newbie needs help with +/-15v psu

[XXLPeanut]

Hello.
Ive build this psu
http://www.albertkreuzer.com/psu01.htm
And now for the question. When I measure my +15v it gives +24.5v, and my -15v gives -14.5v. What could this be? Could my voltage ragulator ( instead of the 4, 1N4001 ) be fried??
Any help will be nice.. Thanks
Nikolaj

 

[Limhes]

Make sure your voltage regulator is a 7815 and no 7825, that might help 🙂

 

[XXLPeanut]

Make sure your voltage regulator is a 7815 and no 7825, that might help 🙂

The writing on top is
B40
0800
3702

I cant find my receipt so im not sure what number they have
thanks for the fast reply limhes

 

[anatech]

Hi Limhes,
You meant 7824, right? 😉

Anyway, a 15 VAC source should give you around 20 VDC, maybe a touch higher. Not 24.5 VDC though!

Make sure your ground trace is okay and measure your actual input voltage to the bridge.

-Chris

 

[XXLPeanut]

I just measured again, at it gives me +26.4 and -18.6. The difference is 45V instead of 30V, which also matches what I measure between + and -.
What do you mean by the ground trace?

Nikolaj

 

[XXLPeanut]

ehm.. this is wierd... when i turn the psu upside down and measure I get a pretty +/- 15V

 

[XXLPeanut]

and now when i connect the circuit to the psu I get +15V and -3.2 V
Any ideas???????
Thanks

 

[anatech]

Hi Nikolaj,
If I had it in front of me, I would be checking all solder joints and the PCB for cracked or damaged traces.

-Chris

 

[XXLPeanut]

Hi Nikolaj,
If I had it in front of me, I would be checking all solder joints and the PCB for cracked or damaged traces.

-Chris

I will do that to start with..
When I disconnect the pcb I get +15 and-18 Volts, but +15/-3 when connected.
Im thinking of disconnecting all potmeters, switches and so on on the pcb and measure the +/- supply each time I connect a new meter or switch.. Could it be a wrong connected meter thats giving me trouble?

 

[dfdye]

Are you measuring this with a load on the rails?

 

[XXLPeanut]

no I havnt connected any instrument to the preamp yet.. only connected the pcb to the psu
Thanks

 

[dfdye]

Try dropping in a dummy resistor or an LED on each rail and see if you are getting the correct voltage then.

 

[XXLPeanut]

Try dropping in a dummy resistor or an LED on each rail and see if you are getting the correct voltage then.

I think one of my friends talked about this as well.. He said I should try a restistor between the output of the 7915 and ground.. Is this what u mean?
In that case what size should I choose? Im a beginner at this so bare with me 🙂
Thank you all for the fast replys

 

[dfdye]

I would put a 3K resistor (or something thereabout) between + and ground and another resistor between - and ground.

 

[BobEllis]

From the link you posted:

The 7915 (at least the "made in Morocco" I used) needs a small load (some mA) to work correctly. If you get funny voltages (-18.4V or so), put a resistor from the 7915 output to ground (2k2 works good). Or it might be a good idea to put the "power on" LED with a 1k5 resistor to the negative rail (take care of polarity!).

What happens when you hook up the pre board says there is an error there - check the polarities of caps, etc.

 

[XXLPeanut]

I would put a 3K resistor (or something thereabout) between + and ground and another resistor between - and ground.

I put in two 4.7k resistors as you said, and it made no difference. What about my idea of disconnecting all swithces and potmeters?

Thanks

 

[XXLPeanut]

From the link you posted:



What happens when you hook up the pre board says there is an error there - check the polarities of caps, etc.

Do you mean putting a LED and resistor in series between +15V and ground?
I already have the two resistors in, but is it enough to put one between +15V and ground?

Thanks

 

[XXLPeanut]

Ok let me just summon up.
With the pcb connected, the outputdiff on the voltage regulator is 51.9V.
The writing on top of it is
B40
0800
3702
I thought I bought the right type but i could be wrong

1. How does this fit in with the +15V and -3.2 V i measure on +/-15V?
2. And should I only have a resistor between -15V and ground? not +15v and ground or how?
3. I should be able to measure + and - 15V at all times right? No matte the load ect. Only the amperes will change?

I havn't connected anything to the pcb input yet.
Any ideas?

Thanks you

 

[BobEllis]

Dig up your order and look to see what you bought - i am not familiar enough to say whether those markings correspond to some one's version of the 7815 or 7915. Go to thte manufacturer's website and download the data sheet. If the data sheet doesn't discuss marking, do a search on teh manufacturer's website for "marking" or "marking conventions"

You should have a load resistor to ground from both positive and negative regulators. You mentioned early on that you got -18 some volts on the negative side - exactly the strange behavior that the source website described as being related to not having a load. These chip regulators may need a bit of a load to make them work, then the voltage should be stable. Anything on the order of 1-3K or so ought to work fine. You can put an LED in series with each of the resistors to

Turning the board over making everything work great says that you have a bad solder joint somewhere or broken trace. Double check your joints. Reheat so that they all come up shiny.

When you hook up your preamp, the negative rail drops to 3V - this says that you have a near short from the preamp's negative rail to ground. Did you build this on a board or point to point? Check for solder bridges and components (especially transistors and electrolytic capacitors) in backwards.

 

[anatech]

Hi Nikolaj,
I still think something odd is happening with your ground (common).

What are your AC voltages going into the PCB?
What rectified DC voltages do you get from your ground (common) on the PCB, before the regulators?

These will help come to a solution.

-Chris