gareth magennis wrote:
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** I once heard about a tribe of African natives who had not figured the cause of female tribe members becoming pregnant.
Witch doctors invented many semi plausible theories, which tribe members were obliged to believe.
Most involved the intervention of spirits and animal gods.
Do you see any parallel here yet ??
Ok, my last post got me thinking.
I posted earlier that 100 ohms in series with around 2nF produced full blown oscillations.
I was scoping the ringing at the time, didn't have audio monitoring connected, and didn't notice that it was motorboating at a few Hz as I didn't want the unit to be oscillating like that for long.
So this recreates the symptoms. Full blown oscillations of around 100kHz and 60v pk to pk dumps the power supply and it motorboats.
The ringing is at the same frequency.
Pic here.
https://imgur.com/YmkTq5N
** What you have is not called "motorboating".
An oscillation that self extinguishes and restarts over and over is said to be " squegging ".
It is mostly a layout and wiring issue causing *positive* feedback.
Make sure no output signal is being coupled back to the input by stray capacitance.
Is the output transformer metal grounded?
All wiring shielded?
...... Phil