Troubleshooting 317/337 Regulator Circuits - diyAudio
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Old 5th August 2006, 08:54 PM   #1
TerryC is offline TerryC  United States
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Default Troubleshooting 317/337 Regulator Circuits

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.
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Old 5th August 2006, 09:00 PM   #2
BWRX is offline BWRX  United States
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Take the regulator out of the circuit and make sure there aren't any shorts and the resistors are all the right values.
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Old 5th August 2006, 10:05 PM   #3
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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?
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Old 6th August 2006, 04:50 PM   #4
TerryC is offline TerryC  United States
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Thanks for the excellent suggestions. I'll post a follow up when I'm done.
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Old 6th August 2006, 09:43 PM   #5
Nisbeth is offline Nisbeth  Denmark
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You know the 337 has a different pinout from the 317, right? (input and output pins switched)


/U.
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Old 12th August 2006, 05:42 PM   #6
TerryC is offline TerryC  United States
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Quote:
Originally posted by 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?
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.
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