Help me save a big repair job! (Karma synth)

[MrDiggie]

Dear good folks of EEVBlog,

At some point in the last several years, Dave and a few other YouTubers got me interested in electronics repair. I have been working to repair a Korg Karma synth by replacing all the switches, encoders, etc., but was pretty devastated tonight when I noticed that a surface mount capacitor had recently come off one of the input boards.

It's board KLM-2215, and it's part "C3". The service manual happens to be online in various places, e.g. here:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/SM_scena/Korg/Korg_Karma_Service_Manual.pdf

The schematic for KLM-2215 is on page 8 and towards the upper left C3 is shown, but the only information provided seems to be "CE". I'm not an electrical engineer and although I kind of suspect it might be there to reduce noise through the common ground plane, I have no idea if that's correct or what I may need to replace this with. The BOM at the end of the service manual seems to be also unhelpful in this instance. I'm worried this probably $.05 part might kill my entire project.

Please help! Ideally someone can explain what I need and why, or even just a link to something suitable on digikey/mouser/arrow wod be nice.

Thanks to anyone who can share their wisdom!

[MrDiggie]

I wonder if removing and measuring C4 might be appropriate as this seems to be a similar part on a 3.3v circuit. Any thoughts on how valid that is?

[JackJones]

Looks like it's an input for an ADC. The maximum input impedance for unbuffered ADC isn't infinite (it's usually around 10ish kOhms?), so the capacitor is there to give it a little boost. It's a pretty standard practice to increase the allowable input impedance.

The capacitor is usually around 10nF in value.

[MrDiggie]

Thank you! I think the closest part I have at hand is around 0.1uF. I will cross my fingers and say some prayers to Zeus to ward off smoke.

[AndyC_772]

From the schematic, it looks like it's just filtering noise from a DC voltage that's set by a user control. The unit will probably work just fine without that cap, but if you do decide to replace it, the value shouldn't be critical and certainly won't make anything smoke.

[mikerj]

Agreed, not a critical part in terms of value, 100n should be fine.  Since it's mounted right next to the pot I suspect it's function is more for ESD protection than noise filtering.

[PA4TIM]

I think you can calculate the value if it is a filter, but if it is indeed on a DC line (can not open the schematics on this thing I use now) it does not matter to much. But more important it will not cause smoke. I would first try without one, if the synth works then try some values, I would start with 100 pF en su go up to 100nF but I assume you do not have a capacitor substitute box to fast try other values.

BTW, if you plan to do more, it can be handy to buy some caps, to banana busses and a multi position switch and make a little substitute box. (you can connect multimeter probes to it or crococlips) You can use it to AC couple audio in a circuit, set a cap over the powerrail of an IC to test things (for instance to see if the ripple on a powerrail needs replacement, or try to find the best value like in your synth. (You could use something like 100pF, 1nF, 4,7nF, 10nF, 47 nF, 100nF, 470 uF, 1uF, 10 uF, 100 uF and 1000 uF)

You can also make one with some resistors so you can limit currents, bleed capacitors, make a filter together with the caps-box.

[MrDiggie]

Thanks all.

You are right, PA4TIM, I absolutely need more parts and tools for test. As I take on more projects and as this is a hobby and not a business, I'm trying to build up my toolkit over time. On this occasion my pre-christmas splurge was on a desoldering gun as I need to replace 70+ through-hole parts without damage and less mental trauma than desoldering usually gives me But I have a ways to go before I have all I need - still working without a power supply and scope, don't have a box full of spare smd parts just in case, etc.

I'm very much still in the phase where I'm encountering problems and learning that if I only had xyz handy I could solve them, so that I can actually justify spending money to buy that and know that it won't sit in a box unused. As our place is small I'm currently "borrowing" our kitchen table as a work bench, so if I buy too much at once there is a decent chance my wife won't let me live to use it either.

Appreciate your help, will percivere...

[PA4TIM]

I use the whole house except the kitchen 

[Chris56000]

Hi!

If you look towards the bottom of page 5 you'll see a note in bold type about 5mm letters:–

"CE 0.1uF CE"

. . .this applies to ANY  "CE" capacitor on any of the sheets, the type you require is 0805 case size, X7R  dielectric, 0.1uF 50V DC, this applies to ALL of these capacitors where "CE" has been annotated!

Chris Williams

[Chris56000]

PS!

There are two main types of SMD ceramic capacitor dielectric:–

"X7R" is a high–permittivity dielectric for general purpose bypassing and supply decoupling applications, commonly found across DC supply lines, etc!

X7R should NOT be used for any usage where stability with time or temperature is needed, and X7R is not suitable for critical tming or waveform shaping applications!

"C0G" is a low permittivity ceramic dielectric, sometimes referred to as "NP0", which has a capacitance tolerance of ±5% or ±10%, is stable with temperature, and is the type to specify for time constants, filters, waveform shaping, etc., etc!

Chris Williams

[MrDiggie]

Wow, thanks, Chris!

You have all been far more helpful than Korg was, who told me repairs are dangerous and not for end users and therefore they don't help with such things. Sigh.

[MrDiggie]

Just bumping this thread temporarily to let you know that with your help I completed this project! I put a short write-up here:
http://reddit.com/r/synthesizers/comments/eqtsoq/i_fixed_up_this_2001ish_korg_karma/

I've been spreading the know-how as I can to others looking for parts for their own projects.

Rock on, eevblog!

[PA4TIM]

Nice job ! I have a Yamaha keyboard I bought decades ago (played piano as a kid) but I do not use it much anymore. Most of the time I play (electric) guitar