identical G05-802 problem with 2 different deflection PCBs

Hello,

I've been a lurker here for a while, quietly absorbing the insight but
I've hit a problem that has me (and my limited knowledge) baffled.

I have a working asteroids game board (2 actually), with which I can
play fine on the scope (also when grabbing the signal from P100
instead of the test points)

When I initially got the machine, I had problems with the monitor
(G05-802) having no horizontal deflection and the spot killer being
active. After checking the PCB, I replaced a couple of transistors on
the board a well as the Q708 & 709 chassis transistors.

Upon powering up, I would lose the 2 chassis transistors in question
and sometimes the F700 fuse. I practically inspected every single
component on the board, but found no extra faults. I looked for
shorts, tested voltages (95V at P500, 30-35 VAC coming in at P100 pins
7 & 10, 12kV for HV etc) but found nothing that looked suspicious to me.

I recently got a new known working/tested deflection PCB as a
comparison/reference point, hoping that would help me see what was
wrong or, at the worst, serve as a working board. Well, no luck there,
when I plugged it in the spot killer was on and I had no horizontal
deflection. I guess I can deduce from this that my problem is outside
the PCB itself?

Powering it up a second time to take some readings, the spot killer
was still on but I'd lost the picture, turning brightness/contrast up
only produced a faint haze towards the top of the screen.

Inspection of the PCB revealed R713 was burnt to a crisp as well as
its neighbours Q705 and Q706 both being shorted. I checked the chassis
transistors, they were fine, I looked for shorts in the sockets, plugs
and cables leading to them but found nothing wrong either.

I replaced the dead components but powering it up now blows F700 when
I'm lucky or the chassis transistors. I'm weary of powering it up
again as I don't feel like destroying components for the fun of it.

I'm at a loss now, not sure where to look outside the PCB. Could the
yoke be at fault? I'm in unknown territory here and would appreciate
any pointers I can get.

Thanks!
Colin

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Received on Sun Nov 29 18:18:41 2009