1512 ref pin +17 VDC above AGND

[bks]

i have two t15 modules. both passed the initial tests per the assembly instructions, but on one of them i noticed almost a volt of DC between pins 2 and 3 of the output. tracing backwards, i found about +17 on pin 5 of U1, which ought to be close to AGND. also, U1 seemed hotter to the touch on the problematic module, compared to the one with insignificant DC at the output and the U1 reference terminal.

[tpryan]

i found about +17 on pin 5 of U1

Make sure you have a good connection between AGND and PGND with the wiring harness installed. Resistance between J3 pins 2 and 4 should look like a short circuit.

Make absolutely sure that R24 and R25 are the correct value and that the solder joints are good around U2, C10, C11, R24 and R25.

If that doesn't work, jumper U2, pin 7 to AGND and see if the DC voltage at U1, pin 6 disappears. If it does, you probably have a bad U2. If it doesn't you probably have a bad U1.

[bks]

six months later...

the grounds are good, the resistors measure correct, and the soldering looked clean.

grounding U2 pin 7 to AGND did not zero the voltage at U1 pin 6, so i replaced U1. U1 pin 6 still measured most of a volt, and i also noticed U4 becomes very hot, very quickly.

help! thanks.

[tpryan]

What voltages do you measure at U1, pins 2 and 3?

Are you absolutely sure that you have D1 through D4 installed correctly?

Are you absolutely sure that R6 through R9 are the correct values?

Are you absolutely sure that R22 through R25 are the correct values?