DM465 Digital Mixing
A small digital processing
module arrived the other day after a mishap in a factory. Not
having seen one before meant a learning curve, but I was able
to see the extent of the problem when I dismantled it.... not
just a "blown resistor". Below is a picture of the
automated bagging machine in which it's used.
And here's a picture of
the DM465 module
You can see from the labelling
it's designed for handling 24 volt DC inputs, but something very
much larger had appeared on one of the inputs. I forgot to take
a picture of the circuit board before I cleaned it up, but below
is a picture of the parts removed from the module. The main part
is a 20-leg surface-mounted chip, type BTS716G which is classed
as a "4-Channel High Side Power Switch". There are
some filter capacitors also. One was completely blown up and
the ones adjacent were in poor shape because of soot and metal
splatter which had turned them into resistors. To check on their
value I removed a good one and found it was about 1nF.
Replacing these multi-legged
surface mounted chips is tricky because if track is damaged either
from the result of the mishap, or when detaching the old chip,
the task of fitting a replacement is an order of magnitude greater.
I've tried various methods of
removing old chips. Usually the best way is to thread a fine
wire under the legs and by heating the legs and pulling the wire
one side of the chip can be pulled away from the solder pads.
Once this is done one can generally heat the remaining legs and
pull them away. Old solder is then removed from the pads and
the new chip fitted by first adding solder to the pad in one
corner then heating this to secure the corner leg. Once you're
happy with the position of the chip you can solder the other
legs starting from the corner diagonally opposite.
I used new 1nF 100 volt
capacitors to replace those that I removed. See below.
As with all jobs of this
type you have to carry out measurements on the repaired board
in case damage has extended beyond the obviously damaged parts.
Thankfully, in this case I found no anomalous readings so all
should be well.
Below is the top view of the
old BTS716G which was 3rd from the left above.... definitely
U/S. I was lucky because other undamaged chips carry the full
manufacturers' code. Sometimes there are no similar chips and
often any shorthand code numbers need to be analysed to identify
the chip. Not always easy because a specific set of numbers can
be used by different manufacturers to mark totally different
products, and it can take ages to identify a replacement chip
and/or to find a supplier or to figure out a suitable equivalent.
On one occasion, many
years ago I had to ask the customer to get on his hands and knees
with a torch to find the missing piece of ceramic material...
he actually found it and the number was clear enough to get a
new one! Some jobs seem impossible, but if there's a similar
machine in the factory this can provide information on just what
was fitted in the charred hole in the circuit board.
What about prices of these sort
of things? Well, RS has this chip marked at £2.99 and Farnell
has them for £2.70. Neither is good value because RS sells
them in 5's making their price £17.94 and Farnell's price
is for 150 making £372.60.
Fortunately, Farnell have them
priced singly and supplied me with just one for £3.83.