On Tue, 18 May 2021 19:16:44 +0100, T i m wrote:

On Tue, 18 May 2021 09:33:03 -0400, legg wrote:

On Mon, 17 May 2021 21:11:45 +0100, T i m wrote:

On Mon, 17 May 2021 11:00:05 -0400, legg wrote:

snip

Front
https://ibb.co/5McshGF

Rear
https://ibb.co/w6Wq6hj


You're running at 220V? OK

Check.

Make sure the choke ends aren't touching as they leave the toroid
center.

Do you mean the opposite ends RL, in as it passes past out? If so, no
they seem to be a good 2mm apart.

https://ibb.co/YQg7Lp4

The wires without magnet wire enamel look pretty close to me.

I think it may be my bad picture, IF we are talking about either of
the pairs of wires as they pass in and out of the ring choke?

You should be concerned that wires exiting core in different
directions are not visibly or physically separated.

Ah, right, then yes, there is a good 2mm between them.

Force
these apart with a flat screw driver and insert biz card
thick paper stock.

A stack of 5 biz cars would fall out. ;-)

Can I just confirm something please? The output choke seems to be
bi-wound, two wires wound side-by-side so in parallel, both going to
the same pad at each end so carrying the output current between them?

So, it wouldn't matter if the 'pairs' of wires touched as such, as
long as the input and output wires didn't as they will be at a
different potential / phase?

They're shorted on the printed wiring, so just simple 2wires in
parallel.

On closer examination of choke, I see it's been impregnated, so
there's little likelihood of fault due to enamel failure or
vibration.

Wires not shorting to core.

Measurable just with my DMM? (if so, I can't measure anything).

You're looking for discoloration, crumbling of core where magnet
wires are forced against core edges due to winding force.

Check.

Discoloration is sign of point-contact heating.

Well I think the output choke windings have been running hot as the
enamel looks darkened compared to those bits that were likely cooler
as they were in the air and had heatsinks in the form of the soldered
connections to the board but I can see no obvious signs of failure of
the enamel.

No sign of cracked cores.

Do I only have one on the board (the big one in the final stage (is
it?)) and if so, not that I can easily see?

If you don't hear grinding noise or see shifting, when manipulated,
then there's probably no issue.

There doesn't seem to be?

Anything serious would have showed up measuring Lp.

snip
Ok, I just pulled these had found:

The 100uF, 25V:
VLoss= 1.8%
Capacity= 90uF
ESR= initially 2.1 ohm then dropped to .69 when repeating the test.

These are often part of feedback network for simplest integrated
switcher ICs. Probably not the culprit, but low side of tolerance
on an electrolytic is indication of end-of-life. Changing esr can
indicated corroded/intermitent inner contact.


The 4.7uF, 50V:
VLoss= 3.0%
Capacity 4755nF
ESR= 5.5 Ohm and stayed around that after several repeats of the test.

So, is that our 'bad' cap?
snip
Small 4u7 can have esr in the mid-ohms range. Probably not an issue
on housekeeping supply, but parts with 1R esr will allow start up
always in cold weather. Check rectifier feeding it for typical
forward voltage. If open or shorted, the unit would only run
intermittently (short would affect Lp measurement, but only if
in-circuit)

Main transformer primary inductance of a forward converter
running off mains will be more than 400uH - typically 2mH.

Ok, I'll try that now ('strike whilst the iron is hot'). ;-)

https://ibb.co/N33cKSf

Lower values suggest shorting of windings or terminal
connections/traces.

Understood.

What I think is the primary (marked in red) was:
.4 ohms, 17.4mH.

That's ok as long as measuring units are correct (common mistake).
Fairly high value for this power level and topology, but only
low side limit is important.

Another winding on the primary side (marked yellow and going to the
small caps etc) showed:
.3 ohms, .08mH

FWIW, the output seems to be made of 3 windings in parallel (marked in
green and tested wired that way) measured:
.02 ohm, .25mH.

No surprises.

Integrated switchers can be finicky when it comes to self-generated
noise. This makes snubbers and clamps pretty important. As a last
resort, check primary parts wired across main primary winding and
secondary side RC before replacing the big integrated switch.

RL