Thursday, August 21, 2008

Thoughts on Capacitors

For today's post, I'm turning over my blog to my husband, 'cause he knows more about this subject than I do.



I recently read a bunch of posts on AH about recapping.
After the laughs and tears subsided, I thought I'd write a few comments.


"I don't get this whole recapping thing"

Obviously you don't. Electrolytic caps have a wet paste in them like a battery.
Like a battery, they can lose their ability to hold a charge or can leak.
They can corrode their conductors from the inside out.
Much like a rusty pail of water, they can appear fine one day and leak the next.
It's well known and accepted that electrolytics do not perform as well over long lengths of time. To not seek that knowledge first before offering a dumb opinion, is itself quite dumb. I've never actually seen a person die from cancer, but thousands do every day.
Btw, how clean do you think that water remains in a rusty pail? ;-)



"You can test the cap in the circuit."

Wrong. Most old caps will read just fine if tested in a circuit or out.
This is because most people are checking them in a static way and are using a capacitance meter. However aged caps can change in how they react to high frequencies and can develop lower resistance than desired.
A cap may measure fine with a meter and fail horribly when actually in a circuit.
Even old polystyrene caps can work 90% fine, but then work poorly at high freqs.
Their "dissipation factor" may not be as good as new and this can affect *just* high freq tracking.




"Replace the tantalums"

On Arps, I wholly recommend this.
But on many other synths, failures just haven't been observed as often.
I've had to replace only one tantalum in Moogs in 15 years.
Replace them if you like, but remember that tants have even more specs to adhere to than electrolytics and they're very expensive.



"Anybody that can solder can replace caps."

What an ignorant statement.
Quite like "anyone that can cut flesh can do surgery."
It's always what someone doesn't know that bites them.
I've seen people replace caps on some synths (Sonic six for example) and not know the holes are not plated through and thus the component required soldering on both sides of the boards.
I've seen people install caps in synths where the leads were left so long, the caps shorted out upon shipping.
I've seen caps installed backwards.
I've even seen full recapping jobs done correctly, but where the synth died because the "tech" screwed up on assembling it correctly afterwards.



"Discharge the caps!"

How ridiculous!
Low voltage caps in low impedance circuits don't need to be discharged.
What little voltage they have will be quickly drained.
Besides not having any voltage left in them about 5 seconds after being powered down, the most they'd have is 20-30 volts on them anyways.
Discharging caps is necessary when working on high voltage devices such as video monitors and TVs. It is not necessary in synths.
A cap in a synth won't kill you. A cap in video monitor with 50KV, might.


"How do we learn if we can't try?"

Try all you want. That's how I learned.
But very few amatuer techs write a followup post how they screwed up their instruments afterwards, so the thought left to many people is that the job is easy.
The fact is, you cannot just decide to do something right. You must practice.


Dentistry, law, medicine, martial arts.. they all look easy from the outside.
But only an idiot tries doing these jobs themselves unless they don't value the outcome.
Most home medical remedies don't work.
Most legal self-defenses fail in court.
Most DIY jobs I've received in are done poorly.
Most people who think they can fight get their butts kicked if the opponent is trained.

It should be mentioned that (imho) Tony at Oakley sound got it right and so did the guy who mentioned lifting traces despite good tools and experience.
I have 34 years of synth experience and still make mistakes now and then.
But the difference with people like Tony and I is that we know how to fix our own mistakes and don't cause as many to begin with.

3 comments:

synthparts said...

In my experience it's not just ARPs that should have the tants replaced.

I've seen OB-X voice boards with old tants that have shorted across power rails. Also seen the same thing on OB Two/Four-Voices, and on the voice boards in Prophet-5s...

rick said...

I've replaced shorted tants on Yamaha CS40Ms (2) as well as OmniII, Quadra, Prophet 5, OB1.

Synthfool said...

I've seen tants go bad in most synths, but Arps are the only brand I'll automatically replace every one without hesitation.
Any that see heat on a power supply too.

It's funny- the JP8 uses mostly 85c caps while the MKS80 uses 105c.
Roland learned. ;-)

Btw, the silkscreen for C118 on the later MKS80 voice boards is backwards.
Install that cap as the board indicates and watch the fireworks. ;-)