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Author Topic: Mystic Marathon  (Read 2744 times)
tim
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« on: November 20, 2005, 04:36:48 PM »

Symptoms
Power on, no boot. Blank screen. LED solid 8. No address lines pulsing. Clock present. Sync present.
Reset line held low.

Tests
Cut watchdog disable jumper. Reset still low. Manually pulling reset high makes game boot and pass
self tests. Reconnect watchdog jumper. Manually pulling watchdog high makes game boot. A look at the
schematics show IC67, IC35, IC65, IC66 and IC51 are part of watchdog circuit. IC67 is
a 7407 (a buffer). Pin 6 is the watchdog output. Pin 5 is an input. Pin 5 is high, pin 6
is low. Data sheet says if pin 5 is high, pin 6 floats. Therefore, the watchdog is not at fault,
since something on the other side of it is pulling the line low. Testing all the parts in the circuit,
I found that shorting the zener diode ZR1 caused the game to boot. The schematics show 6.8 volts
tiggers current flow in the direction of the reset circuit. The voltage on the far side of the diode
is only at +5. Tracing back from ZR1 brings me to the audio opamp. Broken pins. Replaced the opamp.
Still no boot. Looking at the schematics, the broken pin on the opamp should carry +12. It's only
got +5. Hmmmm. The pinouts on both spies and the parts placement page of the schematics are wrong.
They list +5 where there should be +12 on pins 9 and 10.

Fix
Putting +12 on pins 9 & 10 causes the game to boot, but I hear static instead of game sounds.
Putting -5 on pin 12 makes all sounds work. Never trust the schematics. Wink
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