Re: Ceramic Disc Caps for AS2518-61 Squawk and Talk Board

Our two posts were timestamped 53 minutes apart, but not
everything travels at the same speed on USENET.

Capacitors have a number of electrical characteristics, all of
which sum up how they will perform in delicate circuits.
For simple power supply bypassing, values are very rarely
critical at all. Big electrolytic caps are used for "bulk"
bypassing (smoothing out AC ripples), but electrolytic caps
are not as good at absorbing fast transients, such as switching
noise of digital logic. That's why you see so many small ceramic
caps on boards with lots of digital chips. In those applications,
every disc cap is not even required, and the values can change
over a wide range (.47uF to .001uF are used typically.

Next up are DC blocking capacitors. These are used to pass
an AC signal (in pinball machines, usually an audio signal).
Here they can affect the sound. If the capacitor is too small,
low frequencies can be affected. If the capacitor is too large
there is nothing inherently wrong, but the secondary characteristics
of the capacitor can adversely affect the signal in some circumstances.

Finally, capacitors used in RC (resistor-capacitor) circuits
are most critical. RC circuits are used in filters (lots of those
on the S&T board), oscillators and other timing circuits. In these
uses, the characteristics are critical. The value of the cap should
be matched exactly, as should the type and tolerance. These and other
critical circuits (e.g. sample/holds) are where you need to be careful
matching a replacement capacitor. Voltages here are usually pretty low,
but some small value capacitors are so small already they only come in
100 or higher voltage ratings. Again, there's nothing inherently wrong
with putting in a replacement cap with a higher voltage rating in all
but the most critical circuits. And if the replacement does change the
sound, it's not due to the voltage but some other characteristic of
the part.

There are audiophile amplifier designers that spend forever tweaking
every resistor and capacitor in an amplifier until they get it to
"sound right". For the lo-fi audio that comes out of most every pinball
machine, these kind of tweaks are unnoticeable and unnecessary.

The sound can change if the capacitor is out of spec or dried out.
So replacing an old worn out cap with the "same" new cap can still
change the sound.

-Mark
-----
http://pinballpal.com

fordiesel69 wrote:
On Jul 21, 10:29 pm, Mark Clayton <spamuser1...@xxxxxxx> wrote:
So if you don't know what it's used for, why are you discouraging
him from taking the advice already given? C35 is a DC blocking
cap used near the low pass filter on the output of the GI sound
generator chip. The value is non-critical. The voltage is non-
critical. It's not used as part of an RC network, and if the
performance was critical, they wouldn't have used a disc cap to
begin with. No need to make things more complicated than they
already are.

-Mark
-----http://pinballpal.com



fordiesel69 wrote:
On Jul 21, 8:01 pm, "Wingman" <w...@xxxxxxxxxx> wrote:
C35 is the board position.
"Wingman" <w...@xxxxxxxxxx> wrote in message
news:4a664e4f$0$201$bb4e3ad8@xxxxxxxxxxxxxxx
I'm trying to find a .47uF ceramic disc capacitor for an AS2518-61a Squawk
and Talk board. I've located a cap kit for all the others, but it doesn't
include the ceramic disc caps. I've looked at DigiKey, Mouser and Newark,
but nothing.
The reason I need one is the existing one has been broken in half.
Thanks!!
Wing- Hide quoted text -
- Show quoted text -
Would any member here have an old board laying around that could hook
this guy up with a used one?
I have found that many caps on the S&T are used in voice speed and
pitch. Changing the characteristics to somthing other than original
can affect voice. I don;t know what this particular one is used for.- Hide quoted text -
- Show quoted text -

Somehow Mark I did not see your reply. It does make sense now. My
EBC board I got off ebay someone substituted some caps with higher
volts but kept the uF the same. The ones in GPE's kits were 6.3v and
the ones I took out were 100v and 50v. It did cause some wired
voices. I was looking for any kind of a cap for him and found lots
of .47 @ 50v, but none at 16v. So I assume now that the voltage on
this cap would be no problem especially if it is higher.

Could you explain why a higher volt cap changes the characteristics if
used in circuits for voice generation?
.

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