| A good
friend
recently inherited a SHARP, entertainment unit and rang me about fixing
it for
him.
The
only fault (apart from the quality of the
equipment) was that the lamps illuminating the tuner display had
expired.
My
friend tried using a torch to see the
display but got sick of that pretty quickly. I agreed to have a look at
it for
him.
The
unit appeared one bright day, along with a smaller AKAI that wouldn’t
play
CD’s.
My friend had bribed me with a bag of spuds and some
beetroot to repair these units.
The next day I decided to tackle the beasts. The
small unit
was relatively easy, a clean with alcohol and cotton buds to
the laser lens and interlock sensor, got it up and
running.
Suitably enthused, I then tackled the big beast.
After removing a million screws and various parts of
the
horror, I finally got to the display board and the offending lamps.
Both were in series and blown, and I had to do a bit
of
juggling with the meter probes to measure the voltage applied to
the lamps.
Whilst I was doing this I heard a sickening “pop”,
followed
by a familiar smell that comes from IC’s bursting.
I had a good look around all the boards in this
thing but
didn’t see anything obvious.
"Oh well!" I thought "I might as well get the lamps
replaced and
the see what works and what doesn’t".
After a lot of mucking around, I settled on two 12v
lamps,
under run, to give a suitable display colour.
Now to test the beast… I
hooked up
some headphones and was greeted with a loud hum in one channel and
nothing
in the other.
Well, now I knew where to look….
The output
stage was
the likeliest place to start.
That’s when I spotted a tiny sliver of solder, that had
deposited
itself on and
between some wire links on the power amp
circuitry.
I determined later that the short was between the
left
channel output and the negative supply rail. “Oh bother” (or some
other polite term) I said, and rang up my parts
supplier
with a request for an STK4132II output chip.
This was on a Friday morning in between a series of
intense
thunder
storms….
They didn’t get back to me.
A
sudden wave of dread came over me.. "That chip is probably
obsolete
and oh darn," (or again some other polite term),
"what am I going to do now?"
I got onto the 'net and searched for the part number
of the
chip.
A circuit appeared with some data on the chip.
It turned out to be a 20w per channel amp, nothing
all that
special.
I then worried about it all weekend and planned a
possible
option involving LM1875 amplifiers which I knew were easy
to get, and went about designing a circuit board for
them.
I was that desperate to solve my little mishap!
I rang the parts supplier on Monday morning and they
confirmed my worst fears… The chip was no
longer
available!
In the back of my mind I knew I had seen something
with a
similar chip. Then I remembered… My Sony
amplifier had
one!
After removing it from the tangle of cables that
connected
it to my audio and video equipment, the lid was removed
and…
YES..... It had a similar looking chip
but with
a different number on
it!
My heart sank… So near yet so far.
It was an STK4192II.
I
guessed the amp was about 40w per channel so I got onto the 'net and
looked up
the part number.
I got the complete data sheet for it and as it turned
out, it was rated
at 55W
per channel.
What really caught my eye on the data sheet was that
this
entire series from 6 to 60W were pin compatible.
I then checked the circuits for both chips and yes, apart
from minor
differences in the feedback circuits (involving tone
controls being added in one circuit), they were the
same.
I wondered what would happen if I replaced the blown one
with the STK4192II ? Would it
work? I thought it might
but was worried
about the much reduced
supply voltages stuffing up the chip's internal biasing and causing
distortion.
Give it a try? Why not. I
had got
nothing to lose!
The big test! Yes it did
work! No
distortion was evident to my ears (which I judge as pretty good at
hearing
distortion of audio signals). I
breathed a
sigh of
relief. My
friends entertainment unit was up and running again,,
thanks to an old Sony amp (that ironically, was
given to me
a few years previously by the same friend), and,
the internet!
The moral of this story
is to be VERY careful with older equipment using these STK type chips.
It was just sheer luck that I had a compatible part
available, and at no cost (so far).
The SONY amplifier is now waiting to be resurrected
and put back into use.
Has anyone got a spare STK4192II ?
|
