Major havoc repro. Issues

I still need to thoroughly go through the window circuit, but it does seem that the signal makes it to pin 16 of 12d. While probing around I noticed that I had floating pins on 3d(10,12,13) and the leg on y2 that connects to q5 seems to float at times. I currently have a 8mhz crystal populated but have an 8.99mhz on hand, not sure if I should swap it though due to not knowing if this is normal behavior. I wouldn't want to kill the new one if theirs something taking it out. Is this normal? I also noticed there's a thick trace(one of the thickest on the pcb) that runs under the ic labels from 4R to 4E. It produces a barely lit low signal on my probe although it doesn't produce the audio I usually get from a low signal. Hope I'm making progress. Gotta sleep for now.
 
Sorry I haven't contributed to this thread, really busy these weeks and haven't had a chance to dig out the schematics.

Looks like Douglas has you covered though. :)
 
This is definetly the largest project of this kind that I've ever taken on, hopefully I haven't created too many issues for myself. Placing an order for a few components tonight.
 
I'm not sure if this a healthy signal on 9J pin 13, it seems to change when the blanking is going on(45s into the video). I'm not sure how to diagnose any issues before 9J pin 13(signal VYD11?) it seems to be tied to quite a few chips. Is this a data bus? Sorry I'm a total noob, but willing to listen and learn.

https://m.youtube.com/watch?v=Uo10GLSRqTY

I also took a quick video of the weirdness on Y2, sometimes it's almost as if the leg I'm probing floats(soldered in solidly) while not shown in the video it does seem to behave inconsistently. Sorry for the lame videos, never used YouTube much(not sure how to edit) and my iPad has no space for pics/vids(deleting as I go). New dac08 and transistors should be here Saturday.

https://m.youtube.com/watch?v=wZE1gCHpSpU

Thank you guys for all of your help thus far.
 
I'm not sure if this a healthy signal on 9J pin 13, it seems to change when the blanking is going on(45s into the video). I'm not sure how to diagnose any issues before 9J pin 13(signal VYD11?) it seems to be tied to quite a few chips. Is this a data bus? Sorry I'm a total noob, but willing to listen and learn.

You are correct, VYD11 is part of a bus and is used by a whole lot of stuff. One of its main functions is to deliver the Y value to the Y DAC so it can be converted to an analog signal and move the beam up and down. There's a complementary bus (VDX_) that does the same for the X axis. But VDY_ it also is used for things like setting color information, enabling (and creating) the sparkle effect, binary scaling, and even selecting which page of the big vector ROMs at 6H and 6J/K are being accessed. Bit 11 in particular is part of the range for Y coordinates (fed into DAC @ 11D), it is used by the the AVG chip to direct the execution of the vector generator, and of course by the window circuit. So there will be a lot of varying activity on that line.

Whatever is on VDY11 goes into pin 13 of 9J, gets inverted, and sent out of pin 12. You can estimate if this is happening by observing how your probe behaves while on pin 13, and then comparing it to 12. For example, both LEDs are flashing but one is probably brighter because the pin is spending more time in that logic state. Switching to the other pin should show the opposite.

That goes into pin 13 of 7A, and along with pin 12 gets the logical OR operation applied to produce a signal on pin 11. That is used to refresh the flip-flop at 10K, which is being cleared by low pulses on its pin 13. At the proper time, a low blip will come out of pin 8 to allow the rest of the window circuit to sample the Y output at that moment. So make sure you have activity that resembles this on each of those pins, all the way up to pin 16 of 12D.

The rest of the Window circuit is hard to probe without a scope, so just do a visual inspection that you have the right parts installed and all pins soldered.

Then, in the RGB output section, probe pins 11, 12, and 13 of 8K and pins 1, 2, and 3 of 8J. Make sure you have a 74LS00 installed in 8K and a 74LS02 in 8J.


I also took a quick video of the weirdness on Y2, sometimes it's almost as if the leg I'm probing floats(soldered in solidly) while not shown in the video it does seem to behave inconsistently.

Seems fine. The crystal is not 'strong enough' to trigger the logic probe. Check pin 12 of 3D for a nice study buzz. But if your Y2 were not working you'd get no vectors at all.
 
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Awesome! Thank you for the in-depth walk through of the signal. I'll be sure to report back after I find a little time to sit down and take a look.
 
I received some transistors and a new dac today, swapped them out and I'm still having the same issues. I think I'll focus on the window circuit first before I try to tackle the slight "jitter" in the text. I'm trying to wrap my head around all of the "logic" used, I seem to be getting the signals that you've described but when I get to the flip flop at 10k I get a little confused. All the pins used seem to have activity on them, should the "low blip" on pin 8 be a low pulse? Or just float and then send a quick low signal? I also noticed that the bank of dipswitches at sw1 are always "on", I've checked continuity through the switches and to ground and the switches themselves seem to be open. They look like they pass through the quad pokey, pulled it from its socket and get the same result. I've got a ttl databook, printed schematics, laptop and an iPad open to various pages and your instructions, seem to be getting no where but managing to have a good time trying to absorb all of this. I really need to watch some vids showing how to use a scope properly and set triggers. At least I can play the game when I get a little frustrated, even if my dipswitch issue has me on the hardest difficulty. Since I socketed all of my chips, I'm thinking about ordering one of those $20 testers from china. Thank you for bearing with me.

Ps: BOM calls out q7 as a 2n3904 while the schematic says it's a 2n3906, I've got a 3906 in there though.
 
I decided to pull the 5 roms used for return to vax and program them with the rev 3 set. It cleared up the issue I was having with the attract text(no more self test errors), although I do still have a problem with dipswitch bank 1 and a slight jitter in the text(not sure if it's normal). I checked the pull up? Resistors that are tied to the dipswitches and quad pokey, they test low as if the switches are closed(they're open,tested with multimeter) unless I power up without the quad pokey which in that case they test high(bad quad pokey?). I had forgot to mention that when I first performed the adjustment of the pots on the pcb I had to adjust a few to their limit(some didn't seem to adjust enough), could this be an issue with the linear scaling circuit? I seem to remember reading somewhere that a slight jitter in the text can be due to pots adjusted close to their limit. Maybe I'm chasing the wrong issue? Anyway I'm glad the main issue was sorted out, although I'm still skeptical that I have some deep lying issue. Can the return to vax set be accessing another part of the circuit other than the "new" speech section? And not showing itself with the "regular" sets? I was wondering if I can pull the speech ic and if that would kill the speech section while I try and run return to vax to see if that changes anything? Sorry I have extremely limited knowledge repairing boards, just trying to find an easy way of eliminating possible issues. Maybe I'm thinking about this too much?(can't help it) Is anyone else running return to vax without any self test errors, and with fully functioning attract text? Thank you guys for helping me get this far.
 

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