Author Topic: Help me fix my buddy's power supply...  (Read 4545 times)

$uperpuma

Help me fix my buddy's power supply...
« on: November 16, 2006, 11:01:35 PM »
My buddy handed me his SignalFlex power supply... he says he plugged a pedal he suspected to be broken into it and now it doesn't power on.  I've got it open and its pretty straight forward. I'm guessing it probably got shorted. The rectifier is a Fairchild KA7809, which is still available, would the 78L09, or the lm317 work in this capacity? If I can find it, I think I  have a 78l09 on hand but I wanted to check... any other part values anyone would suggest for this part?  other than that, I'll change the caps... theres only 3... then by process of elimination that would only leave the transformer... what are the chances its that beast?  thanks for any help!

Rock on
-Nate

Breadboards are as invaluable as underwear - and also need changed... -R.G.

R.G.

Re: Help me fix my buddy's power supply...
« Reply #1 on: November 16, 2006, 11:17:41 PM »
The 7809 is a 1A chip, the 78L09 is a 0.A chip. Get the big one.

These chips are pretty self protecting except in situations where there is a higher voltage on their output than their input. It's good practice when using a three terminal to put a local 22uF or more cap from input to ground terminal right at the IC as close as you can get it. Ideally there will be a diode reverse biased from the output to the input voltage on the regulator, so if the output is pulled up, the diode dumps the current back into the supply and protects the regulator. A smallish cap, 1uF to 10uF on the output is OK, but don't go overboard there, as it can make the thing oscillate.

If it was shorted, it's likely that the transformer or rectifier diodes may be open or shorted as well. Remove the regulator chip and fire up the supply without any regulator chip. Verify by meter that you have at least 11Vdc on the filter cap feeding the regulator before you bother replacing the regulator. You might also want to measure the AC voltage out of the transformer before the regulators.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #2 on: November 16, 2006, 11:48:57 PM »
there is one cap on the input, 2200uF and two on the output 220uF and .1 uF film... the bridge rectifier and a resistor are the remaining parts :)  so if the bridge rectifier is good, and the transformer is good, then I should have 11ish volts at the 7809's input trace?
Breadboards are as invaluable as underwear - and also need changed... -R.G.

R.G.

Re: Help me fix my buddy's power supply...
« Reply #3 on: November 17, 2006, 08:21:40 AM »
Quote
if the bridge rectifier is good, and the transformer is good, then I should have 11ish volts at the 7809's input trace?
You'll have more than 11V. 11V is where the 9V regulator falls out of regulation. You'll probably have more like 14 or 15V, just guessing.

Do measure the AC output of the transformer first, with no regulator in the circuit. When you have that number, the DC on the first filter cap should be about 1.414 times that number, minus two diode drops in the bridge. If it's not, something is wrong with the transformer (not enough voltage) or bridge (not rectifiying right) or the cap (leaking too much current away).
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #4 on: November 18, 2006, 03:56:06 PM »
OK, with the transformer disconnected I get a couple different readings... I get either 0v AC, 25v AC or 55v AC...(almost randomly but its one of those 3 measurements and nothing else)  on each of the leads... using the ground wire coming from the 110v... and no matter how I try I don't get any readings above a volt on the DC scale...  if its the transformer what specs should I look for in a new one?  110-117v input/ <20v output? how many milliamps should it do to handle 5 outputs?
« Last Edit: November 18, 2006, 04:18:58 PM by $uperpuma »
Breadboards are as invaluable as underwear - and also need changed... -R.G.

R.G.

Re: Help me fix my buddy's power supply...
« Reply #5 on: November 18, 2006, 06:18:13 PM »
Do this:

Unplug the transformer from the wall.
Set your meter to measure ohms.
Measure ohms between the two AC wall plug contacts. Should be at most a few hundred ohms, may only be a few ohms.
Measure ohms between the third wire safety ground on the transformer or core to both AC power contacts. Should be unmeasurably high. If it's not, the transformer is damaged in a way that makes it unsafe. Throw it away if this is the case.
Measure ohms from the AC wall contacts to the secondary wires. Again, should be open circuit.
Measure ohms between the two secondary wires. Should be well under 100 ohms, maybe almost a short.

Given that the transformer gives the expected results above:
Switch your meter to AC volts.
Plug in the transformer, taking care to not expose yourself to the power line voltage in doing so.
Measure the AC voltage *between the two secondaries*, not between them and something else. You do not use the ground wire from the 110V side for doing this, the voltage is only between the two secondary wires. One of the purposes of having transformers is to be able to have a secondary that is NOT connected to the primary power in any way. You should measure on the order of 10Vac to maybe 16Vac.
You will not get any DC. Or if you do, the transformer is toast. The DC is only made by the rectifiers making AC into DC.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #6 on: November 21, 2006, 10:05:54 PM »
with the transformer only connected to the wall, secondary only gives about 3.2 mV with a hearty 124 VAC going in... safe to say its toasty? What would be a good replacement? This one? http://www.mouser.com/search/ProductDetail.aspx?R=F44Xvirtualkey55310000virtualkey553-F44X
Breadboards are as invaluable as underwear - and also need changed... -R.G.

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #7 on: November 23, 2006, 01:03:31 AM »
anyone?
Breadboards are as invaluable as underwear - and also need changed... -R.G.

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #8 on: November 25, 2006, 03:24:35 PM »
last bump, then I'll leave it alone...
Breadboards are as invaluable as underwear - and also need changed... -R.G.

R.G.

Re: Help me fix my buddy's power supply...
« Reply #9 on: November 25, 2006, 03:33:50 PM »
Quote
with the transformer only connected to the wall, secondary only gives about 3.2 mV with a hearty 124 VAC going in... safe to say its toasty?
Yep.

Quote
What would be a good replacement? This one?
That one will work if it will fit physically.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #10 on: November 25, 2006, 03:48:07 PM »
Thanks a ton, RG... the one at Small bear, the F200, I think it is...better alternative sizewise... still enough current to run 5 pedals?
Breadboards are as invaluable as underwear - and also need changed... -R.G.

R.G.

Re: Help me fix my buddy's power supply...
« Reply #11 on: November 25, 2006, 07:59:07 PM »
what's the specified current on the F200?

R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

$uperpuma

Re: Help me fix my buddy's power supply...
« Reply #12 on: November 25, 2006, 08:53:02 PM »
its the mouser equivalent of the one Steve Daniels has...200ma for five outputs...  on the pedalboard he'll be powering a mosfet boost, an uglyface, a Proco Rat, a Tube screamer and a boss Phaser... I've got about a 2 1/2" W x 2" H x 2" L area to work with...
Breadboards are as invaluable as underwear - and also need changed... -R.G.