newbie: troubleshooting power supply

[Guest]

Hi folks,

I shorted a power supply (one rail to another) and blew the fuse in a bipolar (27v, with 48v) power supply for mic pres. 7th circle to be specific.

I replaced the fuse, but I'm getting exactly +/- 32 (instead of 27) and 54.5 on the phantom. It seems highly conicidental that I'm getting exactly 5v extra on the B+/- and 10v on the phantom. Does that ring any bells?

I'm wondering if anyone can think of where to start. I'm also wondering if anyone knows how to test capacitors.

Thanks, and my signature says it all.

kb

 

[Mbira]

Do you have a proper load on the power supply? Is it plugged into the card?

Joel

 

[Guest]

No...does there need to be?

I was under the impression that it put out 27 v (which is regulated on the card to 24v). Should I be afraid to put a card on it with that voltage?

kb

 

[alk509]

[quote author="thearnicasync"][...] it put out 27 v (which is regulated on the card to 24v).[/quote]

If the card has a 24V regulator, then 27V is pretty much the bare minimum it will take... 32V should be OK.

Peace,
Al.

 

[Guest]

Thanks everyone. FWIW, it's supposed to put out +/- 27v, 48v without a load, according to the seventh circle site.

:roll:

kb

 

[Mbira]

I'm a total newbee too-I learned that just last year on my first green-pre with the power supply. It was supposed to do +-15v, but with no load it is at +-18v. put on a nice fat 2 watt resistor (I don't remember that math to figure out what resistance) and it drops.

Black magic I tell you.

I don't know the power supply, but if there are voltage regulators there, check each one and see what voltage is coming out. Go through each component from the transformer to the regulators to see if there is anything weird. I would think that if you are getting the same + and - voltage then there isn't a bad component, maybe there is a resistor that is the wrong value? Did you get +-27 before you shorted tit out?
Joel

 

[Guest]

Yeah! It worked fine until I shorted an output (either the 48v or 27v I don't kow).

Interesting, thanks A CAGILLION for the resistor idea. I just asked over at the seventh circle site if it might be blown resistor somewhere.

Newbies helping newbies. Give me a hug!

:grin:

Kelly

 

[ti2m]

First of all, be careful and don't kill yourself working on this stuff if you don't know what you are doing! Stay away from the 120VAC when the unit is plugged in. The one hand in the pocket rule is a good one.

With that said...

Do you have the PS02? If so, you most likely blew (shorted) the LM317/337 regluators in it. The easiest thing to do is check to see if the regs are shorted input to otput. If so, then there is your problem. Also, make sure you don't have any diodes shorted. It would probably still be blowing fuses if there were any bad diodes.

I don't believe that 32V will harm a 24V regulator, but it may depend on the brand...

The PSU may act a little strange without a load on it, the LM317/337 need to have about 5mA of current drawn out of them to regulate properly, if they don't the voltage won't be right. With a 27V output, a 5400 ohm resistor will draw 5mA. A 4.7K will draw 5.4mA which would be fine and easy to find at Radio Shack (eeeww!). A 10K on the 48V output will be fine. 1/4 watt is also fine for all of these. If the voltages are still high with the resistors from their respective outputs to ground then you have a problem with the regs. They are pretty cheap.

Here is what probably happened (my guess): LM317/337 regs have a maximum input to output voltage of 40V. What that means is that if you have 50V going in, you can only regulate the output down to 10v before the regulator blows up. It sounds like you have 32V going in and you shorted the output to -27V! The negative reg saw a similar thing. I don't have an answer for why the +48 went out... unless it needs a load!

I hope this helps!

Tim

 

[Mbira]

Hey Tim (or anybody),
What is the math to figure out what size resistor to use to put a load on a curcuit? I know everyone here but me knows this, so sorry. :roll:

Joel

 

[Guest]

Tim, thanks so much!

I'm going to study your guess thoroughly. For my own knowledge, I will learn about the regs, i.e. why they can only regulate down to 10v. I haven't gotten that far in the book yet. :grin:

As it turned out, the battery in my multimeter was going bad. I changed it, and voila: 27.1, 27.3, and 48.5.

I guess you live and you learn, but I feel stupid.

Thanks so much everybody, and I'm sorry to have wasted your time.

kelly

 

[Guest]

[quote author="ti2m"]It sounds like you have 32V going in and you shorted the output to -27V! The negative reg saw a similar thing.

Tim[/quote]

Do you mean that the regs saw a sum of 32v + -27v (for 59v)?

kb

 

[ti2m]

>Do you mean that the regs saw a sum of 32v + -27v (for 59v)?

>kb


That is what I was thinking, but it turned out to be a bad DMM battery!

When I said the regs could only regulate down to 10v that was just a "for instance" if you were giving them 50V on their inputs. They will reg down to 1.5V I believe, but then you can only give them 41.5V at their input...if I'm doing my math right.

I'm glad it was a non-problem!

Tim