LA22 power supply...... 28 VAC need help

[Bill Buswinka]

I have a UREI / JBL LA22 compressor - cool piece except for the the
cheap wall wart grafted on to the rack. Supposedly, the person that
sold it to me said, JBL did the modifications and said it had to be
done. Well, the unit sounds nice but I have a loud hum coming from
the power supply ( not in the audio path but it is audible accross the
room). I would love to replace it - and was planning on it -nice
accopian I figured woudl help the noise and the audio.....well,
looking at the wall wart and the specs to go hunting for supply had me
wondering something.....

the input of the supply is, of course 120vac ...but the output is also
AC ...28 VAC 300ma

the power supply is model 120-27-2


I was just wondering why they might have used 27 volts AC instead of
27 DC ? 27vac is not easy to find....could it have been a misprint,
seems like a lot of trouble to have an external supply and then still
have a ACto DC converstion , rectification and regulation
inside...........

would love to get rid of the weak noisy wall wart..............any
help is greatly appreciated.

Thanks!!!!

Bill

[Mike Rivers]

In article > writes:

> I have a UREI / JBL LA22 compressor - cool piece except for the the
> cheap wall wart grafted on to the rack. Supposedly, the person that
> sold it to me said, JBL did the modifications and said it had to be
> done.

> the input of the supply is, of course 120vac ...but the output is also
> AC ...28 VAC 300ma


> I was just wondering why they might have used 27 volts AC instead of
> 27 DC ?

Perhaps it goes into a diode bridge inside the unit and is converted
to a positive and a negative DC voltage. If you put DC into it, you'd
only get a single polarity out and the compressor wouldn't work.

There may be something else wrong with it.


--
I'm really Mike Rivers )
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[Bill Buswinka]

Mike,

thanks for the reply. What else might be wrong iwth it?.....The
unit is working fine other than the noisy supply. So, if there is a
diode bridge inside could I just wire a dual 27 volt supply to the
point after the bridge?

Thanks!

Bill

[Mike Rivers]

In article > writes:

> thanks for the reply. What else might be wrong iwth it?.....The
> unit is working fine other than the noisy supply.

You said "hum." I assumed you meant hum in the audio output, but here
you say "noisy supply" and as I recall you said something about a wall
wart strapped to the back of the unit. Are you talking about
mechanical noise here? That the wall wart itself is actually buzzing?
Nothing to do there but replace it, or remove it to someplace where it
won't be objectionable.

But wall warts are notorious magnetic field generators, so if it's
really in close proximity to the electronics, it could be inducing
electrical hum. Again, the solution is to move it. But if it's
electrical hum (in the audio output) it could be caused by a bad diode
or filter capacitor in the power supply, or a ground problem. Any
number of things.

Sorry, but I don't know everything that you might know that you
haven't told us, so I can't suggest a guaranteed remedy.

> So, if there is a
> diode bridge inside could I just wire a dual 27 volt supply to the
> point after the bridge?

Sure, but I wouldn't be too sure about the +/- 27 volts. Best to open
up the unit and measure the voltage, assuming that it's operating
normally except for the hum.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

[Bill Buswinka]

Mike,

Sorry for not being clear. Thankfully the noise only seems to be
mchanical in nature and the audio seems unaffected. But, so far,
moving the supply has not been satisfactory as the noise is very
audible in the room. So, I guess I will look into seeing what
voltages the unit is using internally......I think I have a schematic
in the manual: ))) Hope this clears things up a bit.

thanks!


Bill

[Scott Dorsey]

Bill Buswinka > wrote:
>
>Sorry for not being clear. Thankfully the noise only seems to be
>mchanical in nature and the audio seems unaffected. But, so far,
>moving the supply has not been satisfactory as the noise is very
>audible in the room. So, I guess I will look into seeing what
>voltages the unit is using internally......I think I have a schematic
>in the manual: ))) Hope this clears things up a bit.

Why not just replace the wart with a better quality transformer? The
Talema toroids are cheap at Digi-Key and should be easy to strap for 28VAC.
And quiet, with low magnetic leakage too.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

In article > writes:

> Sorry for not being clear. Thankfully the noise only seems to be
> mchanical in nature and the audio seems unaffected. But, so far,
> moving the supply has not been satisfactory as the noise is very
> audible in the room.

Well, the solution there is to get the power supply OUT of the room.
Or replace it. Or crack it open and shoot some tar into it. Mechanical
problems are solved by mechanical means.

Occasionally you can get a buzzy wall wart. I'd suggest replacing it
with one of the same voltage rather than trying to modify the limiter
to work with a different power supply. Not only will you need to know
the voltages (which may or may not be on the schematic) but you'll
need to know the current so you can buy a power supply with sufficient
capacity. The only way you'll find that is to get it working with the
existing power supply and measure the current of each DC output.


--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo