On 3/8/2016 11:48 AM, Cursitor Doom wrote:

Well, I've replaced all the flaking capacitors and still no improvement.
A number of people have been suggesting I remove the two resonant caps
(the 30n ones) from the primary circuit and test them. I didn't have any
expectation that this would achieve anything since they tested good in-
circuit, but as we're running out of ideas now I did remove them this
afternoon and they both tested at 31nF a piece and no signs of any
physical damage. I then subbed a couple of common-or-garden mylars of the
same value in their places and re-swept for changes in resonance. Result
was no change in resonance - but a slightly better Q(!!) Also checked the
two 10Meg resistors whilst I was at it and they were fine, too. So I can
only think of making up Dimitrij's winding tester and looking for signs
of any turns shorting in the main transformer.
I'm coming to the end of the amount of time I'm prepared to spend on this
psu as it stands. I'm more and more tempted to mothball the key parts of
it til next year then rebuild it as a conventional non-res converter to a
fresh design. TBH, I'm not prepared to still be testing this thing after
the end of this week, so if anyone has any last-ditch ideas, now's the
time to toss 'em into the mix. Speak now or forever hold your peace.
Thanks, all.


I have no idea what you now consider to be wrong with the PSU. Apart
from a resistor that in your opinion runs too hot even though its well
within its rating I seem to recall amidst your ramblings that the
voltage rails are correct?

It seems to me that the only fault was the diode which was pretty
obvious from the beginning. (Always check for previous repairs).

You persist in not using the correct manual, refuse to test it with an
appropriate load and won't put it in the scope to see if the scope
actually functions.

If you just want to discuss switching supply design go buy some control
chips or an evaluation kit and build some.

On Tue, 08 Mar 2016 12:35:07 -0500, JC wrote:

I have no idea what you now consider to be wrong with the PSU. Apart
from a resistor that in your opinion runs too hot even though its well
within its rating I seem to recall amidst your ramblings that the
voltage rails are correct?

Yes, they're fine. It's not *my* opinion that this is a poor design! I
posted the schematic to s.e.d and the designers there told me that. I've
said all along I know nothing about this type of PSU so I defer to those
better qualified. Having said that, the widespread evidence of charring
and soot residue that appear even worse in real life than in the photos
would seem to indicated that that is not a happy board. The rest of the
scope's boards are still pristine showing no sign of repairs at all.

It seems to me that the only fault was the diode which was pretty
obvious from the beginning. (Always check for previous repairs).

Nothing obvious about it! - apart from the rather poor replacement
technique. The problem with that replacement was very subtle inasmuch as
it wasn't recovering quickly enough at 20kHz. Nevertheless it was still
capable of functioning as a viable rectifier right up to 500kHz.

You persist in not using the correct manual, refuse to test it with an
appropriate load and won't put it in the scope to see if the scope
actually functions.

All the recent tests have been carried out using the actual scope itself
as a load - can't get any better than that. I won't put it in the scope
until that resistor is running cool. It's proximity to other temperature
sensitive components like diodes and its siting deep within the scope
uncooled lead me to believe that it is running far hotter than it should
be - and will be even worse in situ. That *might* just be a design flaw,
but I doubt it.

On 3/8/2016 1:43 PM, Cursitor Doom wrote:
On Tue, 08 Mar 2016 12:35:07 -0500, JC wrote:

I have no idea what you now consider to be wrong with the PSU. Apart
from a resistor that in your opinion runs too hot even though its well
within its rating I seem to recall amidst your ramblings that the
voltage rails are correct?

Yes, they're fine. It's not *my* opinion that this is a poor design! I
posted the schematic to s.e.d and the designers there told me that. I've
said all along I know nothing about this type of PSU so I defer to those
better qualified.

Yes, I read your posts on sed, you didn't tell them this was a Philips
scope, and some replies implied Philips made televisions, jeez.

Philips/Fluke made excellent test equipment and many products beat
Tektronix into the ground but being European never made it across the
pond in large numbers. I've worked in both design and repair facilities
and the folk who couldn't repair ANYTHING were the design guys because
they always spent 3 days criticizing the original design, another week
debating it with their colleagues and then gave it to a repair tech to
fix (in about 30 minutes). You obviously fit into the design mentality,
so good luck and give the scope to someone who will appreciate it.