Re: Re: The Secret Life of XY Monitors

At 04:51 PM 3/17/2001, you wrote:
>For those of you not reading RGVAC...
>...
>
>Al Kossow has asked two questions:
>
>
>----------
>
>2. Also, there has been speculation for years that the deflection
>transistors fail due
>to base-emitter overvoltage due to inductive flyback. Opinions?
>

Being unable to find this message on the newsgroup (maybe someone can post
this?) I shall answer this here. It's a bit rough, but I hope you can
follow it. I am NOT a technical writer unfortunately...

          I believe the answer to this perennial problem is ground fault
related. I strongly suspect that when the ground connections for the logic
board get toasted that the monitor ground reference changes relative to the
power supply or the logic board. This introduces voltage differences that
are far outside of the normal input range for the monitor and the output
transistors blow trying to follow. The monitor has similar grounds to the
logic board, however the power supply grounds to these boards or the
connections at the regulator (SEGA!) deteriorate and thus your ground
potential starts to drift.

         Note that the Atari XY monitor has at least two ground reference
points. There is the chassis ground wire that goes to the main ground bolt
on the transformer box, and then there are the signal grounds that go to
the logic board, these are 'tied' at the monitor but not again at the logic
board. So as the resistance starts to build in the logic board power ground
pins the ground relative to the power supply ground starts to drift, it can
go up to .7VDC volt relative to the power supply ground (at least on our
pinball games). Once it reaches about .7 (actually around .5 in some
cases), then transistors turn on a bit as their bias drifts, they get hot
and then they fry...

         This is the same problem that I identified in Gottlieb pinballs
over ten years ago, (why coils were frying and taking the driver
transistors with them) and it is why I have had our ground connections
repaired/beefed up on the ground circuits for all the XY games we sell. I
have not lost a monitor in a long time as a result.

I guess I should have shared this with the group before, but really I
didn't think of it as the problem had been long gone for us...

John :-#)#

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Received on Sat Mar 17 21:20:50 2001