Re: Help with Defender Repair

On Tue, 12 Aug 2008 18:00:14 -0700 (PDT), BrianDP <bdp222@xxxxxxxxx>
wrote:

Hi. I need some help with a Defender problem I'm having. I bought
this set of boards from Ebay, and I swapped them into a Stargate
Cocktail that I have. Everythign was fine, and then one day I plugged
in the machine, and it came up with a bright green screen. It hasn't
played, or gotten off this green screen since.

I have a friend looking at it, he's an electronics buff. So, he
looked at it, and reports the following:

Defender CPU board - the 6809 CPU has a constant reset on pin 37.
The reset is not being generated by the ROM board or the widget
board.
Because the schematics do not match the boards in question,
impossible to continue troubleshooting any further. If the
schematics
matched the circuit board then troubleshooting could continue.

If anyone can help us with this issue, I'd appreciate any feedback
you've got! Thanks!

-BrianDP

If you were running it in a Stargate Cocktail and it was flipping,
then it is a newer board set.

Check the power supply voltages. As I recall, if the unregulated +12 V
drops below 11.5 V the reset circuit engages and the reset line on the
CPU is engaged. This is to prevent a problem where the CPU would
spontaneously overwrite the CMOS RAM during power down.

The other possibility is the watchdog circuit. This is a timer chip
connected to a flip flop that needed to be toggled by software every
so often or it would pull the reset line down. If the flip flop is bad
it may be pulling the reset line down. You should be able to locate
the watchdog on the schematics and use a logic probe to verity that
the clock chip is working and that the flip flop is not stuck.

The bright green screen is a common tip off in Williams games that the
CPU is either fried or is being locked out by the reset line. That is
just the normal power up color of the video RAM before the CPU sets
it.

You can find a semi-legible scan of the schematics for the later board
at:
http://www.robotron-2084.co.uk/manuals/defender/defender_later_pcb_drawing_set.pdf


Good luck.

ken
If it ain't broke....you ain't playin' with it hard enough!

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