Electronics > Beginners

Potential fault with Transformer Secondaries

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NTB:
Hi,
This is my first post here. I am gradually learning about electronics. I have an audio amplifier and decided to hook up my oscilloscope, which I'm fairly new at, to the transformer secondaries that powers the main output stage. These secondaries have a neutral center tap. Each of my probes are set to 1:1 and the earth clip to the metal chassis which is earthed via the power cord to the power point.
I noticed that the expected AC sine waves were not really uniform. Please refer to attached pics. Sorry about the unintended Rigol....aspect?
So, I have 2 questions, 1. could this indicate that the transformer could be faulty? or 2. could the circuit following the point where the scope was attached be reducing the appearance of a sine wave? Hopefully the later!
Thanks in anticipation of your patience and assistance,
Nigel Brown

perieanuo:
hi,
first, you need to watch some videos about oscilloscope probes, try to use mainly 1:10 probes.unless you want a bad frequency response.
most likely transformer is ok.usually the common point of transformer is't tied to earth, only case is(1'st mistake).
correct those and we'll talk again after.
regards,pierre

Benta:
Looks absolutely correct. What you are seeing is the current peak when charging the electrolytics after the rectifier. This current is so high that it depresses the output voltage from the transformer.

NTB:
Thanks very much, for your advice. I'll look into learning more about probes, ie: using 10:1 setting instead of 1:1.
Ciao,
Nigel

vk6zgo:

--- Quote from: perieanuo on December 07, 2018, 12:46:34 pm ---hi,
first, you need to watch some videos about oscilloscope probes, try to use mainly 1:10 probes.unless you want a bad frequency response.
most likely transformer is ok.usually the common point of transformer is't tied to earth, only case is(1'st mistake).
correct those and we'll talk again after.
regards,pierre

--- End quote ---

Because both channels are showing similar  waveforms, it rather looks like this device uses a 2 diode full wave rectifier as was common in the days of valve/tube rectifiers using power transformers.
They did use the centre tap of the secondary connected to chassis, either directly, or commonly in radios,via a low-ish value resistor to provide " back bias".

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