TECH: Zaxxon sound help needed....

[Dinger2]

I have an upright Zaxxon. When I turn it on all the sounds are fine but
after a few (about 10) minutes some of the sounds fade (BOOM when ship
explodes, and WHOOSHING sound)(I know, that is a lame description, my
apologies)

I know which chip it is (the sound chip in the middle) and I run the
diagnostic on the machine and it tests fine. It is almost as if the
chip overheats and fades out gradually, can this happen? Why do the
sounds work when it is "cold" but as it warms up they don't?

I figured someone out there would know. I know it is something simple I
just can't figure it out.

Please respond to: sol...@aol.com with any input.

Thanks in advance.

Jim

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[Rhett James Barnes]

In article <15f8eaa0...@usw-ex0106-044.remarq.com>, Dinger2
<dinger2...@prodigy.net.invalid> wrote:

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I've had three Zaxxon boards from three different sources. All have come
with this problem. Either I'm very unlucky, or it is an extremely common
problem. I would call it a design flaw on the use of the blocking-cap,
but I'm no pro.

Here's a cut from my very small fix journal describing the problem and my fixes:
-Rhett

****Zaxxon (+Super Zaxxon)
==Problem: Sounds of ship flying, explosions, etc. may be there when the
game is first turned on, but slowly fade over several minutes. Will not
return until the game has been left off for a while. [COMMON]
-Solution 1 (easy): Replace all 2.2 uF/50V caps C69, C68, C29, C28, C38.
Only use basic cheap-o polarized electrolytics (e.g. Mouser
140-XRL50V2.2). Donšt get fancy, as the circuit relies on the leakage
through this cap.
-Solution 2 (better): Replace C69, C69, C29, C28, C38 with non-polar
electrolytics (e.g. Mouser 140-NPRL50V2.2). Next, strap the "output" side
of each cap to the +6V common though a 100kohm resistor. Refer to the
circuit diagram to find the best place to install each resistor.
-Discussion: These caps are used as DC blocking caps in the audio
section. As the feed a high-Z load without any tie-down resistor, they do
not really work as high-pass filters as planned. The capacitor can build
up a charge which eventually biases the OP amp to the point of clipping.
When the caps are fresh, leakage through the cap can prevent the run-away
bias. But if fresh caps are installed that do not have enough leakage,
they will not work.