[Paul[_13_]]

On 8/1/2019 2:30 PM, Scott Dorsey wrote:
In article , Paul wrote:
This one was not powering up, so I cracked it open.

The on-board fuse was still good. There was 120 AC,
and about 330 DC on the first rectifier. But
there were no DC voltages going to the next board.

So it's most likely a power supply issue, but as you
know, modern switching power supplies are very
complex, and are difficult to troubleshoot, especially
without a proper schematic.

Get the datasheet for the PWM controller. Likely most of the power supply
will be copied right off the datasheet, or at least enough for you to
figure out what is going on.

I found the PWM chip. It's a 1271A, PTAC. So it's this one,
if I am not wrong:

pdf.datasheetcatalog.com/datasheet/on_semiconductor2/NCP1271-D.PDF

I have confirmed that I am getting AC and DC off the first main
rectifier, and there appears to be nothing on the secondary of the main
transformer.

I was NOT able to see a 65kHz driving signal on the driver pin 5,
but I assume this is because the oscillator is still off.


If the big switching fet isn't shorted, if there is power on the big
primary-side capacitor, and you don't see physical damage, and it's
not kicking on briefly or crowbarring, then change out the kickstart
capacitor (which will be one or two MFD and right near the PWM controller
chip). If it doesn't fix the problem, it's still the first step toward
diagnosing why the PWM oscillator won't come on.


Is the kickstart capacitor either C6 or C7 on page 18? One of the
1.2nF caps?


I wrote an introduction to repairing switching supplies without the manual
in AudioXPress a couple years ago, and the article is probably worth digging
up.
--scott