Is it the Power Supply or the unit?? Lexicon DC1

[Bob T]

I have a lexicon DC1 ( a high end digital surround sound processor) that
wouldn't power up so I opened it up and found the oem power supply a
NAN40-7610 was not outputting 5V
The supply which is a 100khz switcher would not start and the 5V went to
about 2V and collapsed . It worked fine without a load
Trying to determine if it was the unit or the PS I "helped" the 5V with a
diode isolated external power supply and the unit came up and ran fine.
If I let it warm up I could turn off the AC then back on within a few minute
and it came back up.
If I removed the AC for more than a few minutes it wouldn't start until I
jump started it again.
I loaded the PS with a 3Amp resistive load (which is its rating) and it
comes up fine
When I put a scope on the supply I find that the chopper input to the
transformer oscillates for a few milliseconds that dies then restarts and
collapses over and over. After I "jump start" it the chopper looks fine
Not wanting to make a career of fixing this power supply which I can buy for
$50 I'd be interested in anyone's opinion as to whether its the PS or some
condition on the board that is preventing it from starting and if so what
should I look for.
Also any likely candidates on the PS would be interesting
Unfortunately I don't have a schematic of either the DC1 or the PS
Thanks in advance
bob

 

[James Sweet]

I have a lexicon DC1 ( a high end digital surround sound processor) that
wouldn't power up so I opened it up and found the oem power supply a
NAN40-7610 was not outputting 5V
The supply which is a 100khz switcher would not start and the 5V went to
about 2V and collapsed . It worked fine without a load
Trying to determine if it was the unit or the PS I "helped" the 5V with a
diode isolated external power supply and the unit came up and ran fine.
If I let it warm up I could turn off the AC then back on within a few
minute and it came back up.
If I removed the AC for more than a few minutes it wouldn't start until I
jump started it again.
I loaded the PS with a 3Amp resistive load (which is its rating) and it
comes up fine
When I put a scope on the supply I find that the chopper input to the
transformer oscillates for a few milliseconds that dies then restarts and
collapses over and over. After I "jump start" it the chopper looks fine
Not wanting to make a career of fixing this power supply which I can buy
for $50 I'd be interested in anyone's opinion as to whether its the PS or
some condition on the board that is preventing it from starting and if so
what should I look for.
Also any likely candidates on the PS would be interesting
Unfortunately I don't have a schematic of either the DC1 or the PS
Thanks in advance
bob

Sounds like the power supply. I'd start by replacing electrolytic caps.

 

[Bob T]

Thanks
You right on both counts!!!
while I checked the main caps initially
I just went back and replaced a nichicon 1800/10 and it started right up
Thanks again
I can start enjoying some movies again
Bob

 

[Arfa Daily]

Thanks
You right on both counts!!!
while I checked the main caps initially
I just went back and replaced a nichicon 1800/10 and it started right up
Thanks again
I can start enjoying some movies again
Bob

Just whilst you're at it, I would check that the startup resistor hasn't
also gone high. Check also (and probably replace, for good measure) the cap
that decouples the supply to the controller I.C. , assuming that it uses
that sort of topology on the primary side. It is usually around 10uf at 35v,
and located close to the chip. Both of these items are favourite failures on
most switchers.

Arfa