What Caps to Recap Classic II?

[CompuNurd]

I want to do an entire analog board recap for my Mac Classic II. For the PSU part (there is a big group of caps, and you can see a green stain around the board where they are) I would be using low-ESR. For the rest, which should be the CRT circuitry, I would be using high frequency.

The problem is, I don't know what would happen if I put in regular capacitors in the CRT circuitry. Would it work fine? Would it fail quickly? Would it just not last as long? What would happen?

So, what if I replaced them all with high frequency, or if I replaced them all with low-ESR?

Sure it may not be necessary to recap an entire A/B, but I just want to be safe!

 

[Elfen]

Don't use High Frequency Caps in the Analog board since the CRT Circuit is tuned to those cap values. Do use a higher voltage Cap. There is a difference, and a higher voltage caps will handle surges and stresses the analog board goes through in life. A Short Life for a Analog board is less than 5 years, while a long life is a bit over ten. With higher voltage caps, you can get a board to last you 5 to 10 or even more years - all depending on the stress the Mac goes through (does your area suffer from voltage spikes? Even though I'm in NYC in a Housing Development where electricity is pretty stable 99% of the time, I do have a neighbor that uses heavy industrial tools for his car & engine repair business he's running in his apartment, and his equipment constantly spikes, surges and dips the electricity for the neighborhood! This has cost me a few Macs and PCs throughout the years. DAMN HIM!)

As for the logic board, I did mine in the summer, and its a simple job to recap the board except for 1 cap. It is in a very tight area behind the ports on the right side of the board. For me, this recap brought a dead Classic II I found in the trash back to life though it has no sound, which I blame on a rotted trace I have yet to find.

https://68kmla.org/forums/index.php?/topic/23800-semi-successful-recap-on-classic-ii/

 

[CompuNurd]

The Mac would probably not get used too often, maybe a few times a month. I like to keep the Macs in working order before I put them on the collection shelf, in case I do want to use one or get parts from it.

From what you said, I think general purpose or low-ESR would do the job fine. Probably no need to up the voltage for how I will be using them (sitting on a shelf, fired up once a week or month just to keep the parts moving smoothly).

 

[Elfen]

How ever it is done, or why it is done, you should get the caps with the right values on it.

A cap works something like a switch that you switch on and off constantly but at a much faster rate than you ever can. This creates a frequency within the circuit which is measured in Farads (microfarads, nanafarads and picofarads - microfarads - µf - being the main one of concern here). It is this frequency that determines the height and width of the video on the screen. Put in a different farad value into the circuit, and the height or width of the screen changes. So the farad values need to be the same in the circuit. The Voltage rating is how much the cap can handle. This can be changed to a higher value to make the circuit physically stronger. One of the caps on the Analog board is in the 1000 volt range, this one cap is connected to the high voltage transformer. This one cap is dangerous as it can hold that much power and the reason why working with the analog board can kill you. In replacing this one cap, short out its pins first before desoldering it. A screw driver across the two cap pins is enough to discharge it.

There are other caps on the board that charge up the high voltage transformer, that is why you discharge the transformer.

Since I seen your work, I think you will do well in getting the Classic II recapped nicely. Just take your time with it and get the right cap µf values for it, though you can use higher voltage ratings.

 

[techknight]

A cap works something like a switch that you switch on and off constantly but at a much faster rate than you ever can. This creates a frequency within the circuit.........

This is called a transistor...... not a capacitor.

If you want to study the function of a capacitor it would be easier to google Wikipedia it than trying to explain it here.

 

[Elfen]

This is called a transistor...... not a capacitor.

If you want to study the function of a capacitor it would be easier to google Wikipedia it than trying to explain it here.

It would be easier or you to explain is in words of 3 syllables or less... And Wikipedia can be so unreliable at times.

As I remember it, a capacitor has two plates and a non/semi conductive material (a dielectric) between them. Electricity builds up on one plate until it surpasses the threshold of the dielectric material of holding back the charge and then the charge is set through to the other plate until the charge reaches zero and the dielectric material is able to block again. This, as I say it sounds like, switching on and off at a fast rate.

A transistor is a switch to allow current to go through two pins depending if the third pin of the transistor is activated by a charge or not. Thus the difference is as I see it, a Transistor is either on or of, and a capacitor is pulsing on, off, on, off at a steady frequency depending on the dielectric material and surface area of the plates.

Help us here and correct me if I'm wrong. We all need to learn.

 

[techknight]

It would be easier or you to explain is in words of 3 syllables or less... And Wikipedia can be so unreliable at times.

As I remember it, a capacitor has two plates and a non/semi conductive material (a dielectric) between them. Electricity builds up on one plate until it surpasses the threshold of the dielectric material of holding back the charge and then the charge is set through to the other plate until the charge reaches zero and the dielectric material is able to block again. This, as I say it sounds like, switching on and off at a fast rate.

Technically speaking... Yes. this is true BUT... This would be called "ringing" and it cannot sustain itself. This isnt switching. switching requires control currents, etc. This is simply ringing. and eventually due to the resistances, leakage currents, and other non-ideal situations in the capacitor it settles down. 

Capacitors are great at blocking DC currents, because as soon as you hit the capacitor with one polarity of charge, it will charge up, and the rate at which it charges is in Q, or coulombs. This is directly related to the capacitors value, in farads. As noted: (One farad is defined as the capacitance of a capacitor across which, when charged with one coulomb of electricity, there is a potential difference of one volt.Conversely, it is the capacitance which, when charged to a potential difference of one volt, carries a charge of one coulomb.[2] A coulomb is equal to the amount of charge (electrons) produced by a current of one ampere (A) flowing for one second.) -Wikipedia   

But once it charges in one direction, as long as the voltage polarity potential is stable, it doesnt "bounce around" or oscillate.

Capacitors are great at AC, because in an AC environment they will alternate their polarities allowing the same waveform to pass on the other side from the site, With a tiny bit of phase-lag. The amount of energy that is passed from one side to the other depends on the Reactance (Xc) of the capacitor and its circuit, And its ESR at its given frequency tune. There can never be a reactance of 0, because ESR gets in the way. But this requires again a transistor and some other stuff to create an oscillator. There is quite a few oscillators, Blocking oscillators, colpitts oscillator, etc. 

But this is all irrelevent in the Mac land, (except maybe the power supply control section). In the mac, the majority are all used as filter capacitors. Capacitors make great storage tanks because what happens is if the noise in the supply rail is faster than the capacitors rate of charge in Coulombs, it smooths the supply because the capacitor cannot discharge and recharge at that rate of speed.  The value of the capacitor here is determined on the factor of noise/ripple, AND current that will be used on that supply line.

The higher the frequency component, the smaller the capacitor you need.  

there may be quite a few things that I missed, but thats the majority of it. 

 

[Elfen]

This is very well put, Techknight. Thank you!

 

[CC_333]

Agreed!

Someday, when I'm done with school, I'll know this stuff!

c

 

[techknight]

The only reason I know this stuff, is as an engineer, kinda have to. There is also some stuff that I am supposed to know that I have long since forgotten. Like math formulas. thanks to google, its an easy search. lol.