DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

I tested the terminals for the PSU and got it reading 48, 18, -18. Great! Just a weird thing happening... When I first turned the switch, the onboard LED lit up. I adjusted the 48V trimmer, great! Then the LED went out when I started to turn the 18V trimmer... The terminals were still reading 18V though. I was using my fingernail to turn the trimmers. When I put my finger on the trimmer, the onboard LED would light, when off, the LED turned off. Weird... Whenever I apply pressure to any of the three trimmers, the onboard LED turns on. How can I fix this?

The trimmers do not control whether the LEDs light up or not. The trimmers just control the output voltage. You can adjust it from 1.2V to 21V+.



What I'm afraid happened is you push your fingernail hard on that trimmer and applied a lot of pressure downward that broke some mechanical connection.

Do you have a screwdriver? Try using a screwdriver.

What voltages are you getting?

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I doubt anything broke because I was not pushing hard at all. What I mean is that when not touching the trimmers, the on-board LED is off. When I touch the metal part of the trimmer, the LED pops on. Sometimes it flickers when I don't touch it. I put the outboard LED in to test it. And it doesn't flicker or turn off. It's always on regardless of whether I'm touching a trimmer or not. If I did break something, why would the LED go on when I touch ANY of the three trimmers?

Really weird. Do you have a photo of the unit?

Have you tried adjusting the trimmer screws the other way/direction to get 1.2Volts, and then adjust it back again to get 18V.



Okay... here's a question... without touching the trimmer, what voltage are you getting at the output?

If you touch the trimmer, what voltage are you getting?



Have you checked for any shorts or cold solder in your PSU board? Anything getting extremely hot?

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WEIRDER! It's working now. Both onboard and off board LEDs are on constantly even when I touch the trimmers. My readings are still good. But to make sure... my black pointer thing from my voltmeter should always stay in contact with the ground right? That's the way I measured and adjusted... I just moved the red into contact with whichever I wanted to measure. I hope this is correct!

Yes. The black test leads are the ground and just point the red test lead into where you want to measure the voltage.

My *guess* is you have some cold solder or flaky connection on the PCB and pressing it down with your finger (the trimmer, which probably causes the flaky connection to make contact) completes the connection somewhere on the PCB and the LED lights up. Also, you said it flickers... so that's a sign of a flaky connection. I don't think it has anything to do with the trimmers not working since you said you're getting 18V output.

Run your soldering iron on "questionable solder pads", and make sure you have good connection on everything. Make sure some lead goes through the hole so it makes good contact inside the hole (and not just solder on the bottom side).

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Just to let you know... I went over some solder points until I found the culprit... I believe one of the LED leads wasn't soldered well enough. Thanks for he help!