"Johny B Good" wrote in message
...
On Fri, 18 Jul 2014 02:45:37 +0100, "Arfa Daily"
wrote:




While you're on, don't suppose you've got a schematic for a Mackie
SRM1801 sub have you ? Have combed the net, but nothing lodged with any
of the usual suspects. It has a permanent overload LED, although it
seems to be an indicational problem as otherwise, it works just fine.

Arfa

**Sorry mate. I can't find a schematic for that one.

--
Trevor Wilson www.rageaudio.com.au

No, I couldn't either. Mackie nor Loud Technologies will not help, even
though it's an old unit now. Today, I decided to give it another go, as
one
way or another, it needed to go back to its owner Friday. Strangely, it
now
works correctly, and has done all day. I replaced the comparator I.C. that
drives the LED a few days ago, but as expected, it didn't cure the
problem.
About the only other thing that I did was to give the board a good scrub
down with alcohol, as a lot of the joints had that sort of 'grey, dusty'
look to them when an item has been stored somewhere a bit damp. At the
time,
that didn't have any effect on the fault either, but now I'm wondering if
it
has all dried out, and some moisture under a surface mount component has
gone, and cleared up a leakage path across the print. I'll try it again in
the morning, and if it's still ok, I'll stick the amp back in its cab, and
write out the invoice ... :-)


You need to use a hot air drier after giving a PCB a 'Washdown" with
IPA. It's not sufficient to rely on the stuff evaporating out of the
PCB board 'under its own steam' and making it 'look' like it has dried
up as I discovered when cleaning the carbon smoke damage from my
homebrewed portable sound mixing box which had caused the op-amp
perfect rectifier driven VU meters to show a -10dB reading after I
replaced the burnt out safety resistors in the phantom power feed cct
to the stage box (the result of trusting a coon DJ's shoddy equipment
not to have mains voltage on its supposedly earthed connections).

When I powered the mixer up after the IPA and toothbrush cleaning
exercise, the meters shot over to the end stops. I figured that maybe
I needed to dry it out more thoroughly with a hair drier. I was right!
Once properly dried out, the meters stopped showing any spurious
readings and all was well again.
--
J B Good

Yes, agreed. However, it has only been on very rare occasions that I've ever
suffered such problems after a cleandown, and I guess doing this stuff day
in day out, you get complacent. In theory, the only thing that should make
IPA conductive, is water content, and the stuff I use is 99.7% electronics
grade, and kept in a sealed tin, so it shouldn't be conductive at all - wet
or dry. If the symptoms had changed after the cleandown, I might have
suspected that a drying out was required, but it was identical before and
after. Still, it's all working now and has gone back to its owner. One of
the little mysteries that this trade flings at us from time to time, I guess
.... :-)

Arfa