Seedysea

Green Pre PSU Phantom problem
« on: September 07, 2005, 06:40:27 PM »
I have been building a green pre as my first foray into serious diy.  I am learning a lot as I go, a very steep learning curve it is too.  

I have a problem with phantom on the psu, which is one of PeterC?s boards.  I get +15.3V and ?15.1V at the outputs, so that part of the circuit seems ok, but nothing out of phantom.  That?s 0 V

I read as many threads as I could find and Peter had suggested to someone else to take out the voltage doubler and tripler caps and diodes and reinsert one at a time to check these.  I did this and, although I don?t understand how a diode and a cap can double voltage, I know that this part of the circuit works, which was kind of cool to learn.  I get 25.5V, 51V and 76.5V at each stage, so I can see that it works perfectly.  

These figures are higher than those Peter kindly supplied on the documentation.  Is it possible that the regulator (LM317) is not able to cope with this?   I checked the voltage on the AC side of the first diode, (straight out of the transformer), and get 17.8V AC.   The transformer is a 15-0-15 30VA that I want to use to power four preamps.

Can anyone help educate me here?  What further tests can I make?

BTW, my first pre seems ok, although I haven?t put the ic?s in yet.  I measured the voltages as per Peter?s chart and they seem good considering my voltage input to the board.  I want to get the psu going before I move on.


peterc

Green Pre PSU Phantom problem
« Reply #1 on: September 08, 2005, 02:58:54 AM »
Hi Craig

What is the voltage of your transformer secondary?


Peter
If opportunity doesn't knock, build a door.

Seedysea

Green Pre PSU Phantom problem
« Reply #2 on: September 08, 2005, 06:25:00 PM »
Hi Peter,

I have only measured one side, but that is 17.8VA.  I expected to see 15VA as on the data sheet.  

I wondered if starting with a higher than expected voltage would result in too much voltage at the regulator.  Is this a possible cause of the problem do you think?

The transformer is a toroidal from RS, made by Toroid International. FG range.

What about the primary side?  What would be the effect of too much voltage there?  

I'll measure all the voltages tonight and see if there are any others that seem out of place.

Craig

matta

Green Pre PSU Phantom problem
« Reply #3 on: September 08, 2005, 06:55:32 PM »
Craig,

You will more often than not get more on the secondaries than the quoted spec 17.8VA is about right on 15VA, well at least a decent quality one.

Since after the diode you have a couple caps,a few resistors and a regulator I'd check there.

Before you do ANYTHING... you'll see a green line on the parts overlay, that is a jumper, you need to connect the diode pad to that other pad with a piece of wire, I used insulated hook up wire on top of the board.  You may very well have done this.... so forgive me, but often the small things are missed.... right if you have done so... proceed to my checklist

First, check for no visible shorts around the 48V part of the circuit.
Second, check your resistor values, are they correct? You should have a 220R and 8K2 there.
Thirdly check the orientation of the 2 10uF, 63V caps, are they facing the right direction?
Lastly, try a fresh 317 Reg, there is a chance you could have shorted it or over heated it while soldering.

Hope this helps. Great you are nearing the end, and we will help you get through this. Report back.

Cheers

Matt
Matt Allison
www.matt-allison.com

Quote
We are not going to start thinking of ways to get an octopus to commit a crime, cause that just has failure written all over it – Earl J Hickey

peterc

Green Pre PSU Phantom problem
« Reply #4 on: September 09, 2005, 02:51:22 AM »
Craig

Your voltages look good, & once you connect a load (the preamps), they will probably go down a volt or so.

As Matt says, you need to put in a link between the third diode & the regulator input. The regulator will be able to handle the 76v on the input.

If you have done this, check the 317.

Regards
Peter
If opportunity doesn't knock, build a door.

Seedysea

Green Pre PSU Phantom problem
« Reply #5 on: September 11, 2005, 07:05:48 PM »
Thanks Matt and Peter,

I checked the circuit over and everything looked ok, but I resoldered the reg pads and there must have been a dry joint I didn?t spot as that fixed it.

I was very pleased to see 48V at the output finally, along with + and ? 15V.

So, I put the ic?s in place and plugged it in and the pre works.  I haven?t had time over the weekend to give it a good check, but phantom, the hpf and polarity switches all work.  It doesn?t have as much gain as I expected, and a little buzz that I suspect is a grounding issue.  Since it is only on the bench at the moment, I will try racking it up to see if I can get rid of the noise.  It did sound very nice apart from this though.  Then I have three more to practice on.

I?m on my way to diy addiction and it feels good.  This forum is unique in webland and I look forward to more hours spent learning and hopefully contributing to the threads.  Thanks to everyone who has ever posted about the green pre, I read your posts, and they helped me get this far.


 

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