Tempest X-out voltage stuck

giffnyc

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Resurrecting a Tempest which had been placed in storage working and came out not.

Have done a full cap kit and rebuilt much of the deflection board. Game has spot killer on and plays blind. For a bit, the game was playing with the screen collapsed to thin squiggle in the center of the monitor, but now the spot killer is on.

Have moved down to the PCB and am getting a steady 7.45v on the X out test lug with pot all the way down. Y seems to be deflecting OK. Good steady +5v from the PS and the rest of the voltage from the ARII looks OK.

Turning the X pot will smoothly up the voltage, as far as about 8.5v.

I'm not very familiar with moving upstream - any tips on basic testing that can be done to isolate the fault - do not have a scope or probe, just my trusty DMM.

Appreciate any help. Thanks!
 
Check the MC1495s. A common failure point. Make sure that the DACs output is pulsing and then move the 1495s. I bet you'll find one is dead.
 
Thanks - been looking at the schematics, and poked around a bit - could use a tip on how to test the digital components, for instance, looking for variable voltage at pin 14 of the DAC for X at A10 - its marked "X ref" on the schematics - but not really sure what I'm looking at - any advice appreciated.

Did find a slightly higher resistance at pins 10-11 of the 1495 at c12 - like 3.6Kohms instead of the 2.7K indicated on the schematics - worth socketing/replacing?

Thanks!
 
Well the video out section is analog, so your best bet is to put a scope on all the outputs of each chip going backwards using the schematics.
 
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