Author Topic: By Design or FUBAR fixed?  (Read 2092 times)

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Offline RayJonesTopic starter

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By Design or FUBAR fixed?
« on: May 27, 2011, 02:09:01 am »
Just repaired a spa controller and came across this bypass arrangement over the processor.

It looks a bit like a FUBAR fixed after manufacture, but then again the positive lead runs to a dedicated hole on the PCB.

It certainly provides a low impedance bypass direct to the IC's pins.

I have used under C caps in the past which provide the same end effect, and do not get in the way of servicing should the IC need replacing.
 

Offline Neilm

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Re: By Design or FUBAR fixed?
« Reply #1 on: May 27, 2011, 05:03:31 pm »
I cant't quite make out what that dedicated hole is near. Is there is another one under the purple wire? If so, it could be that the processor fitted has gone obsolete prior to the building of this board. the modification with the cap on top might be the manufacturere using another device for the same role and having to "bodge" it in to get it onto the board and working.

There are a few reasons I can think of why they would do this.
First, they are using up the old stock of PCBs before buying new ones.
Second, they don't want to alter the PCB (for what ever reason) and this is what has to be done to sell the units. Generally this is a dictate from either Management or Accountants.
Thirdly, the design might be so old they can no longer alter the PCB without having to completely relay it. An example of this would be a good, well selling product that has not been touched for years - so old the PCBs were layed out in a very old (and not supported) CAD system or even with tape.

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Online mikeselectricstuff

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Re: By Design or FUBAR fixed?
« Reply #2 on: May 27, 2011, 05:45:08 pm »
My guess is this is a fix for EMC or ESD problems on a poorly laid out PCB- the copper tape as opposed to wire would provide the minimum impedance.
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Offline RayJonesTopic starter

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Re: By Design or FUBAR fixed?
« Reply #3 on: May 27, 2011, 10:28:31 pm »
As it happens I will need to pull the unit out again today as the fix has failed once again  (triac that drives the blower outlet) :'(
I will actually look at improving the heatsinking area as it worked for many minutes then fused the triac again.

I can tell you however that the apparent copper strip is actually a strip of PCB with a groove across the middle where the cap lies, but yeah certainly a very low impedance path.
The red wire also went to a hole with a circle about it, so it seems like by design.

I'd rather not have to pull the PCB itself out as it is buried by many mains wires heading in many directions, so to check below the chip where the power runs will not be so easy.
Fortunately the triac is a flying leads so it is relatively easy to replace - one you get your tongue in the correct positioning.
 


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