General question re: recapping

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evm1024 said:
The easy answer is that you measure the DC across the cap to find the DC bias (harder than it sounds). It may be zero or it may be many volts. If it is zero then any AC signal will drive the cap reversed bias half the time. If there is (oh say) a 1 volt DC bias then any AC signal of 0.707 volts RMS will be 1 volt peak positive and 1 volt peak negative. So on the positive half (assuming sine waves here) you end up with 2 volt across the cap and on the negative half you end up with 0 volts across the cap. (ignoring time constants) Increase the AC voltage just a little and the negative half will bias the cap negative. And then you get whatever bad sounds the cap is "going" to make....
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Well, since that sounds complicated I don't think I'm going to even consider replacing polar caps with non-polar. I was hoping there was some way to identify from a schematic where this could be done, but it sounds to be a case-by-case matter and even at that it doesn't sound like there is a blanket yes/no break-point but that there is some grey "maybe" area...and that makes me itch.

Am I off-base or is my general perception of the determination process accurate?
 
man-bot said:
I would be quite interested to hear about reccomended/best practices for removal of components form PCBs. While I have quite a bit of soldering experience (building tube guitar amps and mic pres) I'll be darned if I didn't manage to be heavy handed and lift some tracks on the PCB when I unsoldered some ICs on my Soundcraft series 2 board (circa 78).

Jim Williams has mentioned the Hakko desoldering tool as a must - anyone care to toss their hat into the ring as to their best technique??

Maybe I was just too impatient!! I'd love to re-cap a few channels to see the effect... but the IC incident put me off for the time being!!
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Don't know if this will help but I thought it might:

https://homerecording.com/bbs/showthread.php?t=309645
 
Dinty, I think you should def. replace LOTS of the caps in both those devices. I would bet you can hear the difference.

Sounds like that "major can" is very likely the one you want to change- probably the filter caps- if it's a cardboard tube with three wires to it, and if the cardboard is oily-looking, def. change it.
 
Thanks. :)

There's no leaking that I've noticed.

Would I likely expect clearer tone, fuller tone, more sound, louder etc...?
 
First thing you will notice will be less noise. Then, perhaps fuller tone, but prob. not louder.

I could be wrong, just taking a stab at it...
 
Loudness is not the key here.

Clean power is so that the noise level drops dramatically.

Part of what made Ray Dolby a success was "thermal" noise from the electronic components, and believe it or not poor quality electrolytic caps were a major part of that noise base.

So get clean power and the noise floor drops making "loudness" more prevalent.
 
GCalo said:
Part of what made Ray Dolby a success was "thermal" noise from the electronic components, and believe it or not poor quality electrolytic caps were a major part of that noise base..
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i would be interested in seeing the source of this statement... and have serious doubts myself of it's veracity...

thermal noise as i understand it applies almost entirely to silicon devices... and occur at frequencies well above the point that supply filters will help...

as to dolby i thought his primary contribution is in the designing of VCA's and applications to multi band use especialy...

as to the polar caps... ime.. when used in the audio path it's almost always a matter of expected dc offset between stages... when upgrading op-amps for instance it's often encountered... best thing IMO is to power the circuit up with coupling cap removed... and insert cap oriented accordingly... also as to bi-polars... becareful some (tantalums in particular) dont like offset voltages... in most cases with older equipment the use of a particular cap is as much about size/space and cost as application so try upping the cap(usually@2X's) for increased freq response... if ya wanna trace for specific bad caps use a scope and signal generator preferable a square wave and look for rounded edges... in a pinch the generator on the scope for adjusting the probes works pretty well it's square wave...
 
OK, so I'm making a shopping list for recapping parts of my M-520.

I'm still somewhat new to this and I need some help on this one topic: Tolerance. The M-520 schematic / parts list does not specify the suggested / required tolerance of any of the capacitors.

Is there a "safe default" tolerance that can be assumed when ordering a replacement capacitor? ±20% maybe? (Guessing)
 
20% is fine. I used these on mine by Cory's recommendation. http://www.mouser.com/ProductDetail/Nichicon/UKT1C102MPD/?qs=sGAEpiMZZMtZ1n0r9vR22e0BBN1kF1oi7uTok%252b14cfw%3d

Not sure when you need a tighter tolerance, or how that comes in to play. I imagine it's like resistors, they'll all work but some are steadier than others.

I might be stating the obvious here but make sure you look inside to see if you need axial or radial for the part you're replacing The power supply tends to have some axial where the channel boards have mostly radial connections.
 
Jeff

Contact Cory. He has a full spread sheet of all the caps you'll need for the 520.

Also, stick with Nichicon caps avaialble from Mouser or Digikey. Mouser seems to have a bigger selection.

The Nichicon caps are listed at 20% but they are really closer to 7% -10%.
These are great caps.

If you want execellent audio quality use the ES series caps.
 
Right on all counts but note that the ES series caps are non-polar. I use the KT series Nichicons for polar audio path lytics, PW's generally for power supply caps.

All the caps on the M-520 are radial.

20% tolerance is totally fine. That simply means that the capacitance value will be +/-20% of the rated value, but as Greg said Nichicon caps seem to always be well under 10% even on their 20% caps...and most are around 7% and some even as low as 5%...very consistent. In most cases even 20% variance will have little if any effect. The EQ circuit is where it is more important to have tighter tolerances, but I never bothered being picky about it. If you wanted to you could always hand-select those and pick ones with the tightest tolerance and the most consistent...this would just make your channel's EQ sections more consistent from channel to channel, but that was more than I felt necessary. Not everybody would agree.

What else ya wanna know?
 
I have to quite drinking!

I can't believe I overlooked that again!!!
 
And, Greg, really...I've TOLD you if you ARE going to drink then for crying out loud quit sipping the stuff that's in the caps...bad.

BTW, though, Jeff, DO loook at those ES caps for audio path non-polar replacements...especially on the mic pre coupling cap. That's the biggest cap on the channel card...470uF/16V.
 
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