[sdiy] is this PSU OK?

[harry]

Hi Eduardo  (and any lurking newbies ... listen in)

Try looking at some signal you are sure of... with each channel.
Try using both probes looking at the same point, they should be the same signal.

You may indeed have a flakey scope channel on an old scope, or even a new
one.  Check that your probes are OK, switch them and see if the problem
stays with the channel.

Then decide which channel you believe....

If you have many modules, try playing with the knobs on the synth and see if
any changes happen... maybe its some glitch of the circuit riding into the scope
?

Try measuring the AC secondary of the transformer  (24VAC) and look at the sine
wave. This should be 16.66mS for a complete cycle (60Hz).  This will tell you if
the timebase is accurate and near calibrated. The look for the 3mS problem
again...

BTW... this is an EXCELLENT reason NOT to hook the earth ground to the
scope.  If you then put the scope ground at a different point, you have a short
circuit
through the probe... BLAM.  There are four ways to avoid this...

1) do NOT allow the secondary of the transformer isolated circuit touch earth
ground
anywhere...

2) Use a ground lift plug on the scope so the scope is not connected to earth
ground.
WARNING this will make the scope case live to whatever potential the probe ground
is... in a 24VAC powered circuit no big deal, but potentially lethal on mains
voltage.  This is what I usually DO... I prefer the shock hazard to the 'hot
metal flying in my face because of a short through the probe' hazard.  As a rule
I never hook up the scope to mains powered equipment....

3) Use a 120VAC isolation transformer to power everything you are testing,
including the audio monitor amp...  If you violate the ground you are at risk
again...
In this case... the scope may be safely earth grounded.  Any voltage will be
across the isolation transformer which can safely er... isolate it.

4) Use a true differential input on the scope (hah very few have that...) and
don't use
ANY grounds on the probes... OR use a differential probe for the scope (these
cost about $500 so I guess you don't have them.  In my work they are a MUST and
we have them all the time. When you look at 480VAC lines with a scope, safety is
absolutely essential !!!

3mS is not related to the line frequency (no way) but if the timebase is wrong
and
its 8.33mS it COULD be the power supply.

This is what I've mentioned before, you need to trust your test equipment... but
not
blindly. Make it prove itself from time to time.

If you find that the channel really is suspect... use the other channel for all
critical
work.  Maybe sometime when you get better at it you can tackle the scope repair.

MY scope was blown from about 1980 to about 1992... when I finally decided that I

had the skill to fix it and shame on me if I could not... Now both channels work,
but
with some chopper noise (that is what was blown up!).  The chopper was
essentially a Diode Ring VCA... something that needed a few years of DIY to
recognise for what it was.... (tee hee)

H^) harry

ElmacacoX at aol.com wrote:

> OK, I think it is some glitch in the scope, but I am not sure since I don't
> know these things very well.
>
> I am using only one channel, although my scope does have chop and alternate,
> I am firmly in one channel.
>
> I removed the 10uf caps from the end of the supply and the chop went away..
> for a little while, I accidentally nudged the calibration pot that is on the
> knob of volts/div and it came back, I moved it back to where it was but the
> gaps continued.  this is why I think it is something with my (very old) scope
>
> then I switched channels and there were no gaps.
>
> thus my dillemma,
>
> which channel is right?
>
> I seem to feel that the problem is not in the circuit at this point.
>
> Harry, No I am not sure I want to connect the mains ground to 0V,  it just
> feels dangerous to me, (especially so after you just said about it!) I said
> about it in case it had some bearing on this problem.
>
> Oh and this doesn't show up when the probe tip is grounded or with the
> scope's test signal.
>
> i have a 5x sweep switch, and it was off at the time of the 3ms observation.
>
> I guess it in't something to worry too much about,
>
> my filter and LFO seem to run very well on it, I think it just means that my
> VCO's have some other problem ;(
>
> thanks for everything guys, If I find anything else out I'll report back.
>
> Eduardo