Mnats PSU odd output voltages?

I'm pretty sure I'm doing something wrong here. I have a Mnats PSU board (30.04.06) that was left off from a 1176 build and now I'm about to recycle it as a variable psu with the 5K trimmers for the spot-on +/-15V to be used on a Studer 169 EQ diy. However, I'm getting only 5.8VDC to 6.1VDC on the side with the LM317 regulator. The other side with the LM337 gives me 36VDC when the trim pot is turned at max. The "failing" LM317 side has a LED that is nicely lit, but the working LM337 side LED starts shimmering only little when the trimmer is nearing the maximum positions (roughly over 30VDC or so).

I can measure 120 ohms over R6 but can't get anything when measuring over R4. I've got a bigger wattage (0.7W?) resistor on R6 and smaller (0.25W me thinks?) on R4, just something I had on hand. Could the problem be here with the R4?

It's running on a 25-0-25 50VAC transformer that just came today from Farnell with lots of other goodies that wait to be assembled Just would like to get one thing working first before I go on with the others.. and the PSU is needed soon already. Any ideas what's going on?

-J

I received your email. Please note that as stated in my board sales post the price is for boards only and does not include individual support.

If you "can't get anything" when you measure R4 why not change that first, then report back to the forum?

Have a look at my newly revised 1176 FAQ, the first answer is applicable to any DIY project (as long as the schematic is verified as correct).

Ahh yes, sorry about the private mail - Didn't remember the whole thing until I read it again in the white market post  :-[

Except for the led, I got it working with the Studer EQ.. now I feel even more embarrassed : Thanks for the reply though!

So what was the fault?

Just calculate the LED current limiting resistor based on the voltage you have selected - there's no reason why it shouldn't work as long as you have your desired voltage at that output.

The LED's only work after you set the voltages to the requirements for your project. Then calculate the resistors needed. I never use the LED's. Altho I did build a bench supply from one that works awesome.

Here is a calculator for the voltage http://www.ohmslawcalculator.com/ohms_law_calculator.php

337 data sheet here: http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/3/LM337.shtml

For some unknown reason I had a 100kohm resistor in place of the 120ohm and that's why I couldn't get a reading with my meter (though I thought I did measure it with 2M area too). No idea how it got there, maybe just picked the resistor from a pack that says 120ohm and didn't measure it. Embarrassing

I tried changing the 390ohm resistor for the -15 side led to 100ohm and 10ohm, even tried making a straight jump without any resistor but it's still only dim when turning the trimmer to full. Well it doesn't affect the use of the PSU so I left it as it is.

JoleFIN said:

I tried changing the 390ohm resistor for the -15 side led to 100ohm and 10ohm, even tried making a straight jump without any resistor but it's still only dim when turning the trimmer to full. Well it doesn't affect the use of the PSU so I left it as it is.

Click to expand...

The LED has surely been damaged by excessive current.

Maybe in the beginning already? It's dim lit, but should it have had a bigger resistor from the very start? The + side has a 840ohm if I remember the correct value, whereas the minus side has a 390ohm. I was guessing there's difference between the LM317 and LM337 but guess there's not.