1 and G1 = 3.71V constant Resistor Measurements: +/- 12V = 8.1K gnd to +12v = 2k7 gnd to - 12v = 5,3k Trying to tune the PSU trimmer does not yield in any change on TP1. Ideas ?
JohnLRice
 Hi, I'm no expert and I only have ideas, no real knowledge, but FWIW: Check that your regulators are the correct part numbers. Check the voltage input to the regulators, it should be the same or (I think) preferably somewhat higher than the desired output voltage. Check to make sure the rectifier diodes are installed with the correct orientation. Best of luck!
janvanvolt
 Regulators are 7912ct and tip31a / 7342 and diodes are the way they should be.
Synthbuilder
 janvanvolt wrote: input ac = 16v AC output +12v and G1 = 8.25V constant output -12v and G1 = -8.94V constant

What are you measuring your AC voltage with respect to? The voltage should be 13V to 18V ac across the output terminals of the power pack.

I think you do not have enough AC voltage going into your unit.

What is the voltage (with respect to ground) across the big smoothing caps? You can measure these most easily at U14 and U10.

The voltage at pin 11 of U14 should be over 15V with respect to ground.

The voltage at the middle pin of U10 should be over -15V with respect to ground.

Check also that you are using AC1 and AC2 to connect to your power pack and AC3 is unconnected.
janvanvolt
 Thanks for the info. Indeed i used the wrong AC inputs. All values now excellent. Next test: audio :-)
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