Strange Waveform on Secondary of Mains Transformer

[Geoff. Barber]

Hi Folks,
Although I'm not an electrical engineer, I've registered on this site in an effort to seek some advice regarding a strange "240v mains" issue I'm seeing in my work shop - I will explain.
The problem first came to my notice whilst trying to locate a 100Hz buzz on my amplifier. I traced to problem back to the 15-0-15v secondary of the mains transformer where I'm seeing a strange "glitch" on the AC waveform (pictures attached). In an effort to try and isolate this, I built a simple replica of the circuit using another transformer connected to a bridge rectifier and a 4700uf capacitor. All looked normal until I connect a load (90mA) across the capacitor and I then see the top of the waveform flatten off and the spike as shown in the pictures. Do I have some king a mains issue as I never seem this before?
Any advice would be appreciated.
Regards
Geoff.

 

[Wilko]

Hi Geoff !
Interesting problem with a few causes ... can you draw the circuit that produces this waveform please

(eg - you mention a bridge rectifier but this pic is an AC waveform, where was it seen?)

 

[Deleted member 26818]

Surely the flat line is due to the capacitor?

 

[Wilko]

Surely the flat line is due to the capacitor?

I'm thinking that too - the transformer saturating trying to top up the capacitor and load, perhaps.

 

[jackhammerJIM]

Very unlikely you have a mains issue . Sounds like a ground loop . What amplifier is it ?

 

[Geoff. Barber]

Hi Folks, The amp is a Marshall AS80R (schematic attached).
I've noticed on my replica PSU set-up, its only when the 4700uf cap is connected is the waveform effected.
When I scope the speaker on the amp, I can see a spike occurring every 20mS (100Hz) and this is what I'm hearing. the 20mS spike seems to be everywhere and de-coupling seems to have no effect. I can even see it at different points on the chassis
Regards
Geoff.

 

[static zap]

If you can measure low volts ac , what is across R301 and associated diodes D301 D302
(whether it how you are measuring or part of the problem - i don't Know )

 

[static zap]

( One oscilloscope I used .... hated any pds between Neutral and Earth
...a winter electric heating issue )

 

[UKMeterman]

Check the continuity of the scope ground clip to the case.

 

[dmxtothemax]

For the top of the wave form to flatten off like that, then either the transformer is not supplying the required current ( impedance too high ), possibly a cheap Chinese transformer, or the filter capacitors are not supplying the current they are required too, possibly they are too large, so they cannot charge up quick enough, or they are faulty. Try a better bigger current transformer, try smaller filter cap's say 2200 uf.
I doubt that an earth loop would cause the waveform distortion.