[sdiy] PSU grounding problem

ganesha goaqihai at yahoo.co.uk
Mon Aug 6 00:45:45 CEST 2012

Thanks for your reply.
I tried that but still, If I check the center tap with my scope, I see a large sinus. Next, I disconnected the transfo from the pcb and tried every possible connection for the center tap.
Yellow - grey
blue - yellow
red - blue
red - grey

Nomatter what I try, the center tap always has a sinus. My scope is set to 5V/div and it goes 2 divisions up and 2 divisions down.

Am I missing something? If I connect my scope to the 0V of any other synth I own, I always see a flat line...

----- Original Message -----
From: Oren Leavitt <obl64 at ix.netcom.com>
To: ganesha <goaqihai at yahoo.co.uk>
Sent: Sunday, 5 August 2012, 23:32
Subject: Re: [sdiy] PSU grounding problem

On 8/5/2012 4:19 PM, ganesha wrote:
> Hi,
> I'm making my own psu for the first time. I used this schematic from Jürgen Haible:
> https://dl.dropbox.com/u/28535891/jh_matrix_fx_power_supply_sch.pdf
> If I measure the voltages with my DC voltmeter, everything seems to be ok. I have +15v and -15V.
> However, I noticed that if I check the 0V/ground with my oscilloscope, I see a sinus.
> Could it be that I connected the transfo incorrect?
> I use this transfo : http://www.velleman.eu/products/view/?country=nl&lang=en&id=2516
> It has 2 secondary windings:
> red yellow
> blue grey
> It says that blue and red are both the start of the secondary windings.
> I connected yellow and grey together (both ends from the secondary windings) for the 0V. Is this perhaps the problem? should I have connected both starts (blue-red) together or perhaps the end of the first secondary winding with the start of the second secondary winding?
> Thanks for the help!

Sounds like you have one of the secondaries reversed. Try yellow-blue 
for the center tap.

- Oren

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