Troubleshooting 317/337 Regulator Circuits

[TerryC]

I'm attempting to fix a preamplifier, and have discovered that the negative supply volatage for the main op-amps is only -1.2 vdc, when it should be around -16.4 vdc. The negative regulator circuit uses a standard LM337 topology with two resistors for output adjustment, and electrolytics at the input, output, and adjustment pins.

Given that the input voltage is a solid -28v, the output voltage is down to -1.2 volts, and the voltage at the adjustment pin is around 2mv, which component(s) is likely bad? I will likely replace all components, since the cost is not really an issue, but it would be great if I didn't have to replace the LM337.

 

[BWRX]

Take the regulator out of the circuit and make sure there aren't any shorts and the resistors are all the right values.

 

[Christer]

If you have -1.2 V, it sounds like the regulator is working but the adjust node is somehow shorted to ground. Is there a Cadj capacitor there that might have shorted?

 

[TerryC]

Thanks for the excellent suggestions. I'll post a follow up when I'm done.

 

[Nisbeth]

You know the 337 has a different pinout from the 317, right? (input and output pins switched)


/U.

 

[TerryC]

If you have -1.2 V, it sounds like the regulator is working but the adjust node is somehow shorted to ground. Is there a Cadj capacitor there that might have shorted?

I removed the adjustment capacitor and the output voltage came back up to -16.4v. I then replaced this capacitor, and the complimentary capacitor in the +V regulator circuit, and all is working fine again. Thanks to all for your input.