[sdiy] Noisy Power Supply

[Needham, Alan]

Scott,
The following may be useful to you ... ?
http://www.electro-tech-online.com/electronic-theory/212-using-oscilloscopes.html

2ms/division - maybe 5 divisions per pulse = 10ms per pulse

   f = 1/t = 1/10e-3 = 100Hz --- maybe --- so it sounds like mains hum
  
You say you have the scope ref. connected to supply ground...
is that the mains earth ? is earth connected to anywhere else on the synth?
(either PSU pos or neg or via a cable to an amplifier ?)
You seem to be describing symptoms of poor smoothing AND the positive rail
being earthed (so no signal seen there, relative to earth).
What do you see across the power rails (neg to pos)?

If you can disconnect one leg of the suspect capacitor (power OFF of course),
use your meter on ohms test to get some idea of how long it takes for the
cap to charge (meter reads open circuit), swap the meter leads round and test
again. Do this as often as you like to get an idea of charge speed, if you
have another big cap, try that for comparison -- I would expect a couple
of seconds each way round for 1000uF, longer is better! bigger caps take
longer to charge.
This test IS a bit tongue-in-cheek but it's better than nothing !
NOTE that different meters will give different results, as will different
ranges on the same meter, so use one range, one meter.

Alternatively ... swap the cap for a new one and see if things improve ?
this could end up proving the old cap was OK and a rectifier diode has
failed - all four diodes could be tested with a meter !.

Other thoughts... it hums - does it get hot ? if not it could just be a
cheap transformer with loose laminates - mount it on rubber pads to reduce
vibration transferred to your cabinet.
[no offence to power-one fans - I STILL haven't seen one !]

	Alan - troglodytes of the cave unite !


# -----Original Message-----
# From: synth-diy-bounces at dropmix.xs4all.nl
# [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Scott
# Sent: 21 June 2007 3:57 PM
# To: synth-diy at dropmix.xs4all.nl
# Subject: RE: [sdiy] Noisy Power Supply
#
#
# The supply is an HAA-15-3-A.  The smaller one that works fine is
# HCC15.1.A (1amp at 12 v, .8 at15).
#
# BY saying the positive is flat, I do mean that there was no ripple on
# the scope.  That was only on the negative rail.  As far as the scope's
# reference, I'm afraid I don't really know how to use a scope!  I just
# have the input going to the positive (or negative) and the ground input
# going to the ground on the supply.  From the looks of it (from what Ive
# seen from observing my synth through it) it looks like the little spike
# on the negative side goes down about 1/2 volt.  The time knob on the
# scope is on 2ms and I get maybe two spikes on the screen at a time.  I
# know that converts into frequency somehow..
# There is no earth ground on the supply.
#
# Why swap supplies?  The smaller one sure is lighter and takes up less
# valuable cabinet space!  It's because I've got about 26 modules in that
# thing.  I've only ever had about 16-18 plugged in at a time.  I never
# ran into a problem w/ not having enough power, but from what I've read,
# I needed a bigger supply.
#
# One last thing, someone asked is this only when it is under a load, and
# it is actually the opposite.  When I plug it in w/ no load, or w/ only
# one low powered module, the problem isn't there.  As soon as I plug in a
# few more modules, I see the spike.  
#
# I can also hear a hum from the  supply itself..  (not through the
# electronics, but the thing is vibrating a little)  It does have two big
# caps on it.  I suppose one could be bad.  It is just a used EBAY supply.
# I'd write the guy and complain, but shipping was almost as much as the
# supply itself, so it's almost cheaper to buy a new (used) one.
#
# I guess there is no way to test a cap, with just a multimeter or scope,
# is there?
#
# Thanks for all the replies!  I could have never built my synth in the
# first place w/out this group!

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