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Model 1 video issues

So I think I have possibly stumbled on to something. Foscusing on Z32 (replaced with no change in symptoms) and Z66 has revealed a possible internal short on Z66 between pins 7 (ground) and 12, the output back to pins 2 & 3 on Z32. Studing the schematic, Z12 & Z50 take other outputs from Z66 (pins 6 & 8 into pins 2 & 3 on Z12/50 ), but no shorts detected on those two outputs. Appears all 3 outputs of Z66 feed Z12/32/50 in same manner.

Have not cut, desoldered or removed anything yet. In my haste to buy chips, I accidently purchased an NTE7411 for Z66, but reality is that it is a 74LS11. Can a non-LS chip be substiuted here?

Same question for the 74LS93....can a 7493 be used? I did use a LS-chip on Z32, but have several 7493 on hand.
 
So I finally found time this evening to sit down and replace that Z66 chip. I did order the correct LS series chip just after my last post above.

It cured the problem of the vertical sync issue! Nice and clear screen. But now another issue...the display seem to flicker/flash in a timed loop. And at same time a blank, square box briefly appears in the center of the display. Currently the 16k RAM chips are removed, so the complete screen is filled with the @-something characters. One thing I did not reinstall is a small ceramic filter cap tied between pin 7 (GND) and pin 8 (output 3Y). This cap is not shown on the schematic, but was added at some point. Next session I will add this back in and see what happens.

Still I am very pleased with results. It's slow going as I usually tinker with this type of thing in the winter months. I'm under the gun to get stuff done outside while weather is still nice. When the wife suggests WE tear out and re-do the landscaping in front of the garage, what that really means is she picks the plants.....and I get to do everything else!
 
Well the verdict is in....this thing appears to be alive. At least to the extent of my knowlwedge for this machine, which is limited. The mentioned filter cap had no effect on the flicker issue when held against the pins of the chip. May or may not soldering back on. I likely will as it was there for a reason I guess.

But reinstalling some functioning 16K RAM chips ended the flicker and produced a functioning screen. Let it idle for about 20 minutes with no screen anomalies. All keys still function. Other than the 5v voltage regulator getting borderline hot, no other chips were more than warm to the touch.

Next step is to get it buttoned up back in the replacement case, and try out a cassette player.
 

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