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A simple, but important replacement for Analog Board Capacitor C1 in 128-512k Macs

T-Squared

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Since C1 is a unicorn for many people (3.9uf, 25v), on a whim, I decided to figure out what to replace C1 with on the analog board.
(I was recapping the analog board to make sure that the startup problem I've been having is not related to it.)

It turns out that a 4.7uf 450v non-polar capacitor works just fine. It seems easier to get.

Be careful, though, because you also have to take into account the size. Don't get the tiny ones, even though their voltage rating says otherwise. I got one that was about half the size of the original.
 
The 3.9 uF non-polar cap is special in that it is also rated for high frequency, because it has to deal with the deflection frequency in the yoke. A normal non-polarized capacitor is NOT rated for that high of a frequency and can potentially fail and cause lots of damage to the analog board. The reason the original capacitor is so large is because the internal plate layout is different to account for high frequency use.

There is only one suitable substitute I know of, a 3.9 uF polyester film capacitor rated for RF microwave applications. The only problem with these are the physical size, so you have to get a bit creative mounting them.

The 4.7 uF capacitor you subbed in is probably running far outside its operating envelope. I wouldn't recommend using it at all, it is not a suitable replacement.
 
so I looked for one reasonably-sized.
This isn't what he was trying to tell you. Yes, the one you used is large in size because it is rated for 450v! Higher voltage ratings makes a cap larger in size. But that has nothing to do with the frequency it is able to work at.
 
I had some info beforehand to not use a small capacitor (since that's like trying to use a penlight bulb on a spotlight), so I looked for one reasonably-sized. This one also happened to be low-impedance.

That's not really a correct analogy. There are several things that determine capacitor sizing, and the actual farad rating is low on that list. But if you have two differently sized capacitors that are both rated at say 2200 uF, they will both store the same amount of energy.

Voltage rating, polarity and ESR rating all have a significant effect on capacitor size. All three change the order of the plate arrangement inside the capacitor body, and even more so if you're trying to combine multiple different attributes.

In a deflection application, you want as low of an ESR as possible.
 
Isn't it the case that caps can fail if you happen to operate them at their resonant frequency ? As reactance falls of a cliff at that point.

Is the CRT horizontal frequency enough to do that in this case ?

So isn't the ESL more significant in this case ?
 
Still don't understand why a film cap wasn't used. Heck, I used to see very large orange-drop caps in vintage CRTs in pretty much the same application.
 
Still don't understand why a film cap wasn't used. Heck, I used to see very large orange-drop caps in vintage CRTs in pretty much the same application.

They were sometimes used. I have worked on SE analog boards which had strange axial/radial film capacitors in that spot that were cylindrical.
 
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