Electro capacitor agigng question

[mac]

I have a distortion circuit that came in a guitar my parents bought me in the 70s, the FAIM circuit I posted long ago.
It has a couple of Toshibas 2sb56 and eight 10uf electro caps which do not seem to be of good quality.
I have a bunch of 2sb56 to make a copy but I'm not getting the exact sound when I use 10uf caps, the copy sounds more bassy. I guess this is because of electro aging.

How much capacitance was lost over the years, an estimate, considering that in the last 25 years it was not turn on.

mac

[PRR]

Zero to 100%. No way to tell from here.

You could remove and measure the old caps.

You could try 5uFd 2uFd 1uFd new caps.

[frank_p]

I changed electro caps in a 60's radio and measured them.  Capacitance augmented and ESR also.  I compared the bad caps with the good ones with this setup (since I don't have an ESR meter):
http://www.youtube.com/watch?v=-CTXZ5lsnjA

Basically what is done there is that you use the cap as a power filter, the less it is effective at filtering, the more the square wave will show up, the more ESR might be in there.
Play with the load resistor and the series resistor that the guy traced on the paper until you find a good combination.
By changing the extern series resistor and matching it to the good cap curve you'll find the delta ESR that the old cap had acquired.

The radio worked well after I changed the bad old caps.

[R.G.]

What often happens is that the insulating oxide layer inside the cap gets thinner. This accounts for the increase in capacitance. The chemical changes in the electrolyte that accompany this raise the ESR. You also get thin spots where the insulating voltage is lower, and where increased leakage happens. This can be modelled as a lower parallel resistance around the capacitor.

You can fake a going-bad cap by tinkering the cap value, adding series resistance, and adding parallel resistance. There are other minor effects as well.

It's in general not possible to say it's changed X amount in Y years with any accuracy. Storage conditions and operating history have too great an effect for that to be accurate.

[frank_p]

and adding parallel resistance.

I didn't know that this one was varying too.  Think I understand why I was scratching my head.  

The guy in the video is a bit misleading: he's mixing the ESR and the parallel one I think.

[R.G.]

Yeah. What happens when they form the insulating oxide is that they put cleaned aluminum into an electrolyte bath and put a voltage from the electrolyte to the aluminum. Initially, it all conducts. But the current direction is such that it causes oxide to grow on the aluminum. There is a punch-through voltage where even if there is an oxide layer, the voltage causes current to flow - and make more oxide.

As long as the current is low enough, the heat formed when the oxide is made lets more insulating oxide form. If the current gets too high, the heat is destructive to what's already there. So forming the insulator is a matter of applying low-enough current and increasing voltage until the oxide is thick enough to withstand the voltage you want.

This happens in reverse as the cap ages. The oxide gets thinner, and thinner insulator equals less voltage resisting ability and more capacitance.  Also more leakage if the voltage is near the breakdown voltage for that section of oxide. This is actually self-healing for quite a while. Leakage causes more oxide to form as long as the leakage isn't too big. That's why using a cap near it's rated voltage keeps it going longer than sitting on a shelf.

But there are other decay mechanisms too, I think. Eventually, the oxide gets thin enough that the leakage current gets big enough to heat spots destructively, and the thin spots get worse - fast. The leakage runs away, the whole cap overheats, smoke and boiled electrolyte spews out, and darkness ensues. I watched this happen right in front of me one time with an old tube unit I was reviving. Fortunately I was riding the variac knob and watching the input current so I could stop it with leakage and not have to clean electrolyte off the ceiling.

[mac]

After reading about leakage, I power the circuit on and did some readings. In and out caps are not blocking DC, they read 0.2V at any time. The others seem to be leaking too.

BTW, the circuit is this, much like a Sekova Fuzz but the first 2sb56 is at 8.7v because it has a 47k at the emiter; it does some magic.



mac

[Morocotopo]

Another example of premium, bulletproof building techniques by the guys at FAIM!!



(I´m from Argentina, as is Mac, so I´m familiar with the FAIM brand stuff. Let´s say that their guitars are, hmmm, questionable. Bronze frets! Plastic nuts! Spanish guitar-like tuners! Recently they got into the market a sort of replica of Brian May´s guitar! Bizarre.)

[mac]

I desoldered one of the caps, it read 4uf...

Another example of premium, bulletproof building techniques by the guys at FAIM!!

(I´m from Argentina, as is Mac, so I´m familiar with the FAIM brand stuff. Let´s say that their guitars are, hmmm, questionable. Bronze frets! Plastic nuts! Spanish guitar-like tuners! Recently they got into the market a sort of replica of Brian May´s guitar! Bizarre.)

Yeap. The guitar is crappy, although cool for a 5 yrs kid. I still have it, I dont know why...
But this distortion circuit is nice.

mac

[Mark Hammer]

Is that a circuit board or a bunch of teenage components playing "Twister"?

[LucifersTrip]

Is that a circuit board or a bunch of teenage components playing "Twister"?

we've become too accustomed to antiseptic modern builds....

[frank_p]

Is that a circuit board or a bunch of teenage components playing "Twister"?

we've become too accustomed to antiseptic modern builds....

This photo is a nice reminder to the fact that to do something it does does not have to be perfect to do it.
But I must admit that it look more like a pre-production prototype. 

[mac]

Is that a circuit board or a bunch of teenage components playing "Twister"?

... this board is better looking than my builds!!!  

This photo is a nice reminder to the fact that to do something it does does not have to be perfect to do it.
But I must admit that it look more like a pre-production prototype. 

it looks horrible, but it'll be well hidden into a box again in a few days, after some cap changes and minor mods 

this is the schematic if you want to give it a try.
someone noted that this is much like a Sekova Fuzz, so i realized that the real output must be taken from the junction of the 470pf and the 22k, possibly through the tone pot of the guitar. and the big ceramic disc could be 0.1uf, not 1uf, label is hard to read. this is the sound i remember from the old days.
of course there is a big volume drop. i plan to use both outputs, but i have to compensate the real output with a 5x-7x amplifier stage.



mac