DM465 Digital Mixing Module

 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.

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