Archived from groups: rec.audio.tech (More info?)

Hi All,

I mailed this to Alesis support, but evidently, they don't give out
such info. If anyone here is familiar with such problems and can help,
I'd appreciate it.

-pasted question-

I have an AI-3 converter which has a very minor problem with the LEDs.
The clip light for the third channel will glow continuously, as if
something is leaking a slight voltage to it. It usually takes a few
minutes after the unit powers up to reach a visible intensity, and
stays there from that point on. The rest of the unit (as well as the
other lights) functions properly.

This unit is well beyond warranty and I am considering carefully
replacing some parts to see if it fixes the light. I notice a group of
four 14 pin, surface mount LM339 chips right near the front panel
connector (marked from U53-U56). Are these a likely culprit for a
problem with the clip LED's? If not, what other parts would you
suspect?

----------------
There are 8 clip lights as well as 8 signal present lights on the
panel.

Please reply in the group (and Thanks!)

PS- I realize I'm getting into some small stuff here. I've got a
magnifier/light on a boom and a rework station which I could use some
practice with. - I'll be real careful

Archived from groups: rec.audio.tech (More info?)

As it takes a few minutes for the fault to appear, it may be as something
heats up. If so, you might be able to find it using freezer spray. Cool
the faulty part and it should put out the LED.


Gareth.



"Cauley Felps" <cauley@nomail.com> wrote in message
news:nhj9v0pe321bbqvop5hlj8de4pv7j5cmdh@4ax.com...
> Hi All,
>
> I mailed this to Alesis support, but evidently, they don't give out
> such info. If anyone here is familiar with such problems and can help,
> I'd appreciate it.
>
> -pasted question-
>
> I have an AI-3 converter which has a very minor problem with the LEDs.
> The clip light for the third channel will glow continuously, as if
> something is leaking a slight voltage to it. It usually takes a few
> minutes after the unit powers up to reach a visible intensity, and
> stays there from that point on. The rest of the unit (as well as the
> other lights) functions properly.
>
> This unit is well beyond warranty and I am considering carefully
> replacing some parts to see if it fixes the light. I notice a group of
> four 14 pin, surface mount LM339 chips right near the front panel
> connector (marked from U53-U56). Are these a likely culprit for a
> problem with the clip LED's? If not, what other parts would you
> suspect?
>
> ----------------
> There are 8 clip lights as well as 8 signal present lights on the
> panel.
>
> Please reply in the group (and Thanks!)
>
> PS- I realize I'm getting into some small stuff here. I've got a
> magnifier/light on a boom and a rework station which I could use some
> practice with. - I'll be real careful

Archived from groups: rec.audio.tech (More info?)

In article <nhj9v0pe321bbqvop5hlj8de4pv7j5cmdh@4ax.com>,
Cauley Felps <cauley@nomail.com> wrote:

> Hi All,
>
> I mailed this to Alesis support, but evidently, they don't give out
> such info. If anyone here is familiar with such problems and can help,
> I'd appreciate it.
>
> -pasted question-
>
> I have an AI-3 converter which has a very minor problem with the LEDs.
> The clip light for the third channel will glow continuously, as if
> something is leaking a slight voltage to it. It usually takes a few
> minutes after the unit powers up to reach a visible intensity, and
> stays there from that point on. The rest of the unit (as well as the
> other lights) functions properly.
>
> This unit is well beyond warranty and I am considering carefully
> replacing some parts to see if it fixes the light. I notice a group of
> four 14 pin, surface mount LM339 chips right near the front panel
> connector (marked from U53-U56). Are these a likely culprit for a
> problem with the clip LED's? If not, what other parts would you
> suspect?
>
> ----------------
> There are 8 clip lights as well as 8 signal present lights on the
> panel.
>
> Please reply in the group (and Thanks!)
>
> PS- I realize I'm getting into some small stuff here. I've got a
> magnifier/light on a boom and a rework station which I could use some
> practice with. - I'll be real careful

It might be a faulty capacitor or faulty chip. A faulty chip would be
very sensitive to heat but a faulty capacitor would be more sensitive to
how long it has been powered. Also check for acid flux that has oozed
off a soldered heatsink - it conducts a little.

Archived from groups: rec.audio.tech (More info?)

On Mon, 24 Jan 2005 21:18:10 -0800, Kevin McMurtrie
<mcmurtri@dslextreme.com> wrote:

>A faulty chip would be very sensitive to heat but a faulty capacitor
>would be more sensitive to how long it has been powered.

Kevin (or anyone),

I got the freezer spray. It's looking more like a matter of power on
time like you mentioned. It may even be related to activity as well.
Having it sit idle and unconnected, it takes quite a while to even
develop a mild glow.

I froze most of the stuff near the LED panel connector after I got a
glow, and it had no affect. I also tried heating different areas with
a hair dryer before the glow started and couldn't get it to act up any
faster.

I've got a couple questions if you don't mind answering-

There is a total of 30 small electrolytics (20-47uF,10-1uF). Are these
the usual suspects for a cap related LED problem. (there are really
small surface mount caps as well)

The LED in question comes off a surface mount resistor, which looks to
come directly from the 2nd output of one of the comparators. I'm not
sure how far back or what part of the circuit the leaky cap would be
in or whether I have the skill to trace it. The board is also pretty
cluttered with silk screening.

There's no sign of thermal compound mess or anything. The only
heatsinked devices are a pair of regulators way over on the side.

Without a schematic, what would most of you guys do?

BTW- The unit is 8 channels of analog to digital ( I guess where the
clip detection circuit is) and 8 channels of digital to analog. The
caps are sort of "grouped". I guess I could swap all the 47uFs near
the analog ins if that sounds like a good start.

-Thanks Again

Archived from groups: rec.audio.tech (More info?)

A healthy comparator is either on or off and cannot be partially driving the
LED. If the chip you mentioned is, in fact, a comparator and the LED is
conected between +V (or ground) via a resistor to the comparator output, and
nothing else is connected to this output, I would suspect that comparator to
be faulty.

Or, one of the signal inputs to the comparator may actually have
oscillations or noise on it, meaning the LED is being switched on and off
very fast, looking like it is glowing slightly. If you have access to an
oscilloscope, have a look and see whether we are talking DC offsets or
noise.


Gareth.


> The LED in question comes off a surface mount resistor, which looks to
> come directly from the 2nd output of one of the comparators. I'm not
> sure how far back or what part of the circuit the leaky cap would be
> in or whether I have the skill to trace it. The board is also pretty
> cluttered with silk screening.
>
>