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Old 24th October 2006, 01:35 PM   #1
fred76 is offline fred76  Philippines
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Default lytics slightly 'raised' in dac?

Hi,

I wanted to replace the psu caps to better low esr types and digital decoupling caps to Oscons, and analog decoupling to Panasonic FM types in a DAC... The problem is the pcb tracks are double sided, so apart from the bottom I need to solder 'em on top of the pcb too . Does cap mounting with the leads raised up ~0.5cm okay, so that I can also solder them on top? I mean would the increased inductance because of slightly longer leads be negligible and not affect performance that much? Hopefully it is not that significant especially for the Oscons on the digital side. I mean wouldn't want to replace even with high performance Oscons when the stock Xicon miniature lytics would be actually perform better because it is mounted "flat" on the pcb with very short leads. Thanks.

fred
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Old 24th October 2006, 02:32 PM   #2
singa is offline singa  Singapore
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Hi,
If this is a comercially manufactured pcb there is no need to raise the caps as the holes are plated through,meaning top and bottom are connected by plating the insides of the hole linking them together.

Of course you do have to check after removing the originals whether they are still connected as the plating may break or torn off if you are not experienced in desoldering.
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Old 24th October 2006, 02:57 PM   #3
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Due to space issues, my player's main filter caps are raised about 7mm. I've always intended to sort this out eventually but is it really so bad to raise the caps?
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Old 24th October 2006, 07:38 PM   #4
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If the main filter caps are on the line side (seeing 50/60 Hz or whatever your wall power is) then it's probably not a big deal electrically - by spacing them 7 mm above the board you have 14 mm extra lead length that you wouldn't have if they were flat on the board. This causes a small amount of extra inductance which is insignificant at 60 Hz or so.

On the other hand if these caps are on the output side of a switcher and see high frequency ripple (100 kHz or more) then the extra inductance (ESL - equivalent series inductance) could be significant and would worsen your units' transient response. This could cause undesirable performance in the system.

Another perhaps more important issue is that of mechanical stability. Obviously you are now supporting the component by its leads, which is not the intended design. If the unit gets too hot this could be a problem, or if it experiences severe mechanical stress (i.e. is dropped). You could break the cap out of there and this could destroy connected circuitry - which would be very bad.

Also - speaking entirely from inside my rectum - I wonder if an overvoltaged cap which would pop its lid open normally would also spew electrolyte out its bottom if it were not supported by the board. This could cause more destruction than a normal cap explosion might, or worse could cause a safety issue. But I suspect this behavior would be unlikely with the standard "K" top cap design which is designed to vent easily.

Just some thoughts, hope that helps.

p.s. fred76 it is really easy to remove the via rings when desoldering thruhole components. This is why I hate them and prefer to stick with surface mount. Check to make sure the two sides are still connected using an ohmmeter and if they are you're golden and can just solder on the backside. Otherwise you will have to solder to both sides which is no fun. Usually, if you have a narrow enough iron, you can apply solder on the topside of the board to both legs, and then hold the iron so that it contacts both legs at the same time; then push the component down quickly as you remove the soldering iron. You can get much closer to the board that way.
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Old 25th October 2006, 09:27 AM   #5
fred76 is offline fred76  Philippines
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Thanks for sharing guys...

I've tried to desolder 3 supply caps, they are a very tight fit. When the cap slightly lets go with the leads halfway up (with a lot of effort!), I still needed to slightly desolder the top side of the pcb for the cap to be completely removed. I actually needed assistance in holding the pcb so that I could pull off the caps without damaging anything by accident. And it's a pain especially when space is tight. I need to get a better soldering iron/station with temp control, and a much smaller/sharper tip. Mine's just a 20W hobbyist gun type with a button to go 200W temporarily.

regards,
fred
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