Phil Allison,

a while ago I believe you posted about a peculiar trait in (SMPS) JBL Eon
amps, whereby you can't run them up on a Variac safely.
Since then I've had 2 powered satellites and a powered sub where I noticed
the same thing.

I had the sub in today and replaced a blown bridge rectifier. I was fairly
sure the power amp was OK, no shorted devices, and a blown amp is unlikely
to take out the bridge.

I usually check these things with a Variac and 100W lightbulb in series, and
sure enough in all 3 cases it looked like excessive current draw at a fairly
low voltage, so stop things right there.

So I tried the repaired sub today on 240v with the lightbulb and it was
clear that the excessive current was going to continue.
However, on disconnection from the mains, both power supply rails held
indefinitely at around + and - 25v, indicating there was probably no short
in the power amp.


Much as I hated to do so, my only option was then to just connect the device
to the mains and turn it on, and it didn't blow up and is now back with the
customer.


I think this was something to do with a soft start relay not kicking in
unless the full supply voltage is used, but can't quite figure out how that
would work in practice.

Maybe you could remind us? I don't have schematics for these things.




Cheers,


Gareth.

That kinda rings a bell. Also, in working on TVs which all have switched mode power supplies now, you usually can't use a variac.

How to try to keep the smoke in ? I have thought of that and the answer might be to remove the main filter off the line recifiers. Temporarily replace it with about a 33 uF. Then a modded light bulb tester, like with two higher wattage bulbs in parallel.

With some timkering you can probably find some comfortable values that'll allow the circuit to power up fairly normally but yet keep the fire brigade away.

I remember a certain model of Sony TV, didn't have a switch mode but it used a startup cap from the main B+ down to about a 20 vlt source to run the horizontal osxcillator which was then powered off the flyback. No way to start those on a variac either. Also Zeniths that had the A-11345 startup module would not slow start. I'm pretty sure RCAs from CTC 88 through 92 wouldn't either.

I don't like it when manufacturers do that but whaddya gonna do ?

Well I know what we're going to do, outsmartem and fix their **** so they don't sell more of it ! LOL

wrote in message
...

That kinda rings a bell. Also, in working on TVs which all have switched
mode power supplies now, you usually can't use a variac.

How to try to keep the smoke in ? I have thought of that and the answer
might be to remove the main filter off the line recifiers. Temporarily
replace it with about a 33 uF. Then a modded light bulb tester, like with
two higher wattage bulbs in parallel.

With some timkering you can probably find some comfortable values that'll
allow the circuit to power up fairly normally but yet keep the fire brigade
away.

I remember a certain model of Sony TV, didn't have a switch mode but it used
a startup cap from the main B+ down to about a 20 vlt source to run the
horizontal osxcillator which was then powered off the flyback. No way to
start those on a variac either. Also Zeniths that had the A-11345 startup
module would not slow start. I'm pretty sure RCAs from CTC 88 through 92
wouldn't either.

I don't like it when manufacturers do that but whaddya gonna do ?

Well I know what we're going to do, outsmartem and fix their **** so they
don't sell more of it ! LOL




I really don't like SMPS's because I don't know enough about them, I am way
out of my comfort zone here.

But, most modern audio equipment (I only know audio equipment) using SMPS's
use small IC's as the controller, which are designed to tolerate almost
anything thrown at it.
They will only attempt to power up once they have enough voltage to do so,
and will shut down if excessive current draw occurs, without destroying
anything.

You can usually power them up with a Variac and lightbulb, but with the
JBL's you can't, because I believe they have added some relay circuitry that
makes this not possible.
I'd just like to know what that is.




Gareth.