On Friday, 5 March 2021 at 21:12:38 UTC, wrote:
gareth magennis wrote:
====================
** 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
This is an issue that has been ackowledged by the company as an issue in the early production units.
This is one of those units, it HAS a design problem.
When I first noticed the oscillation, it was with a dummy load on the bench with a scope probe.
The customer has complained of exactly the same thing I have now demonstrated by connecting 100 ohms in series with 22nF on the output on the bench.
I have spent quite a few unpaid hours trying to find a solution to this, whilst assuming the company has done all of this already and issued a mod to rectify the problem.
I have other repairs that need my attention now, this is not a hobby of mine, though I appreciate the knowledge gained by this experience greatly.
And the customer still hasn't got his unit back.