[sdiy] PSU grounding problem

[ganesha]

Thanks for your reply.

I have a very simple / cheap scope with only one connection for the probe called Y input. I also have a switch with 3 positions wich is the reference DC - ground -AC.
This switch is set to DC and I connect the Y-input probe to the center tap.

This is how I always use the scope. F.e. If I  plug in a jack cable into a vco saw out. I set the switch to dc and connect the probe to the tip of the jackplug and than I see the saw waveform.
If I do the same but connect the scope probe to the 0V of a synth, I see a flat line.
Now I connect the probe the same way to the center tap /0V of my transfo but I see the sinus. This is confusing me.



________________________________
From: bbob <fluxmonk at gmail.com>
To: ganesha <goaqihai at yahoo.co.uk> 
Cc: "synth-diy at dropmix.xs4all.nl" <synth-diy at dropmix.xs4all.nl> 
Sent: Monday, 6 August 2012, 1:03
Subject: Re: [sdiy] PSU grounding problem


what is your measurement reference?  ie, what 2 points are you connecting your scope to: the transformer center tap, and what else?


On Sun, Aug 5, 2012 at 6:45 PM, ganesha <goaqihai at yahoo.co.uk> wrote:

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>
>Cc:
>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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